methodology for polymer supported synthesis towards
TRANSCRIPT
Methodology for Polymer Supported Synthesis Towards Combinatorid Chernistry
Leah E. Begleiter
A thesis submitted in conformity with the requirements
for the degree of Master's of Science
Graduate Department of Chernistry
University of Toronto
Toronto, Ontario, Canada
O Copyright Leah E. Begleiter, September 1997.
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Abstract
This thesis is a sumrnary of research conducted since September, 1995. It is divided
into three chapters, Introduction, Results and Discussion, and Experimental.
The research described is concemed with the development of methodology for
polymer supported synthesis. New dithiol linkers for liquid-phase synthesis have been
prepared by linking lipoic acid to polyethylene glycol derivatives. These linkers have been
used to bind aldehydes to the polymer resin towards the application of Umpolung chemistry
on the resin bound dithianes. The unique solubility properties of polyethylene glycol have
been exploited to obtain both optimum reaction conditions and ease of purification. The
potential application of this methodology to combinatorid chemistry has also been
addressed.
lin Memory of Rose Gasman
Table of Contents
........................................................................................... Abstract ii ................................................................. List of Figures and Tables vi
................................................................................... Abbreviations vii ............................................................................. AcknowIedgments ix
....................................................................... Chapter 1 Introduction 1 1
1.1 Combinatorial Chemistry ............................................................ 12
1.1.1 Library Formats ........................................................... 12
1.1.2 Combinatorial Chemistry and Drug Discovery ........................ 14 ................................................ 1.2 Solid-Phase Synthesis 15
........... 1.2.1 Advantages and Disadvantages of Solid-Phase Synthesis 16
............................... 1.2.2 Resins Used for Solid-Phase Synthesis 17
................................................. 1.2.3 Solid-Phase Reactions 18
1.2.3.1 Diol Linkers for Aldehydes and Ketones ................... 18
1.2.3.2 Palladium Catalyzed Coupling Reactions .................. 21 ............................................................. 1.3 Liquid-Phase Chemistry 23
..................................... 1.3.1 Resins for Liquid-Phase Synthesis 23
......... 1.3.2 Advantages and Disadvantages of Liquid-Phase Chemistry 23
................................................. 1.3.3 Liquid-Phase Reactions. 24
1.3.3.1 Liquid-Phase Oligosaccharide Synthesis ................... 24
1.3.3.2 Liquid-Phase Synthesis of Sulfonarnides .................. 25
1.4 Development of Polymer Supported Synthetic Reactions ....................... 27
1.4.1 Orthometallation .......................................................... 27 ....................................................... 1.4.2 Dithiane Chernistry 28
................................. 1.4.2.1 Preparation of 1, 3-Dithianes 28
................................ 1.4.2.2 Deprotonation of Thioacetals 30
........... 1.4.2.3 Electrophilic Addition to 2-Lithio- l,3-dithianes 31
......................... 1.4.2.4 Deprotection of the Thioacetallketal 32
Chapter II Results and Discussion ...................................................... 34
............................................. 11.1 Polymer Supported Orthometallation 35
....................................... II . 1.1 Dopamine Receptor Antagonists 35 ............................................. II . 1.2 Orthometallation Reactions 35
..................................... 11.2 Polymer Supported 1. 3-Dithiane Chemistry 38
................................................................. 11.2.1 Objectives 38 ............................................. 11.2.2 Solution-Phase Chemistry -40
....................... 11.2.7.1 Reduction of Lipoic acid to Alcohol 40
............................... 11.2.2.2 Synthesis of Masked Dithiols 42 .............................. 11.2.3 Polyethylene Glycol as a Solid Support 44
11.2.3.1 Optimization of Conditions for Linking an Alcohol to
MPEG ................................................................... 45 ................................... 11.2.3.2 Ester Linkage to Polymer 47
........... 11.2.3.3 Reduction of Resin Bound Disulfide to Dithiol 48
................. 11.2.3.4 Thioacetalization of Resin Bound Dithiol 50
........................................................ 11.2.4 Amide Chemistry 53
11.2.4.2 Synthesis of MPEG-DOX-Amine .......................... 53
11.2.4.3 Amide Linkage to Polymer ................................. 54 .............................................................. 11.2.5 Other Resins 55
11.2.5.1 Analy tical Techniques for Polymer Supported ............................................................... Molecules -57
11.2.5.1.1 FTIR& ATR ...................................... 57
.................................. 11.2.5.1.2 Gel Phase NMR 58
11.2.5.1.3 Solid State NMR .................................. 58
11.2.6 Conclusions .............................................................. 58
.................................................................. Chapter III Experimental -60
..................................................... Appendix A Selected Spectral Data 72
....................................................................................... References 111
List of Figures and Tables
Figure 1.1
Figure 1.2
Figure 1.3
Figure 1.4
Figure 1.5
Table II . 1
Table 11.2
Table IL3
Combinatorid library formation ............................................ 5
The integration of combinatonai chemistry
and drug discovery ........................................................... 7
Wang resin and Rink amide resin ......................................... 9 ........................................................... Carbonyl reactivity 19
............................. Lithiated dithiane as an acyl anion equivalent 19
........................ Optimization of ether Iinkage to polymer (PEG) - 3 7 ....................... Optimum reaction time for ether Iinkage to (PEG) 38
Optimization of thioacetalization reaction .................................. 42
Ab breviations
A Ac
ATR Bu
Bn
O C
cat . cm
DCC
BHT
DME
DMF
DMSO E+ EI
Et
eq. Fmoc FTIR
g h
HRMS Hz IR
LRMS
M
m
Me
meq
mg m ml
m o l
angstrom
acetate
attenuated total reflectance
butyl
benzy1
degrees Celsius
catalytic
centimeter
dicyclohexylcarbodiimide
butylated hydroxytoluene
ethylene glycol dimethyl ether
N,N-dimethylforamimde
dimethyl sulfoxide
electrophiie
electron impact
ethyl
equivalent
9-fluorenylmethoxycarbonyl
fourier transform infra-red spectroscopy
gram hour
high resolution mass spectrum
hertz
infra-red
low resolution mass spectrum
rnolar
meta
methyl
milliequivalent
rniiligram
megahertz
millili ter
millirnole
vii
Mn
mol
MOM
MPEG
MS
M/Z
NMR
Nu-
P
P PEG
Ph
prod.
R r.t.
ref.
s.m.
t
ternp.
THF TLC TMS UV
X
Nurnber average molecular weight of a polymer
mole
methoxymethyl
monomethy ether of polyethylene glycol
mass spectrometry
mass to charge ratio
nuclear magnetic resonance
nucleophile
generic protecting group
PXa polyethylene glycol
phenyl
product
generic alkyl group
room temperature
reference
starting matenal
tertiary
temperature
tetrahydrofuran
thin layer chromatography
tetramethylsilane
ultraviolet
generic element
vüi
Acknowledgments
1 would like to extend my sincerest thanks to my supervisor Prof. Rob Batey. His
knowledge, guidance, and encouragement have been immeasurable. 1 have leamed a great
deal in the time I've spent in "the Batey" lab.
1 have had the opportunity over the last two years to work with a great group of
people in the lab. Bruce, your knowledge and experience have been a great resource to draw
upon in the lab. Thank you for your guidance and your friendship. Denny, you've been a great friend over the past two years. Thanks for al1 the good advice (and for doing
everything first). Dave, Avinash, Bijan, Santha, Zesis, and Mustafa, thanks for making the
lab a fun place to be. Thanks for the great conversation if not for the choice of music.
1 am grateful to Dr. Andrew MacMillan for his speedy work in reviewing my thesis.
This thesis could not have been completed without the work of the technical staff.
Dr. Alex Young for Mass Spec, Tim Burrow for NMR and Patricia Aroca-Ouellette for solid
state NMR. Thank-you Tim and Patricia for al1 of your hard work, for your tireless
patience, and for the expert advice. Special thanks to Dan Mathers for his assistance with
FTIR spectrometers and other analytical support work, your expertise has been invaluable.
To my many colleagues outside the lab thanks you for making my time at U of T an
interesting enjoyable experience. To the Lautens crew thank for your advice and friendship
and your open chernical shelves. To Tom, thanks also for the insightful and often heated
conversations. To Kluger's groups, Macmillan's group, and the Jones group thanks for the
warm smiles and conversations in the halls. Special thanks to Enn (and the Kauffman
family) for their generous hospitality and to Jody for making sure 1 got to the gym.
Thanks to my friend Mikaela for the great advice chemistry related and other wise.
And for listening as only a good friend does.
Steven, your love and support have meant so much to me these past two years.
Thank-you for al1 of your help and your understanding, and for always having a hug for me
when 1 needed one.
To my family and especially my grandparents Molly and Michael Begleiter and Jack
Gasman thank-you for your love and caring, and for believing in my abilities. To my
brother David thank-you for making me laugh and for the late night discussions about
science and the Simpsons and everything in-between. Mom and Dad 1 don? know were to
begin; you have taught me so much over the years. You have always loved me and
supported me through every challenge. For that and so much more 1 love you and thank-
you.
Thanks and best wishes to dl.
NOTE TO USERS
Page(s) missing in number only; text follows. Microfilmed as received.
Page 10
Chapter 1: Introduction
1.1 Combinatorial Chemistry The term combinatorial chemistry is an umbrella term which has been used to
describe a variety of different techniques. Generally, this term refers to the techniques used
to simultaneously produce libraries of compounds of diverse structure by applying a similar
sequence of reaction steps to a series of analogous reagents. Many libraries of biomolecules
such as polypeptidesl, oligonucleotides* and oligosaccharides3 have been produced by
combinatorial chemistry but the work discussed here will focus on the synthesis of "small
molecule" l ib ra r i e~ .~
Combinatorial libraries have been synthesized utilizing many different techniques.
Libraries have been made by standard organic reactions in solution, for example the 80
compound library of ester and amides made by a team at Glaxo.5 Most of the effort in the
field of combinatorial chemistry has been directed at polymer-supported chemis'q also
known as solid phase-chernistry.
1.1.1 Library Formats Compound libraries have been developed and screened in different formats.6 Some
libraries have been made by the "split and rnix" technique in which pools of compounds-are
synthesized t ~ g e t h e r . ~ When used in combination with polymer supported chemistry this
technique produces mixtures of compounds however, each bead of polymer in the mixture
should contain only a single product. In this technique multiple reaction vessels are each
submitted to a similar reaction applying a different reagent to each. This is depicted by the
reaction of the polymer with Al, A2, and A3 in the Figure 1.1. The products of al1 the
vessels are combined and mixed and then split again into separate vessels. A second reaction
step is applied to al1 the vessels again using a different reagent (B 1-B3) in each. This system
of mixing the products and splitting them into separate vessels again ensures that al1 of the
different possible products are being formed. One could proceed through as many steps as
desired, mixing and splitting after each step until the desired compounds are obtained.
1 combine
@ = Polymer Support
Figure 1.1: Combinatorial library formation by "Split and Mix" method
"Deconvolution" of the library occurs during screening. Al1 of the final pools are
screened individually to determine which has the highest activity or the contains the
compounds that are reactive to the screen. If for example the pool resulting from reaction
with B 1 shows the greatest activity, then one looks a generation back to the pools that were
combined before the final reaction, that would be the individual pools of Al, A2, and A3.
These individuals pools are al1 subjected to reaction with B 1 since it gave the highest activity.
These three separate pools are then tested again to determine the compound showing the
greatest degree of activity. The last two steps in making the most reactive compound have
been determined. The more synthetic steps used to f o m the compounds the more steps
needed to deconvolute the system. In this way one c m proceed backwards through the
synthesis step by step finding the reactions at each stage which lead to the most active
molecule. Tagging, or encoding strategies have also been used where a tag is added to the
polymer support indicating each step in the sequence that the polymer bead is being exposed
to. Tagging strategies have included the use of oligonucleotides where the addition of each
nucleotide base in the chain signifies a set of reaction conditions to which the bead was
exposed.8 Only a very srnall proportion of the loading capacity of the polymer is used to
build the oligonucleotide tag as it can be rnultiplied by PCR for analysis. Perhaps the rnost
elegant chernical tagging method to date cornes from Still's method uses halogenated
arornatics that are coupled directly to the polymer via alkoxy spacers. The tags were read by
cleaving the aryl halides from the polymer and determining which ones were present by
electron capture gas chromatography. Non-chernical tagging methods have also been
investigated. Nicolaoul* and Moranl 1 have studied the use of micro chips for encoding the
synthesis steps. The chips are written and read by high frequency signals.
Libraries have also been developed in spatially separated arrays. A variety of
techniques have been developed for this, including the use of polymer pins in 96 well
rnicrotiter plates and the use of lithography techniques on polymer coated plates. l 2 The
compounds in these libraries can been screened individually to determine the structure of
highest activity .
1.1.2 Combinatorial Chemistry and Drug Discovery Combinatorial chemistry has grown largely in response to pharmaceutical producers'
need to find new lead structures for drug development. l 3 The advent of high throughput
screening techniques has pushed the biological and pharmacological portions of drug
discovery to proceed ever faster. Combinatorial chemistry serves as a means for chemists to
meet the needs of the biologists for more compounds to screen. It is necessary as well for
the combinatorial chemist to work with the biologist and pharmacologists to develop libraries
that will be suited to the biological screens and vice versa. The following schernatic (figure
1.2) shows a portion of the drug discovery process and the role that combinatorial chemistry
can play in that process.
Identification of a Biological Target
/ Cornbinatorial 1 Chemistty
Clinical Trials rn Figure 1.2: The integration of combinatorial
chemistry into the drug discovery process.
The first step in the drug discovery process is to select a biological target. The next
step involves the development of a screen, or a method for testing the activity of the potential
drug candidates against the initial target. Diverse compound libraries produced by
combinatorial chemistry can be applied to the biological screens in order to find structures
which show biological activity, known as lead compounds. With these lead structures
identified it is possible to return to combinatorial chemistry in order to produce anaiogs of the
lead structures. These analogs are then submitted to the biological screen in order to obtain
optimized lead structures. These compounds may then proceed to further pharmacological
and clinical tests.
1.2 Solid-Phase Synthesis The term solid-phase synthesis refers to the synthesis of molecules on solid polymer
supports. To cany out this type of synthesis a substrate is attached to a po!ymer resin and
then reacted in such as rnanner as to synthesize the desired compound. The concept of
synthesizing molecules on a polymer support was first applied to the synthesis of
p0ly~e~tides.14 The landmark paper published in 1963 by Merrifield describes the synthesis
of a tetrapeptide on a copolymer of chloromethylated styrene crosslinked with divinyl
benzene; the copolymer becarne known as Merrifield's resin (1).
1.2.1 Advantages and Disadvantages of Solid-Phase Synthesis
Solid-phase synthesis became the standard for peptide synthesis and has since been
investigated for broader application to small organic molecule synthesis. The primary
advantage of solid-phase synthesis is the ease of purification of the products. The final
product remains bound to an insoluble polymer, this allows the polymer to be washed with
aqueous and organic solvents in order to remove any excess reagents that may remain in the
reaction solution. The washing steps can be carried out quickly and easily by vacuum
filtration making this system faster and simpler than purification by chromatographie
techniques. The ease of purification afforded by this technique also allows for the use of
excess reagents to drive reactions to completion. The remaining reagents are simply washed
away in the purification step. The repetitive nature of the purification steps for solid phase
synthesis make this technique arnenable to automation. This is a major advantage in the
preparation of combinatorial library synthesis on solid-phase.
One difficulty associated with solid phase synthesis is the need to drive al1 reactions
to completion. The purification technique cannot separate one resin bound substrate from
another. Therefore it is necessary to ensure that al1 molecules of the product have reacted in
the expected manner at each step in the reaction sequence, otherwise partially completed
molecules and side products will remain in the product mixture. One way of addressing this
problem is by using excess reagents to drive the reactions to completion. This however does
not solve the problem of side products on the resin. Generally reactions must be developed
that proceed cleanly (without side products) and in high yield in order to be useful for solid
phase synthesis.
Another disadvantage of solid-phase synthesis is the analysis of intermediate
products in a reaction scheme. This is particularly important during the development of new
methodology for solid phase synthesis. Because the products are attached to the insoluble
polymer, it is not possible to anaiyze the intermediates by standard NMR spectroscopy, mass
spectroscopy or other analytical techniques normally used for assessing reactions. Some
techniques have been developed for analysis of resin bound products. These include FTIR
analysis, gel phase NMR, and solid state NMR. These techniques are as yet not well
defined and the results obtained are often arnbiguous. Elemental analysis has also been used
but the accuracy of this technique is known to be poor in comparison to results obtained
from actual yields after cleaving products from the resin. In many cases products are
cleaved from the resin and then analyzed. This process is tedious as it requires cleavage after
each reaction in order to anaiyze intermediates. Also, it gives little information about the
degree of loading on the polymer and about the resin bound intermediate.
Other factors which must be considered when designing a solid-phase synthesis are
the steps to link the substrate to the polymer and to cleave the final product frorn the
polymer. Furthermore, al1 of the reaction conditions used must be compatible with the
polymer support. The loading capacity of the polymer must be considered in order to
maintain a balance between the scale of the reaction versus the amount of product required.
Loading on the polymer is an important consideration. If the loading level is too low then a
very large amount of polymer would be required in order to produce a sufficient amount of
product while if the loading capacity is too high then intra-resin reactions may start to occur
between two molecules bound side by side on the resin.
1.2.2 Resins Used for Solid-Phase Synthesis
Numerous resins have been developed for solid-phase synthesis. The first and
probably most cornrnonly known is Merrifield's resin. As previously mentioned it is a
copolymer of styrene and divinylbenzene functionalized with benzyl chloride functionalities
for linking substrates to the polymer. This resin is polymerized in droplets which leads to
spherical beads of polymer. These beads are then sieved through mesh to achieve uniformity
of bead size. This polymer is available cornmercially in a number of mesh sizes however
200-400 mesh is the size generally used for solid phase synthesis. The resin swells in
solvents such as THF and DMF and it is known to swell to as much as five times its dry
volume. Many vaiations of Merrifield's resin have been developed to give different
terminal functionalities for linking substrates to the polymer by varying chernical techniques.
The Wang linker (2) displays a hydroxyl functionality for linking substrates to the polymer.
The Rink amide linker (3) displays an Fmoc protected amine functionality. This linker is
particularly useful for forming amide bonds to substrates.
(2) (3) Figure 1.3: Wang resin (2), Rink amide resin (3)
Another cornrnonly used polymer for solid-phase synthesis is Tentagel resin (4).
This resin is a copolymer of polystyrene with crosslinks of divinylbenzene and grafts of
polyethylene glycol. The advantage of this polymer over Merrifield's resin is its ability to
swell in dichloromethane, water and acetonitrile. This is a result of polyethylene glycol
grafts which are incorporated into the polymer matrix.
1.2.3 Solid-Phase Reactions
1.2.3.1 Di01 Linkers for Aldehydes and Ketones Leznoff and coworkers published a series of papers describing the use of a 1,Zdiol
linker on Merrifield's resin for protecting dialdehydes and diketonesls. The 1,2 dimethyl
acetal(5) was prepared from Merrifield's resin (1) and the sodium salt of 2,2-dimethyl-1,3-
dioxolane-4-methanol (Scheme 1.1). This compound was then hydrolyzed with dilute HCI
to give the 1,2- di01 linker (6). The 1,3-di01 equivalent was prepared in the sarne manner
from the sodium salt of 2,2,5-trimethyl-m-dioxane-5-methanol, for use with stericaiy
hindered dialdehydes. The di01 linker was then reacted with excess dialdehyde (in this case
terephthalaldehyde or isophthalaldehyde) in the presence of m-benzenedisulfonic acid to give
the monoprotected product (7). Other dialdehydes and diketones have been used for
sirnilar reactions. In al1 cases excess dialdehyde or diketone were used to prevent protection
at both functional groups which would result in crosslinking of the polymer.
dilute HCI Dioxane:water, (1:l)
Scheme 1.1
A Wittig reaction was carried out on the monoprotected diaidehyde, with
benzyltriphenylphosphonium bromide, to give the formylstilbene (9), (Scheme 1.2).
Crossed aldol condensations with acetophenone and sodium methoxide gave chalcones
(10). Cleavage of the chalcones from the resin gave 3-(p or m)-formyl-phenyl-1 phenyl-2-
propen-1-one. Grignard reagents were reacted with the resin bound dialdehydes to give
monosubstituted products of type (11). Under standard (non-polyrner supported) reaction
conditions this reaction would give a mixture of mono, and di-substituted products that
would be difficult to separate. Cleavage of the final product from the resin is achieved by
deprotection of the aldehyde under acidic conditions.
i)PhMgBr
2 (W3Br
Base
(11)
(1 0)
Scheme 1.2
cis and trans (9)
Leznoff was able to achieve synthetic transformations on solid-phase that would not
Na+ -0Me
be possible under solution chemistry conditions, namely the mono-reaction of syrnmetricd
difunctionalized molecules. While the acetal linker for aldehydes and ketones demonstrates
fairly broad use for many different types of reactions subsequent to linking to the polymer, it
should be noted that the acetal is extremely acid sensitive. Al1 reaction conditions must
proceed under neutral or basic conditions in order to be compatible with the linker. This
imposes substantial limitations on the scope of reagents that can be used.
1.2.3.2 Palladium Catalyzed Coupling Reactions In order to make solid-phase synthesis an effective route towards the synthesis of
new molecules, it is necessary to develop an arsenal of reactions. Currently many reactions
which are well known in their solution phase incarnation are being developed for use on
polymer support bound substrates. l 6 One example of this is Pd catalyzed coupling
reactions. These reactions are often used in total syntheses for the formation of new C-C
bonds. A wide variety of substrates are known for these reactions. The solid-phase
counterpart of the Heck reaction has been developed by a group at the Bristol Meyers Squibb
pharmaceuticals Company. 17
The Wang resin (2) was chosen as the solid support for this reaction sequence.
Wang resin like Merrifield's resin has a matrix of polystyrene crosslinked with divinyl
benzene. The terminal functionality in the case of the Wang resin is the ether linked benzyl
alcohol. The terminal alcohol functionality is used to attach either Cvinylbenzoic acid (12) or 4-iodobenzoic acid (15) to the resin (2) for subsequent Heck reaction (Scherne 1.3). The
resin bound olefin (13) was reacted with aryl halides with Pd2(dba)3 and Et3N in DMF at
100 OC to give the product (14). As well the resin bound aryl halide (16) was found to
react under the same conditions with alkyl and aryl olefins to give products of the type (17). Similarly, solid-phase variations of the Stille Pd-catalyzed coupling between aryl iodides and
alkyl tin compounds,18 and the Suzuki Pd-catalyzed coupling of aryl halides and arylboronic
acids have been reported.19
DCC, DMAP, DMF, 60°C /
Ar-X, Et3N, DMF, Pd2(dba)3, P(~-To I )~ 100 O C , 20 hours
\ DCC, DMAP, DMF, 60°C
= Wang resin
Scheme 1.3
R-CH=CH2, Et3N, DMF, Pd2(dba)3, P(2-Tol)3 100 O C , 20 hours
The repertoire of synthetic transformations on solid phase is growing continually. At
this time computer databases of solid phase reactions are being developed to catalogue all the
reactions which have been done on solid-phase.
1.3 Liquid-Phase Chemistry Liquid-phase chemistry is the term that has been used to describe the synthesis of
molecules on soluble polymer supp0rts.2~ Liquid-phase chemistry is similar to solid-phase
chernistry in that the reactions are carried out with the substrate bound to a polymer. Yet, it
is sirnilar to traditional solution-phase (non-polymer-bound) chemistry in that the reaction
components are in solution during the course of the reaction. Liquid phase chemistiy has its
ongins in peptide synthesis in rnuch the same way as solid phase chemistry. The fïrst
example of the use of soluble polymer supports in organic chemistry was the synthesis of
pentapetides on PEG by Mutter in 1972.21
1.3.1 Resins for Liquid-Phase Synthesis A few soluble polymers have been used for liquid phase chernistry; these include
polystyrene, polyvinyl alcohol and a copolymer of polyvinyl alcohol-poly ( 1 -vinyl-2-
pyrrolidinone). However, the most cornmonly used polyrner for liquid phase chernistry is
polyethylene glycol. This polymer is most often used in its monomethylated variation with
an average molecular weight of 5000. This polymer is soluble in chlorinated solvents,
water, and acetonitrile, and insoluble in diethyl and dibutyl ether. MPEG (5000 average Mn)
is somewhat soluble in THF, DMF, EtOAc and EtOH. The solubility properties of this
polymer allow the advantage of having the substituted polymer in solution during the
reaction but also allowing it to precipitate for ease of purification.
1.3.2 Advantages and Disadvantages of Liquid-Phase Chernistry. Liquid phase synthesis incorporates the advantages of both traditional solution
chemistry and solid-phase chernistry. Like soiid-phase chemistry the work up and
purification procedure for polymer bound products is considerably simplified. The polymers
used are soluble in some solvents and not in others this allows for the use of precipitation
and recrystalization techniques for purification. Again this allows for the use of excess
reagents to drive the reactions to completion. Also automation of these repetitive procedures
is possible which should greatly expedite the synthesis of new compounds. This is
particularly useful for application to combinatorid chemistry and the drug discovery process.
Unlike solid phase chemistry, when using soluble polymer supports the reaction
environment is homogeneous. The homogeneous conditions should make it much easier to
adapt reactions frorn solution-phase to liquid-phase. It is also possible to use known
heterogeneous reaction conditions such as Pd on carbon hydrogenations which would not be
possible for solid phase chernistry. Another way in which liquid phase chemistry is similar
to solution chemistry is the ease of analysis of the reaction products. Due to the solubility of
the polymer in chiorinated solvents such as deuterated chlorofonn it is possible to dissolve
the products while still bound ta the polymer and acquire NMR spectra. The spectra
obtained of the polymer bound molecules show strong peaks in the range of 3.5-4 ppm from
the methylenes of the polymer but by appropriately expanding the spectrum it is possible to
see the peaks resulting from the molecule attached to the polymer. It is possible through the
NMR of the resin bound products to determine whether the desired transformation has
occurred and to what degree.
There are still a few factors to consider when applying liquid phase chemistry. The
linking and cleaving form the polymer adds two steps to the overall number of steps, and it
is necessary to consider the compatibility of the reaction conditions to the resin. The
reactions chosen shouId be compatible with the solubility of the resin, Le.. solvents and
temperatures. The reaction conditions should be chosen such that they will not affect either
the polymer or the bond holding the substrate to the polymer. The linkage between the
polymer and the resin should be chosen such that it is stable to a variety of reaction
conditions. However, the reactions used to link the substrate to the resin and to cleave it
from the resin should be simple and high yielding.
1.3.3 Liquid-Phase Reactions
1.3.3.1 Liquid-Phase Oligosaccharide Synthesis Krepinsky and CO-workers have developed methodology for synthesizing
oligosaccharides on a soluble polyethylene glycol support (Scheme I.4).*2 They developed
a derivative of monomethylated polyethylene glycol for linking sugars to the polymer. The
derivative was the MPEG-DOX-CI (20) linker, which was made by WiHiamson ether synthesis between MPEG (19) and a,cx'-dichloro-p-xylene (18). This intermediate can be
used to form an ether linkage to a carbohydrate hydroxyl(21). The protecting groups of the
hydroxyl can be removed and following further manipulations a second carbohydrate
molecule can be added to afford a disaccharide (22). The MPEG-DOX group c m bbe
removed by vigorous hydrogenation with H2 and Pd black, in ethanol(23).
NaH, Nal, THF, 96 hours
*
(20) MPEG-DOX-CI
Ph
'Y++, HO
NPhth 0-
NaH, Nal , THF 50 OC, 48 hours
Hz, Pd black Ethanol
I
NPhth 0-
(23) Scheme 1.4
1.3.3.2 Liquid-Phase Synthesis of Sulfonamides Janda and coworkers have developed a route for the synthesis of sulfonamides on a
solubIe polymer support (Scheme 1.5).*3 MPEG (19) was chosen as the soluble polyrner
support for the synthesis and was functionalized through the terminal hydroxyl with 4-
(chlorosu1fonyl)pheny lisocy anate (24). This reaction was done in dichloromethane with
dibutyltinlaurate as a cataiyst and afforded the carbamate product (25). The sulfonylchloride
terminal functionality was then reacted with a prirnary amine in dichloromethane to give the
resin bound sulfonarnide (26). The carbamate linkage to the PEG was severed using 0.5N
NaOH to give a free amine (27).
Dibutyltinlaurate (cat.) CH2CI2 -
RNH2 Pyridine , CH2CI2
r/
(26) Scheme 1.5
This liquid phase synthesis was designed as a combinatorial synthesis for producing
diverse sulfonarnides. In order to demonstrate the combinatorial utility of the synthetic
sequence, a library of seven sulfonamides was made. Diversity was incorporated by using a
variety of amines, for addition to the sulfonylchloride. It was found that amines that were
nucleophilic such as isobutylamine reacted well to give the desired products in high yield,
Less nucleophilic amines such as phenylamine reacted more slowly but good yields could be
achieved under increased temperatures and longer reaction times. The resin bound
intermediate products of the reaction sequence were analyzed by NMR and the yields were
determined after cleavage from the resin. This synthetic sequence demonstrates the utility of
liquid-phase synthesis for small molecule combinatorid chernisy.
1.4 Development of Polymer Supported Synthetic Reactions
If polymer supported chemistry is to become the method of choice for the
development of combinatorial libraries for dmg discovery, it is necessary to develop
variations of reactions for accomplishing synthetic transformations on the polymer support.
The first step towards accomplishing this goal. is to develop methodology for polymer
supported variations of well known solution phase reactions. The reactions that are chosen
should be arnenable to combinatorial chemistry, i.e. there should be elements of the reaction
wherein diversity can be incorporated. Two general reaction types were investigated in this
project, orthometalIations*4 and dithiane25 Umpolung chemistry. Before developing
polymer supported methodology it is important to understand the solution phase versions of
these reactions.
1.4.1 Orthometallation
Extensive research has been done towards the selective synthesis of aromatic
compounds due to their utility as pharmaceuticals and for agricultural purposes. One classicai method for introducing substituents ont0 an aromatic ring is electrophilic aromatic
substitution. A downfall associated with this type of reaction is the regiospecific preparation
of polysubstituted products, particularly the synthesis of contiguously substituted 1,2- and
l,2,3- systems.
In 1939-1940 Gilman and Bebb26 and Wittig and Fuhrman27 independently
discovered anisole ortho-deprotonation by n-BuLi. This procedure showed great promise
for the regiospecific construction of polysubstituted heteroaromatic compounds. The
directed orthometallation reaction (Scheme 1.6) proceeds by coordination of the heteroatom
of the directing group to a allrylIithium base. Deprotonation of a site ortho to a directing
group follows to give an ortho lithiated intermediate (29). Upon treatment with an
electrophile the lithium species will give disubstituted products (30).
DMG
E
(29) (30)
Scheme 1.6
The directed metalation group {DMG) generally contains a heteroatom which is a
good coordinating site for lithium but which is not vulnerable to deprotonation by the strong
base. Carbarnates, oxazolines and ethers such as MOMO are commonly used. Strong
lithium bases are generally used for this reaction. These bases exist as aggregates in organic
solution therefore additives such as basic solvents are often used to break up the aggregates
in solution to increase their reactivity.
A wide variety of electrophiles have been used in this rnethod including aikyl and aryl
halides, carbonyl compounds, 02, etc. This reaction has the ability to produce diverse
molecules by the addition of different electrophiles. This diversity of products makes the
reaction suitable for application to combinatorial chemistxy.
1.4.2 Dithiane Chemistry The formation of thioacetals and thioketals is a prevalent reaction in the literature?g
These compounds are used in many cases as protecting groups for aldehydes and ketones
respectively. Since the 1960's much work has been done to exploit the use of 1,3-dithiane
compounds as acyl anions.
The normal charge distribution for carbonyl compounds involves a slight increase in
the amount of negative charge at the oxygen and a slight increase in positive charge at the
carbon. As such, the general reactivity pattern of these molecules is to act as electrophiles
allowing nucleophilic attack at the carbonyl carbon (Figure 1.4).
Figure 1.4: Carbonyl reactivity
A lithiated dithiane may serve as an acyl anion equivalent. In this case the carbonyl
carbon (or its equivalent) c m act as a nucleophile and react with electrophiles (Figure 1.5).
This temporary reversal of charge distribution has been given the term Umpolung. This term
was first used by Corey and Seebach29 to describe this phenornenon and has since become a
cornmonly used term by organic chemists.
Figure 1.5: Lithiated dithiane as an acyl anion equivalent
1.4.2.1 Preparation of 1,3-Dithianes The synthesis of 1,3-dithianes developed out of steroid chemistry, where the
thioacetai was used as a protecting group for aldehydes and ketones. A variety of methods
have been described for synthesizing thioacetals. The most common method is the addition
of a 1,3-propane dithiol to an aldehyde in the presence of an acid and a dehydrating agent.
Early rnethods included the use of HCl in ether or p-toluenesulfonic acid in benzene with
azeotropic distillation. In the 1950's Fieser developed an alternative route to
thioacetaiization using a Lewis acid as condensation catalyst and dehydrating agent
combined. For his experiments, Fieser used boron triflouride etherate as the
condensing/dehydrating agent.30 While many Lewis Acids have since been found to be
more generally suited to catalyze thioacetalization reactions, by tradition, BFyEt20 is still
cornrnonly used.
The order of reactivity for various carbonyl groups to fom thioacetalsketals is
generally aliphatic aldehyde > aliphatic ketone > aromatic aldehyde > aromatic ketone.
Depending on the catalyst chosen a certain degree of chemoselectivity can be achieved.
Aluminum trichloride is one of the most active catalysts of this type? It is able to convert
aryl and diaryl ketones to thioketals readily. However, this reagent is not suited to sensitive substrates such as carbonyl compounds with a-protons, and virtually no chemoselectiv.ity is
found with this reagent. ~etrachlorosilane32 and trimethylsilyl chloride33 have also been
found to be powerful catalysts for this reaction. The silicon based Lewis acids are known to
be slightly less powerful than aluminum trichloride but more selective and less problematic in
terms of side reactions. While many Lewis acids have been examined for use in this reaction
titanium tetrachloride has been found to be the most versatile reagent.34 This reagent has
been used to convert nearly al1 types of aldehyde and ketone to thioacetals and thioketals
respectively. One example is the protection of the phenyl methylketone (Scheme 1.7.)
96%
Scheme 1.7
Preactivated ketones and aldehydes have been used successfully in the formation of
thioacetals and thioketals. Acetals are readily converted to thioacetals using magnesiurn
bromide as a ~atalyst .~s Even acetals of aromatic ketones are readily converted to thioacetals
using this reagent. By using acetals in an exchange reaction, thioacetals which would not
readily be formed can be made under relatively rnild conditions (Scheme L8).
OMe ~ / \ i MgBr2, Et20
n SH SH
92%
Scheme 1.8
One final general route to forming thioacetais is through the use of activated
dithianes. Preactjvated reagents containing b0ron,3~ s i ~ i c o n , ~ ~ and have been used for
the formation of thioacetals and ketals. These reagents work on the principal that a weak
bond is formed between the sulfur atoms and the boron or silicon. In the presence of an
aldehyde or ketone an exchange reaction occurs to give more stable C-S bond and a more
stable B-O or Si-O bond. In the case of the 2-stanna-1,3 dithiane compounds the Sn-S bond
is more thermodynamically stable than the Sn-O bond so these reactions require a further
activating agent, (Scheme 1.9). High chemoselectivity can be achieved by these methods as
well acid sensitive functionalities are not affected.
100%
Scheme 1.9
1.4.2.2 Deprotonation of Thioacetals 2-Lithio-1,3-dithiane is the most cornmonly used metalated thioacetal. While
metallation with other counter ions such as potassium and magnesium have been perfomed,
little use has been made of these compounds.39 2-Lithiodithianes are made using n-butyl
lithium in THF, (Scheme 1.10). The temperature and reaction time Vary depending on the
electronic and steric influence of the group R on the thioacetal. The pKa of the dithiane
proton depends on the nature of the R group.
R H n-BuLi, THF R X ç ~ ç
S S L u - u Scheme 1.10
1.4.2.3 Electrophilic Addition to 2-Lithio-1,3-dithianes Electrophilic addition to 2-lithiodithianes is a highiy versatile and useful reaction; it
has appeared often as a method of C-C bond formation in total syntheses. Some examples
of the types of reactions that have been done with 2-lithiodithianes are shown in Scheme
1.1 1.
Scheme 1.11
The intrarnolecular addition of and alkyl halide to a Zlithiodithiane was instrumental in the synthesis of 6cx-Carba-PGIz, an analog of prostacyclin (Scheme 1. l Q 4 0 Dithiane
(31) was treated with LDA to form the 2-lithiodithiane (32) which cyclizes directly to fonn
the product (33).
(32)
Scheme 1.12
Another example of the use of 2-lithiodithimes in synthesis is the synthesis of
laurencin by Masarnune (Scheme 1,13)?l
Scheme 1.13
Masamune uses the 2-lithiodithiane as a masked aldehyde. The Umpolung effect is used to
add the masked carbonyl functionaiity as a nucleophile to the epoxide on molecule (34) to
give the alcohol(35). The dithiane is then deprotected to expose the aldehyde which is
funher reacted to give the final product laurencin.
1.4.2.4 Deprotection of the Thioacetal/ketal There are two cornrnon routes for removal of the dithiol protecting group (Scheme
1.14). The first is a hydrolytic desulfurization which unmasks the carbonyl functiondity
(36). This reaction is most cornrnonly carried out using Hg0 however many alternative
reagents have been used to achieve the same transformation.42 The second method for
deprotection is a reductive desulfurization which can be carried out using Raney Ni.43 This
reaction would yield a reduced compound (37). Other methods of desulfurization include
conversion to gem difluoride. Treatment of the thioketal with 1,3-dibromo-5,5-
dimethylhydantoin in pyridine-HF yields the corresponding gem-difluoride.44
HgO, 35%HBF4 THFIwater
(36)
Raney Ni H H
(37)
Scheme 1.14
Chapter II: Results and Discussion
The object of the work presented in this thesis was to develop synthetic methodology
which could be applied to combinatoria1 library synthesis. The rnethodology wu developed
for polymer supported synthesis with a particular focus on the incorporation of diversity
throughout the synthetic steps.
11.1 Polymer Supported Orthometallation The object of this project was to develop rnethodology for polymer supported
ortholithiation23 reactions and to apply that methodology to the synthesis of a combinatonal
library of compounds of the general structure (38).
11.1.1 Dopamine Receptor Antagonists Aromatic compounds of the general structure (38) have demonstrated activity as
dopamine receptor antagonists.45 These molecules are of interest for the development of
antipsychotic pharmaceuticals. The synthesis of these molecules through a combinatonal
orthometallation reaction would allow for the synthesis of a library of analogs. This library
could then be screened for activity.
11.1.2 Orthometaliation Reactions Compounds of the general type (38) could be synthesized by the series of steps
shown in Scheme II. 1. The functionalized protected phenol (39) could be obtained in two
ways. Phenol could be protected and then functionalized in the desired manner to give a
structure of type (39), or a comrnercially available functionalized phenol could be protected
and used directly. The protecung group chosen should be such that it activates the molecule
for ortholithiation. Ortholithiation followed by in situ trapping with CO2 would give an acid
of the type (40). In the fina1 step acid (40) would be reacted with an amine to give an
amide (38).
Snieckus and coworkers have demonstrated that a number of hydroxyl protecting
groups can be used to direct ortholithiation. It was thought that with introduction of an appropriate spacer, a polymer could be used as the directing group for the ortholithiation.
This would allow the substrate to be bound to the polymer during the ortholithiation and
electrophilic addition steps and potentially during further elaboration of the molecule.
Synthesis on the polymer support would give ail of the advantages of purification and
potential automation for the process. Deprotection of the phenol would both cleave the final
product from the resin and also expose the hydroxyl functionality. The hydroxyl
functionality could be converted to the corresponding methyl ether which is an essential
feature of the bioactivity of molecules of this type (38). Before developing the methodology for this synthetic sequence on solid phase,
mode1 studies were carried out in solution. Phenol was protected with MOMC146 to simulate
linking the phenol to the poiymer. The phenol was added to a flask containing NaH and
THF. After 15 minutes, MOMCl was added and stirred until the reaction was complete by
NMR analysis (2.5 h). The protected phenol was ortholithiated with t-BuLi at O "C for 1
hour, CO2 was flushed through the reaction solution to yield the acid product (41).47
OH I NaH.. MOMCl MOMO I MOMO OH
Scheme 11.2
With the acid in hand the remaining step was to form the amide bond with a primary
amine (Scheme 11.3). Initially attempts were made to form this bond by a direct DCC
coupling48 The reaction was tried with t-butylamine and with isoproplyarnine. The
reagents were added at O O C and the reaction was allowed to warm to room temperature. In
both cases the formation of the desired product was not observed. By contrast benzoic acid
and isoproplyamine were successfully coupled under identical conditions. One possible
explanation for the inability of the acid (41) to couple with amines is the steric hindrance
imposed by the MOM protecting group; another is the possibility of hydrogen bonding
between the carboxylic acid proton and an oxygen of the MOM group. This interaction
could prevent the bulky DCC molecule from activating the acid. A more classical approach
to forming an amide by conversion to the acid halide and then coupling was also tried. Acid
(41) was converted to the corresponding acid chloride. The acid was dissolved in
dichloromethane and oxalyl chloride and pyridine were added.49 The product appeared to be
rnissing the methyl and methylene peaks from the MOM group suggesting that a deprotection
was occurring during the reaction.
MOMO OH
Wo MOMO OH (COC1)2,
pyridine, CH2CI2 deprotection -
Scheme 11.3
This project was abandoned at this point in favor of pursuing the work which is to
follow. Given the short time spent on this project, it is difficult to draw conclusions.
Clearly, further work would be required to achieve the objectives initially set out.
11.2 Polymer Supported 1,3-Dithiane Chemistry
11.2.1 Objectives
Three objectives were addressed in this project. The first objective was to develop a
new dithiol linker for polymer supponed chemistry. The second objective of the project was
to develop methodology for synthesizing thioacetals on solid-phase and carrying out 1,3-
dithiane chernistry on these thioacetals. Methodology was also to be developed for cleaving
the products from the polymer. The final objective of this project was to create a small
combinatorial library to demonstrate the utility of both the new linker and the methodology.
Scheme IL4 depicts a general approach towards achieving the stated objectives. In
the fust step a protected 1,3-dithiol could be linked to a polyrner by a functionaiity attached
to Cl or Ca. The polymer supported dithiol would then be deprotected to yield a free dithiol.
This would accomplish the first goal of developing a polymer supported dithiol linker for
aldehydes and ketones. The dithiol could then be reacted with an aldehyde or ketone to form
a thioacetal or thioketal respectively.50 Diversity may be incorporated into the molecules
produced by using different aldehydes or ketones for linking. The use of a polymer
supported di01 for linking aldehydes and ketones to a polymer backbone was reported by
Leznoff as descnbed previously.l5 The resin bound molecules were then subjected to
further chernical transformations, however, the scope of these reactions was limited by the
acid labile acetal linker. The proposed thioacetal linker should be stable to a wider rage of
conditions thus providing more possibilities for reactions with the resin bound aldehydes and
ketones. One important type of reaction which c m be done with thioacetals and not acetals is
Umpolung type chemistiy. Thioacetals can be deprotonated with a strong base and then
reacted in situ with an electrophile. Again diversity may be incorporated into the molecules
as a wide rage of electrophiles are known for this type of ~hernistry.2~ With the second
objective of the project acheved three options are available; the products can be cleaved by
one of two available methods, or the resin bound thioketal c m be further reacted. The
products can be cleaved by reductive desulfurization to yield a reduced product. In this case
the linker is "traceless" as there is no functional group remaining at the point of attachrnent.
The other cleavage method is hydrolytic desulfurization. In this case the product ketone is
released as is the dithiol functionalized polymer which can be recovered and reused. The
third option is to do further chemistry on the resin bound thioketal before desulfurization.
The transformations would depend largely on the R and E groups of the thioketal. It has
been demonstrated that diversity can be incorporated into this synthetic sequence at nearly
every step making this synthesis perfect for the development of diverse combinatorial
libraries .
desulfurization R E HXH
Scheme 11.5 depicts the use of lipoic acid for forming the polymer supported dithiol.
Lipoic acid (42) contains a cyclic disulfide with a pendant 5 carbon chain and a terminal acid
functionality. The disulfide can be reduced to pive a 1,3 dthiol and the terminal acid
functionality could be used to link the molecule to the polymer backbone. This compound
has al1 of the necessay components of a good dithiol linker, a latent I,3-dithiol functionality
and a functionalized chah for linking the molecule to the polymer. One other advantage of
using lipoic acid is the fact that it is comrnercially availability.
S-S @ s-s '-OH - LOH - L o - c
(46) Lewis acid
(45)
l i) base ii) E+
(47)
Scheme 11.5
11.2.2 Solution-Phase Chemistry
11.2.2.1 Reduction of Lipoic acid to Alcohol The first step in the proposed reaction sequence was the reduction of lipoic acid (42)
to the corresponding alcohol (43) (Scheme 11.6). The literature precedent for this reaction
states that the reduction c m be achieved with catechol borane, however the product is
described as being "notoriously unstable" and full data for the compound was not
available51 Many attempts were made to synthesize and puri@ this compound.
BH3-THF (1 eq) S-S THF, r.t. S-S
VvviroH * L.Jv-44~ O
(42) (43
Scheme 11.6
The general procedure used for synthesizing the cornpound was adapted from the
procedure of Kabalka. Borane-THF ( lM, lrnmol) was added dropwise to a solution of
lipoic acid (1 mmol) in THF (2.5 ml). The solution was left stirring under N2 for 10-1 5
hours while monitoring by TLC. TLCs of the reaction mixture showed the disappearance of
the lipoic acid spot and the appearance of a single new spot running slightly higher on the
plate. When the lipoic acid was no longer visible by TLC the reaction was worked up in the
following manner. Water (5 ml) was added to quench the reaction the aqueous layer was
washed with ether (3 x 5 ml) the organic extracts were combined and washed with bnne (2 x
5 ml) then dried over MgS04 and dried in vacuo. If the product was thoroughly dried after
the usual aqueous workup, polymerization of the product began to occur. The resulting
polymer is pale yellow in colour and has elastomenc properties. If some solvent remained in
the product, it did not polymerize. Cnide NMR spectra of the of the reaction products were
obtained which confirm the presence of the desired product (43). The product (43) was
purified by flash chromatography at this stage. However, when it was dried following flash
chromatography polymerization always occurred. The work up and solvent removal steps
were done under a variety of conditions including the exclusion of light and/or water but in
al1 cases the product was still unstable and polymerized. Presumably the polymerization is a
radical process initiated by homolytic fission at the S-S bond. BHT (butylated
hydroxytoluene), a radical inhibitor, was added to the reaction mixture prior to work up,
however the product still polymerized. In al1 cases, the polymer that formed was insoluble
in organic solvents, and in aqueous acid and base solutions. It was found that the addition
NaBH4 in ethanol to the polymer would cause it to dissolve. It is known that the disulfide
bond of lipoic acid can be reduced to the dithiol by ~ a I 3 ~ 4 5 ~ It was therefore thought that
the polymer was formed by an intermolecular disulfide bond formation and that these bonds
were then being broken by the NaE3H4 as the polymer dissolved.
11.2.2.2 Synthesis of Masked Dithiols
It was clear at this point that the most direct route to the ether linkage between lipoic
acid and a polymer backbone was not possible. Alternative routes were sought, the first of
which involved the protection or masking of the disulfide bond as a more stable functional
group. The disulfide bond was reduced to the corresponding dithiol using NaBH4 in ethanol
at O OC (Scheme 11.7).53 The dithiol acid (48) could then be reduced to the corresponding
alcohol (49) using borane-THF. This substrate, however, is not suitable for forrning an
ether linkage to a polymer as SN^ displacement by the S would result.
Scheme 11.7
Two protecting groups for the dithiol were synthesized (Scheme 11.8) the first of
which was the thioacetal(50) formed from benzaldehyde and boron trifluoride etherate in
di~hloromethane5~. Addition of the thioacetal alcohol (51) on to the polymer would yield
one of the intermediate products of the initially proposed route. However, if this advanced
intermediate was used to link to the polymer further steps would be needed to make this
sequence amenable to combinatorial synthesis. The disulfide would have to be deprotected
at a later stage and then reprotected using a variety of aldehydes in order to incorporate
diversity into the products, thus adding two steps to the synthetic sequence. This compound
would also be useful for testing methodology which appears after the thioacetalization in the
reaction sequence. The second protecting group that was synthesized was the
dibutyldithiastanane compound (52). This compound was synthesized from the dithiol
(48), BuzSnClz and triethylarnine in dichloromethane.55 Compounds of this type are
known to undergo exchange reactions to form thioacetals when in the presence of an
aldehyde and a suitable activating agent.3g It was thought that once compound (53) was
linked to the polymer the dibutylstannyl portion of the molecule could be easily replaced with
a variety of aldehydes allowing for the desired diversity.
Bu. Bu
Scheme 11.8
The two protected dithiol acids (50) and (52) were reduced to the corresponding
alcohols (51) and (53) respectively. Both of the products were stable and polyrnerization
was not a problem. Experiments were done to form an ether linkage between the alcohol of
the thioacetal(51) and benzylchloride. These experiments were meant to mode1 the ether
linkage formation ont0 the benzylchloride functionality of a polyrner back bone. A variety of
conditions were exarnined in an attempt to optimize the ether linkage of the protected dithiol
alcohols to the polymer backbone, (Scheme 11.9). Based on previous studies THF was
known to be a good solvent for deprotonation reactions of this type. Examples of ether
linkage formation ont0 Memfield's resin were known to work best in DMF, due to its ability
to swell the resin, thereby exposing more of the reactive sites. In al1 experiments the
deprotonation was done in THF, followed by the addition of the BnCl in D m , to simulate
the addition of the swollen resin. Of the three bases tested, KH and NaH afforded similar
yields while KHMDS gave much lower yields. It was also found that the addition of 18-
Crown-6 as a phase transfer catalyst improved the yield of the reaction. The best yield
obtained in this series of reactions was 70%, for the case where KH and the alcohol were
used in three fold excess cornpared to the BnCI, with the solvent system as described
previously, and 18-Crown-6 was used as an additive (10 mol %). One experiment was done
to link the Sn protected compound, (53), to BnCl. The yield was only 24% using 3
equivalents of KH, THF and DMF as previously described; the remainder was starting
material. The Sn compound (53) was also used in three fold excess relative to the BnCI.
KH (3eq) DMF/THF 18-Crown-6 (cat)
BnCl (leq)
KH (3eq) DMFRHF ç:S".Bu D
BnCl (leq)
24% (55) Scheme 11.9
11.2.3 Polyethylene Glycol as a Solid Support Monomethylated Polyethylene glycol (MPEG) was chosen as the support for this
solid-phase synthesis due to ifs unique solubility properties. This resin is soluble in
dichloromethane and water, moderately soluble in THF and ethanol and insoluble in ether.
Therefore it is possible to cany out reactions in organic solvents having the reagents and the
resin bound substrate in solution. The resin and bound substrate can then be precipitated
from the solution and washed with ether to remove excess organic reagents. MPEG (19) was functionalized with a,al-dichloro-p-xylene (18) to give MPEG-DOX-CI (20)
according to the procedure by Krepinski (Scheme 11. 10).22b Monomethylated polyethylene
glycol was dissolved in dry THF with heating. The solution was then cooled slowly to room temperature. NaH was added to deprotonate the alcohol followed by Na1 and a,a'-
dichloro-p-xylene, ten minutes later. The reaction mixture was stirred for 96 hours. A
standard procedure was used for working up al1 PEG derivatives. The reaction solution was
filtered to remove any solid impurities and the residue was rinsed with dichioromethane to
redisolve any PEG derivative that had precipitated. A large excess of diethyl ether was then
added and the reaction mixture was cooled to 4 OC for -20 min. The precipitate was then
collected and redisolved in a minimum of hot ethanol. The solution was filtered again and
then cooled to 4OC until the PEG derivative had precipitated. This precipitate was collected
and dried in vacuo. The product (20) has a terminal benzyl chloride moiety which was
intended to facilitate the ether linkage to the lipoic alcohol derivatives. The plana phenyl
ring of this chain extension is thought to add a more rjgid quality to the end of the polymer.
Furthemore, this end piece would be highly soluble in organic solvents thus preventing the
chain end from getting wound up inside the bulk of the polymer, and exposing the reacting
site.
NaH, NaI, 96 hours CI
Scheme 11.10
11.2.3.1 Optimization of Conditions for Linking an Alcohol to MPEG A senes of experiments were run to optimize the conditions for linking an alcohol to
MPEG-DOX-Cl (Scheme II. 11). Commercially available cinnarnyl alcohol(56) was used
as a mode1 compound to simulate the masked lipoic acid compounds in order to expedite the
optirnization process of the ether formation reaction. For this reaction NaH was used as the
base and Na1 was used to convert the benzyl chioride of (20) to a benzyl iodide in an in situ
Finklestein reaction. The reaction was optirnized for nurnber of equivalents of reagents,
solvent, temperature, and the use of additives. Table II. 1 is a surnrnary of the results of the
optirnization experiments;
(56)
Eq. NaH 6.0 6.0 3.5 6.0 6.0 6.0
12.0 6.0 12.0 6.0 6.0
6.0 6 .O 6 .O
6 .O 6 .O 6 .O
Scheme 11.11
Eq. Na1 Solvent 1.5 THF 0.2 THF 1.5 THF 1.5 THF 1 S THF 1.5 THF
1.5 THF 0.2 THF 1.5 THF 0.2 THF 1.5 THF
1.5 Toluene 1.5 DMF 1.5 DME
1.5 THF 1.5 Toluene 1.5 Toluene
3 4
(57)
Temp. 50 50 50 50 50 R.T.
50 50 50 65 65
65 65 65
65 8 5 8 5
Additive a - - 7
HMPA 18-cr-6
-
- 18-cr-6 18-cr-6
18-cr-6
18-cr-6 18-cr-6 18-cr-6
15-cr-5 18-cr-6 15-cr-5
Table 11.1
a: Where phase transfer catalyst was used 3eq were added. b: % yields were caiculated based on 400 MHz NMR relative integration ratio of protons on
C3 an Cg.
In al1 cases 3 eq of cinnamyl aicohol were used and the reaction time was 48 hours.
The best yields were obtained when 6 eq of NaH and 1.5 eq of Na1 were used and
18-Crown-6 was used as an additive. Although the resuits obtained in toluene were slightly
higher than in THF tne difference is not significant based on the method of yield
determination. Based on these results another set of experiments was run to evaluate the
time of reaction completion. A reaction was run under the conditions described in
experiment No. 12 in Table II. 1. Aliquots were taken from this reaction at 12, 36, 72,96,
and 120 hours. These aliquots were worked rip in the standard rnanner and the yields were
determined. The result for the sample at 96 hours is thought ta be an anomaly. The percent
yield seems to drop slightly with after 48 hours. Clearly there was no advantage by very
long reaction times in fact there was even a disadvantage, hence 48 hours was chosen as the
standard reaction time.
Determination of Optimum Reaction Time
Time Elapsed 12
36
% yield
45 5 1
46
Table 11.2
Although the yield for the ether linkage reaction did improve over the course of the
optirnization experiments even the best yields were only in the range of 60-65%. This added
to the fact that a number of reaction steps would have to be done in solution phase to prepare
the lipoic acid for linking to the polymer made the ether linkage an undesirable route.
11.2.3.2 Ester Linkage to Polymer One alternative to an ether linkage was to form an ester linkage directly with the lipoic
acid. This was achieved in high yield of 94 %. This reaction was done with Cs2C03 as the
base in refluxing acetonitrile,56 using 3 equivalents of lipoic acid, (Scheme 11-12). The yield
was determined based on the degree of conversion of starting polymer MPEG-DOX-CI to
the lipoic acid functionalized polymer. This was determined based on the relative integration
of the protons of Cg and C2.
1 O
S-S
O 94% 7 5 3 0 9
(58)
Scheme 11.12
11.2.3.3 Reduction of Resin Bound Disulfide to Dithiol With the lipoic acid attached to the polymer the next step was to reduce the disulfide
bond to the corresponding dithiol and then form the desired thioacetal. While the reduction
of the disulfide to the dithiol had been a simple and high yielding step in solution phase, the
resin bound equivalent was considerably more difficult. The solution phase reaction
employed NaBH4 at low temperature (O OC) in ethanol to achieve the desired transformation.
If the reaction was not run at low temperature, over-reduction occurred leading to
degradation of the starting material. The analogous reaction could not be run on the PEG
supported lipoic acid due to the incompatibility of the required temperature of the reaction
and the solubility of the PEG derivative at O O C . PEG and its derivatives are only slightly
soluble in ethanol; PEG will dissolve in ethanol at high temperatures but precipitates out at
low temperatures. The PEG was insoluble at temperatures low enough to achieve selective
reduction with the NaBH4. Alternative reducing agents were required to achieve the desired
transformation (Scheme II. 13). An attempt was made to reduce the disulfide with
NaCNBH3 in THF; however, no reaction was observed. Lithium tri-t-
butoxyaluminohydride is know to be a very mild and selective reducing agent.57 This
reagent, synthesized from L i a & and 3 equivalents of t-butylalcohol in THF>8 was added
as a solution in THF to the resin bound lipoic acid. Again no reaction was observed.
*/SRMe NaCNBH, THF No Reaction
O (58)
No Reaction
Scheme 11.13
A precedent for the reduction of disulfides with Ph3P in a 4: 1 dioxane water solution
was found.59 The reaction was modified from the literature procedure by extending the
reaction tirne from 20 minutes to 10 hours, and doubling the equivalents of Ph3P in order to
get the best yields on the resin bound lipoic acid. Two equivalents of Ph3P were added to a
solution of resin bound lipoic acid in dioxane and water. The solution was heated to 40 O C ,
then a few drops of WC1 were added (Scheme II. 14). The reaction was found to proceed in
high yield after 10 hours.
Ph3P, HCI (cat) Dioxane:Water (4:l)
(58) O
Scherne 11.14
Care was taken when carrying out this reaction not to add too much HCl. HCI was
used to catalyze the reaction as the Iiterature precedent States that without acid catalyst the
reaction proceeds extremely slowly (a reaction completed in a few hours with acid required a
few days without). However, strong acid was capable of hydrolyzing the ester linkage to
the polymer affording a small amount of the compound MPEG-DOX-OH, (60) and lipoic
acid. NMR spectra of the dithiol(59) showed a srnall peak at the same chemical shift as the
methylene adjacent to the OH on compound (60). The integration of this peak showed that
there was less than 5% of this compound relative to the desired dithiol. To confirm this
hypothesis the compound (60) was independently synthesized based on the literature
procedure by ~ r e p i n s k i . ~ ~ MPEG-DOX-Cl(20) was dissolved in 10% aqueous Na2C03
and heated to 70 O C for 16 hours. The standard work up for PEG derivatives yielded the
product (60) (Scheme II. 1 5).
(60)
Scheme 11.15
11.2.3.4 Thioacetalization of Resin Bound Dithiol With the dithiol in hand the next step was to form the thioacetal. The thioacetal of
lipoic acid had previously been made in solution phase with benzaldehyde and the Lewis
acid, BF3-Et20. Attempts to form the thioacetal from the PEG bound lipoic ~ c i d gave the
product (61) but approximately 50% starting material remained (Scheme II. 16, Table 11.3).
Three different Lewis acid were tried under varying conditions, however, the yields did not
irnprove significantly.
Lewis Acid CH2CI2, PhCHO
(59) O
(61) O
Scheme 11.16
No.
1
2
3
4
5
7
8
Lewis Acid
BF3-Et20
BF3-Et20
Tic14
Tic14
Tic4
Tic14
TMSCl
equivdents
1.30
2.00
2.00
O. 13
0.50
O. 13
1.50
ddehyde
1.50
2.00
0.25
10.00
10.00
10.00
5.00
temp b
r.t.
r.t.
- 10°C - r.t. -10°C - r.t.
-10°C - r.t.
- 1 O-+40°C
r.t.
prod1s.m.d % overall e
53/47 47%
60140 49%
5 6/44 30%
54/46 31%
5 3/47 51%
57/43 59%
57/43 50%
Table 11.3 Thioacetalization Reactions "yC
Al1 of the reactions were mn over night.
Where temperatures were varied over the course of the reaction, the mixture was stirred
at the initial temperature while the Lewis acid was added after which time the solution
was ailowed to warrn to its final temperature.
One equivalent of the lipoic acid dithiol bound through an ether linkage to the resin was
used in ail cases.
The ratio of product to starting material was calculated based on the relative integration of
the peaks for the protons on C9 of each of the compounds (product (61) and starting
material (59)).
The % yield overall was determined based on the relative integration of the peaks
corresponding to the protons of C9 and those of OMe. This value gives the overall
conversion of MPEG starting materiai to thioacetal derivatized MPEG after three steps.
It is not possible to separate the desired product for the starting matend as both are
resin bound compounds. A number of attempts were made to increase the yield of the
thioacetalization reactions by resubmitting the mixture of product and starting materials to the
reaction conditions. The mixture of compounds frorn experiment 2 was resubmitted to the
same condition as described for experiment No. 2 (Table 11.3). The result was a slight
decrease in the ratio of product /starting material to 5 1/49. Based on the integrations of the
methylene next to the ester linkage to the substrate, the amount of materiai bound to the resin
did not Vary. This elirninates the possibility that the reaction conditions were causing the
degradation of the ester linkage that might lead to product loss. The mixture of product and
starting material resulting from experiment 3 was resubmitted under the same conditions as
those used in 3. Again a small drop in the product to starting material ratio was observed,
ref.
30
30
60
60
60
60
3 3
(54146). The mixture of products from this reaction was submitted to the same conditions a
third time resulting in a further decrease in the ratio of product to starting material, 36/65. It
is clear that resubrnitting the mixture of product and starting material will not improve the
yield of this reaction, further study is required to improve the efficiency of this reaction
Reactions involving the conversion of acetds into thioacetds are known in the
literature.35 A reaction of this type was carried out on the PEG bound dithiol (59). The
PEG bound dithiol(59) was added to dry MgBr2 (3 eq) in THF. The solution was heated
slightly until the PEG derivative dissolved. The solution was then cooled to room
temperature and benzaldehyde dimethyl acetal(5 eq) added. The mixture was left stirring for
18 hours. The product was worked up by the standard procedure and the proton NMR was
obtained. Based on the relative integration of the nonaromatic proton originating from the
acetal and the OMe protons, there appears to be less than 20% product (61) the remainder
was starting material.
MgBr, THF,
$0;.
The ester linkage has been a successful and speedy route to a polymer bound dithiol.
While there are limitations to its usefulness due to the reactivity of the ester it may still prove
useful for a wide variety of reaction conditions. Efforts will continue towards improvement
in the degree of conversion of the thioacetalization step.
11.2.4 Amide Chemistry While linking the lipoic acid to the polyrner by an ester linkage was a quick route into
a solid support bound dithiol, it was known that this product would be unstable to the
conditions of transformations required for the 1,3-dithiane chemistry. In particular the ester
linkage would be vulnerable to cleavage by BuLi or other strong base used for the
deprotonation of the thioacetal. In contrat an amide linkage would be more resilient to such
conditions.
Lipoic amide (62) was reduced to the corresponding dithiol (63) (Scheme II. 18).
This reaction proceeded smoothly and with high yield under the sarne conditions that were
used for lipoic acid. Both NaBH4 and Ph3P conditions achieved good yields.
1)NaBH4, ethanol S-S O O C HS SH
+NH* or t
2)Ph3P , HCI (cat) LJ,,,Pyw
O Dioxane:water (4: 1) (63) O (62)
1) 89% 2) 92%
Scheme 11.18
The solution phase reaction for forming the thioacetal on the lipoic amide was also
c h e d out giving a near quantitative yield (Scheme II. 19).
PhCHO,
99% (64) 0
Scheme 11.19
11.2.4.2 Synthesis of MPEG-DOX-Amine The formation of an MPEG-amine derivative was known in the literat~re.6~ The
general procedure was adopted for the formation of the MPEG-DOX-Amine derivative. The
MPEG-DOX-Cl(20) was converted to the corresponding phthalamide (65) using potassium
phthalamide and N d in DMF (Scheme 11.20). This conversion occurred quickly (under 3
hours) and in high yield. The Phthalamide (65) was then converted to the amine (66)
using hydrazine monohydrate in ethanol. This reaction proceeded in good yield after 12
hours.
K'phthalamide, Nal, DMF, 120 O C
Scheme 11.20
11.2.4.3 Amide Linkage to Polymer The MPEG-DOX-amine was linked to lipoic acid via a DCC coupling (Scheme
11.21). An excess of lipoic acid and DCC were used in this reaction. Solutions of the lipoic
acid and the DCC in dichioromethane were cooled to O OC and then combined and allowed to
stir for 1 hour at room temperature. This mixture was then filtered and the filtrate was added
to a solution of MPEG-DOX-amine in dichloromethane at room temperature. In the initial
experiment the reaction was allowed to run for 10 hours. It was found that two products
resulted from the reaction. The major product was the desired product (67) while the minor
product (68) was the result of DCC coupling between the lipoic acid and terminal hydroxyl
of the remaining unfunctionalized MPEG. The identity of the side product was determined
based on a comparison of the NMR spectrum of the mixture of compounds to the spectrum
of pure (68). Compound (68) was prepared by DCC coupling of lipoic acid directly to
MPEG by the same conditions described above (Scheme II.22). The DCC coupling to
amines is known to occur much faster than the corresponding reaction to alcohols therefor by
shortening the reaction time to two hours the desired product could be obtained without the
ester side product. Work is underway for reduction of the polymer (67) to the
corresponding dithiol.
flyMe Lipoic Acid s/ [p~"" ' H2N \ DCC, CH2CI2 \
+ (67) 0 0°C-r.t.
scheme IL21
Lipoic Acid DCC, CH2CI2 S -S
OMe t
0°C-r. t.
Scheme 11.22
11.2.5 Other Resins Other polymer resins have been considered as the solid support of the dithiol
chemistry. Experiments have been done to form ether linkages from lipoic alcohol (in
solution) and other alcohols to Merrifield's resin. Unlike PEG, Merrifield's resin is an
insoluble polymer. Al1 reaction are done with the resin swollen in a suitable solvent
followed by the addition of the appropriate reagents. The following experiments were done
on Merrifield's resin:
i) NaH (3eq), THF S-S ii) Merrifield's resin DMF
i) NaH (3eq), THF ii) Merrifield's resin DMF
1)KH (3 eq), THF ii) Merrifield's resin DMF
>
(73)
Scheme 11.22
The lipoic alcohol (43) was prepared from Lipoic acid and BHyEt20 in THF. In
order to prevent the alcohol from spontaneously polymerizing the product was stored as a
solution in THF. Approximately three equivalents of the lipoic alcohol in TW were added
to a flask containing NaH previously washed with THF. Following 30 minutes of stirring,
the solution was transferred by cannula into a flask containing Merrifield's resin swelled in
DMF. The mixture was left stirring overnight. The polymer product (69) was washed with
one portion of CH2C12, four portions of a 1: 1 solution of DMF:water, three portions of
DMF and then another three portions of CH$& This sequence of washingh* is intended to
shrink and swell the polymer so it will release any trapped unreacted material washing away
both aqueous and organic soluble unreacted reagents. A sirnilar procedure was carried out
on the m-cresol(70) and on the dinitrobenzyl alcohol(72). Visibly the colour of the
polymer had changed from white to pale yellow during the course of these reactions. The
product polymers were analyzed by IR and gel-phase NMR however the resulting spectra
were arnbiguous and it was not possible to clearly state the results of these reactions.
A reaction was done to link lipoic acid to Memfield's resin through an ester
linkage.63 Lipoic acid was added to NaH in THF this solution was added to slurry of
Merrifield's resin swollen in D m . Na1 ( I O mol 5%) was added to the solution to increase
the rate of the reaction by converting the benzyl chloride of Merrifield's resin to the more
reactive benzyl iodide. The product of this reaction was then subjected to the conditions
required for reducing the disuifide to a dithiol. Ethanol was üdded to the derivatized
Merrifield's resin and the mixture was cooled in an ice bath. Sodium borohydride was then
added slowly to the flask. The reaction was left stimng overnight as it w m e d slowly to
room temperature. The work up procedure for this reaction was the same as described
previously for the ether linkage reaction.
S-S i) NaH (3eq), THF ii) Merrifield's resin DMF, Nal
C
(42) O (74) 0
Scheme 11.23
While polymer products have been obtained from these reactions, difficulties remain
in analyzing the products. Generally when developing methodology on solid phase the
products are cleaved from the resin after each step and then analyzed in solution. This
method of analysis gives little information on the degree of loading on the polymer.
11.2.5.1 Analytical Techniques for Polymer Supported Molecules A number of analytical techniques have been explored for their use in analyzing
sampies of derivatized Merrifield's resin. Among these are MS, FTIR, and NMR. 16
11.2.5.1.1 FTIR & ATR It has been suggested that the degree of loading onto the polymer can be deterrnined
by the disappearance of the H-C-Cl stretching vibration at (1250 cm-' ) in the IR spectrum.64
Furthermore, if a calibration curve is constructed then the degree of loading c m be
quantitativeiy determined, FTIR spectra of a KBr pellet composed of 10 % Merrifield's
resin was found to have a medium peak at 1264 cm-' and a smaller peak at - 1250 cm-' . The IR of compound (69) was performed under the same conditions, also as a KBr disc
containing 10% polymer. The spectrurn obtained for this compound had less defined peaks
and had a more rolling appearance. A broad but weak peak at 1254 cm- was visible with no
other significant peaks around it. An IR spectrum of the compound (75) was also obtained,
this spectrum showed a peak at 1265 cm- l . The peak was again broad and weak and had no
other Peak near by. It is difficult to judge with any certainty the significance of these spectra.
Also spectra were obtained by ATR-IR (Attenuated total reflectance). The solid polymer
swollen in THF was laid down as a thin layer on the ATR crystal and the spectrum obtained.
This procedure was perfonned for Merrifield's resin and the reacted polymer but the
resulting spectra showed no improvement over the spectra obtained from KBr pellets.
II.2.5,1.2 Gel Phase NMR Another anaiytical technique tried was gel phase N M R . ~ ~ Samples of Merrifield's
resin and substituted Merrifield's resin were swollen in different solvents ( CDCl3, DMSO,
and benzene d6) and transferred to NMR tubes. Proton spectra of the "gel phase" samples
were run. A sample of the derivatized resin (69) was run in the sarne manner. Al1 spectra
showed extremely broad peaks and there was nothing to distinguish the derivatized resin
from the underivatized,
II.2.5,1.3 Solid State NMR The final analytical technique studied was solid state NMR. 1 3 ~ NMR spectra of
Merrifield's resin and of the substituted resin (74) were obtained. It was possible to
identifj the CH2 adjacent to the Cl on the unsubstituted resin. The CH2 of the substituted
resin is observed shifted slightly downfield. In order to quantify these results it would be
necessary to determine the signal intensity ratio relative to the weight of polymer used. Solid
state NMR seems to be a promising technique for the analysis of the derivatives of
Merrifield's resin.
11.2.6 Conclusions In conclusion, the first objective set out in this project, has been achieved. A new
linker for carrying out dithiol che,mistry on a polymer backbone has been made by linking
lipoic acid to a modified PEG polymer through both ester and amide linkages. The lipoic
acid has subsequently been reduced to fonn the corresponding dithiol. The second objective
of this project has been partially met. Thioacetals have been formed using the new dithol
linker. Although the Urnpolung chemistry has yet to be established, the thioacetal alone may
serve as a useful protecting group for linking aldehydes and ketones to a polymer backbone
for solid phase synthesis. Most of the results discussed in this section refer to chemistry
done on PEG based polymers. The advantage of analyzing the intermediates directly by
NMR affords this resin superior qualities for developing this new methodology. It should
be noted that as the analytical techniques are improved for analyzing products linked to
insoluble polymers this methodology may be uansferred to solid polymers such as
Merrifield's resin or Rink amide resin.
Chapter III: Experimental
General Experimental Unless othenvise stated, al1 reactions were perforrned under nitrogen using
either flame-dried or oven-dried glassware. Reaction solvents were distilled prior to use,
under an inert atmosphere. DiethyI ether (ether) and THF were distilled from sodium wire /
benzophenone. Dichloromethane was distilled from CaH2. DMF was successively dried
three times for 24 hours over 4 A molecular sieves and stored under N2. Acetonitrile was
dried over 4 A molecular sieves overnight and stored under N2. Al1 reagents, unless
otherwise noted, were purchased from Aldrich Chemical Company, Fisher Scientific Limited
or BDH. PEG (Monomethylated, 500 mw) was obtained from Aldrich as was Merrifield's
resin (200-400 mesh, 2% crosslink lmeq CUg).
Flash column chromatography on silica gel (60 A, 230-400 mesh, obtained from
Whatman Company) was performed with distilled hexanes, distilled ethyl acetate, reagent
grade dichloromethane and spectrograde acetone (Caledon). Anaiytical thin-layer
chromatography (TLC), was performed on pre-coated silica gel plates (Alugram SiL GAJV254 obtained from Rose Scientific Limited), visualized with a UV254 larnp
(Spectroline, Longlife Filter) and stained with 20% phosphomolybdic acid in ethanol
(commercially available), cent molybdate (17.3 g MoO3, 14.4 mL concentrated NH40H in
48 rnL H20, slowly added to 7.6 g of (NH4)2Ce(S04)4*2H20, in 100 mL of 50% A2SO4,
then diluted to 500 rnL with H20). Solvent systems associated with Rf values and
chromatography are reported as volumetric ratios. Purification of PEG derivatives was
performed with distilled dichloromethane and ether and reagent grade absolute ethanol.
Al1 1H and 13C NMR spectra were obtained on 200 / 50 MHz Varian XL, 400 / 100 MHz or 500 /125 MHz Varian Unity spectrometers in CDC13 (referenced at 8 7.24 and
77.00 ppm for 1~ and 13C, respectively). Features of peaks in the 1H NMR spectra are
labeled in brackets proceeding each chemical shift in the following order: multiplicity (s =
singlet, d = doublet, t = triplet, q = quartet, p = pentet, h = heptet, m = cornplex multiplet),
integration, coupling constant, identity of proton (if possible). Proton NMR for PEG
derivatives were obtained frorn concentrated solution of the PEG derivative in deuterated
chloroform. Generally 64 or more transients were acquired and integrals were assessed on
expanded portions of the spectra after drift control adjustment. FT-IR spectra were recorded
on a Perktn-Elmer Spectrum 1Oûû or a BOMEM-Hartmann and Braun, Michelson Series
spectrometer. Samples were loaded as films on NaCl plates or as pellets of KBr for FT-IR, ATR sarnples were prepared from a sluny of the solid polymer in THF. Low resolution
rnass spectra were recorded on a Bell and Wowell21-490 spectrometer, and high resolution
spectra were recorded on an AEI MS3074 spectrometer.
Yields of PEG derivatives were determined based on relative integrations of the
assigned peaks in the proton NMR spectra.
Preparation of 6,s-Dimercapto-octanoic acid (48)53
Method A Sodium borohydride (1 .O04 g, 26.5 rnmol) was added slowly to a solution of lipoic acid
(0.503 g, 2.43 mmol) in ethanol (5 ml) at O O C under a nitrogen atmosphere. After 3 hours
the reaction was acidified to pH 2 with aqueous HCl(2 M). The solution was then extracted
with dichloromethane (3 x 5 mi), and the cornbined organic extracts were washed with brine (15 ml). The resulting solution was dried over MgS04 and the solvent removed in vacuo.
Flash chromatography (silica, gradient 20% - 50% EtOAc / Hexanes) provided the title
compound (0.449 g, 88%) as an oily.
Method B To a solution of lipoic acid (0.206 g, 1 .O0 mmol) and triphenylphosphine (0.275 g, 4.86
mrnol) in 1,4-dioxane:water solution (4: 1,5 ml), was added 2 drops of aqueous HCl(2 M).
The solution was heated to 40 O C and stirred for 5 hours. The reaction mixture was cooled to
room temperature then extracted with diethyl ether (3 x 5 ml). The organic extracts were dried over MgS04, and the solvent removed in vacuo. Flash chromatography (silica,
gradient 15% - 50% EtOAc / Hexanes) provided the product (0,191 g, 92%) as an oil.
R f = 0.35 (30% EtOAc A-iexanes); IR (neat) 3358,3193., 2929,2859,2553, 1706, 1659, 1462, 1417, 1267,933., 675. cm - [ ; 1H NMR (400 MHz, CDC13) 6 11.73 (br s, lH,
COOH), 2.89 (m, lH, Hg), 2.66 (m, 2H, 2Hg), 2.35 (t, J =7.2 Hz, 2H, 2H2), 1.86 (m,
IH, H7), 1.75 - 1.41 (rn, 7H, 2H3, 2H4, 2H5, 1H7), 1.33 (t, J = 8.0 Hz, IH, CBSH), 1.29
(d, J = 7.6 Hz, lH, CgSH); 1 3 ~ NMR (50 MHz, CDC13) 6 180.5, 43.2, 39.7, 39.1, 34.3,
26.9, 24.7, 22.7; LRMS (EI) d z 208 (Mf), 191, 157, 140, 123, 113, 101, 95, 87, 81,
73,67,61,55; HRMS (El) rnlz caIc. (Mt) 208.0592, found 208.0596.
Preparation of 6,8-Dimercapto-octanoic acid amide (63)
S-S HS SH
L,A/,,~NH~ 8 WNH, 2
O 7 O
Method A Sodium borohydride (1.00 g, 4.89 mrnol) was added slowly to a solution of lipoic amide
(1.003 g, 4.88 m o l ) in ethanol(20 ml) at O OC under a nitrogen atmosphere. After 3 hours
the reaction was acidified to pH 2 with aqueous HCl(2M). The solution was then extracted
with dichloromethane (3 x 20 ml), and the combined organic extracts were washed with brine (40 ml). The resulting solution was dried over MgSO4 and the solvent removed in
vacuo. Flash chromatography (silica, gradient 20% - 50% EtOAc / Hexanes) provided the
product (0.901 g, 89%) as an oil.
Method B To a solution of lipoic amide (1.00 g, 4.87 mrnol) and triphenylphosphine (1.91 1 g, 7.27
mrnol) in 1,4-dioxane:water solution (4: 1,20 ml), was added 2 drops of aqueous HCl(2
M). The solution was heated to 40 OC and stirred for 5 hours. The reaction mixture was
cooled to room temperature then extracted with diethyl ether (3 x 20 ml). The organic
extracts were dried over MgS04, and the solvent removed in vacuo. Flash chromatography
(silica, gradient 15% - 50% EtOAc / Hexanes) provided the product (0.929 g, 92%) as a oil.
Rf= 0.30 (100 % EtOAc ); IR (neat) 3359,3193,2930,2858,2360,2340,1658, 1632, 1462, 1416, 1293, 1229, 1 138,677,649 cm-l; lH NMR (400 MHz, CDC13), 6 5.76 (br
s, 1H, NH), 5.51 (br s, lH, NH), 2.90 (m, lH, H6). 2.76 - 2.58 (m, 2H, Hg), 2.22 (t, J =
7.2 Hz, 2H, H2), 1.87 (m, lH, H7), 1.77 - 1.38 (m, 7H, 1H7, 2H3, 2H4, 2H5), 1.33 (t, J = 8.0 HZ, IH, CgSH), 1 .28 (d, J = 7.6 Hz, lH, C6SI-I); 3C NMR (100 MHz, CDC13), 6 172.6, 40.1, 36.6, 36.1, 32.9, 23.9, 22.4, 19.6; LRMS (EI+) d z , 205, 174, 157, 140,
123, l l3,95, 87,8 1,72, 67,59, 55; HRMS (EI+) mk calc. (MHf) 208.0819, found
208.08 19.
Preparation of 5-(2-PhenyI-[1,3]dithian-4-y1)-pentanoic acid
To a solution of acid (48) (0.300 g, 1.44 mmol) in dichloromethane (5 ml) was added
benzaldehyde (O. 161 ml, 1.59 mmol) and boron trifluoride etherate (0.195 ml, 1.59 rnmol).
The solution was stirred at room temperature for 12 hours. To quench the reaction, water (5
ml) was added, and the product was extracted with dichloromethane (3 x 5 ml). The
combined organic extracts were dried over MgS04, and the solvent removed in vacuo.
Flash chromatography (silica, 15% EtOAc 1 Hexane) provided the title compound as a
mixture of diastereomers (0.33 1 g, 77%, ratio of diastereomers 9: 1 by NMR) as an oil.
Rf = 0.45 (1 : 1 EtOAc: hexanes); IR (neat) 2932,2260,2341, 1706, 1496, 1451,1418, 1271, 1072,934,731,696,668 cm-'; 'FI NMR (400 MHz, CDC13) 6 10.0 (br s, IH
COOH), 7.58-7.25 (m, 5H, aromatic), 5.24 (s, O. lH, Hg rninor diastereomer), 5.14 (s,
0.9H, Hg), 3.17-2.9 1 (m, 3H, Hg, 2Hg), 2.70 (m, rninor diastereomer), 2.34 (t, J = 7.2
Hz, 2H, Hz), 2.16 (m. lH, lH7), 2.05-1.85 (m, minor diastereomer), 1.76-1.38 (m, 7H, 1H7, 2H3, 2H4, 2H5); 'SC NMR (100 MHz, CDC13), 6 179.9, 138.8, 128.7, 128.5,
127.7, 52.5, 46.1, 35.9, 33.9, 32.8, 32.6, 25.7, 24.5; LRMS ( E h ) m/z 296, 205, 187,
153, 145, 122, 114, 101, 87, 81, 73, 67, 55; HRMS (Eh) m/z calc. (M') 296.0905,
found 296.089 1.
Preparation of 5-(2-Phenyl-[1,3]dithian-4-y1)-pentanoic acid amide (64)
To a solution of amide (63) (0.301 g, 1.45 mmol) in dichloromethane (5 d) was added
benzaldehyde (0.163 ml, 1.60 rnmol) and boron trifluoride etherate (0.197 ml, 1.60 mmol).
The solution was stirred at room temperature for 12 hours. To quench the reaction, water (5
ml) was added, and the product was extracted with dichloromethane (3 x 5 ml). The
combined organic extracts were dried over MgS04, and the solvent removed iri vacuo.
Flash chromatography (silica, 50% - 100% EtOAc / Hexane) provided the title compound as
a mixture of diastereomers (0.43 g, 99 %$ ratio of diastereomers 9: 1 by NMR) as an oil.
R f = 3.5 (EtOAc); IR 3384,2928,2360, 165 1, 1495, 145 1, 1416, 1029, 732,697 cm-]; lH (400 MHz, CDC13), 6 7.56-7.23 (m, 5H, aromatic), 5 -62 (br s, lH, NH), 5.43 (br s,
1 H, NH), 5.23 (s, 0.1 H, Hg minor diastereomer), 5.13 (s, 0.9H, Hg), 3.10 (m, rninor
diastereomer), 3.05-2.90 (m, 3H, Hg, 2H8), 2.70 (m, rninor diastereorner), 2.26-2.12 (m,
3H, 2H2 H7), 2.10-1.85 ( m minor diastereomer), 1.76- 1.39 (m, 7H, H7, 2H3, 2H4, 2 H 9 , 1.39-1.20 (m, diastereomer); I3C (100 MHz, CDCl3) G 175.2, 138.9, 128.9, 128.8,
128.6, 52.6, 46.2, 36.0, 35.7, 32.9, 32.6, 25.8, 25.3.
Preparation of 5-(2-Phenyl-[1,3]dithian-4-yl)-pentan-l=01 (51)
To a solution of dithioacetal(50) (0.561 g, 1.89 rnrnol) in THF (5 ml), was added borane-
THF (lM, 2.2 rnmol). The reaction mixture was stirred at room temperature for 3 hours.
To quench the reaction, water (-5 ml) was added slowly to the solution. The mixture was
extracted with diethylether (3 x 5 ml), and washed with bine (10 ml). The combined
organic extracts were dried over MgS04, and the solvent removed in vacuo. Flash
chromatography (silica, 20% EtOAc/ hexanes), provided the product as a mixture of
diastereomers (0.419g, 78%, ratio of diastereomers 9: 1 by NMR) as an oil.
Rf = 0.40 (30% EtOAcIhexanes); IR (neat) 337 1, 3059, 3028,2899, 179 1, 1770, 1683,
16 16, 1600, 1520, 1472, 1 137, 914 cm-l; lH NMR (400 MHz, CDC13), 7.56-7.23 (m, 5H, arornatic), 5.24 (s, 0.1 H, Hg minor diastereomer), 5.14 (s, 0.9H, Hg), 3.6 1 (m, 2H,
Hi), 3.12 (m, diastereomer), 3.06-2.92 (m, 3H, Hg, Hg), 2.66-2.74 (m, diastereomer),
2.16 (m, IH), 2.07- 1.82 (m. diastereomer), 1.74- 1.22 (m, 10H, Hz, H j , H4, H5); 1 3 ~
NMR (50 MHz, CDCl3) 129.2, 128.9, 128.7, 128.5, 128.2, 63.2, 52.9, 46.7, 36.6, 33.2,
33.0, 26.3, 26.1; LRMS (EI+) m/z 282, 249, 191, 153, 131, 12491 , 55; HRMS (Er+)
rd. calc. (M+) 282.1125, found 282.1 125.
Preparation of 5-(2,2-Dibutyl-[1,3,2]dithiastanninan-4-yl)pentanoic acid
( 5 2 )
To a solution of dithiol(48) (0.695 g, 3.34 mmol) in dichloromethane (15 ml) was added
Bu2SnC12 (1.062 g, 3.50 rnrnol). Triethylamine (0.98 ml, 7.03 mmol) was added dropwise
to the solution while stirring. The mixture was stirred at room temperature for 14 hours.
The reaction mixture was diluted with dichloromethane (10 ml), then washed with water (2 x
10 ml). The organic layer was dried over MgS04 and the solvent removed in vacuo. Flash
chromatography (silica, gradient 15% - 20% EtOAcI hexanes) provided the product (1.199
g, 82%) as an oil.
Rf=0.40 (30% EtOAcIhexanes); IR (neat) 2941, 2669,2361, 1738, 165 1, 1596, 1558,
1505, 1377, 1341, 1179, 1076, 1021,959,940,845,748,745 cm-'; IH (200 MHz, CDCl3) 6 1 1.14 (s br, 1 H, COOH), 3.2 1-2.49 (m, 3H, Hg, 2Hg), 2.34 (t, J = 7 Hz, 2H,
HZ), 2.00-1 .O7 (m, 20H, 2H7, 2H5, 2H4, 2H3, 2[C&C&CI&CH3]). 0.79-0.08 (t, 6H, Z[CH3]); l3C (50 MHz, CDC13), 6 181.6, 41.4, 39.5, 37.7, 34.4, 28.7, 27.2, 27.1, 26.0,
25.2, 19.7, 19.6, 13.9; LRMS (EI+), m/z 439, 383, 365, 349, 325, 29 1, 275, 259, 243,
206, 185, 139, 123, 95, 85; HRMS (Ei+), m/z calc. (M-H+) 439.0787, found 439.0782.
Preparation of 5-(2,2-Dibutyl-[1,3,2]dithiastanninan-4-yl)pentan-l-ol (53)
Bu. Bu Bu. Bu s.sns
-OH 6 4 2
7
To a solution of acid (52) ( 0.188 g, 0.427 m o l ) in THF (2 ml) was added borane-THF
1M 0.5 mmol). The reaction mixture was stirred at room temperature ovemight. To quench
the reaction, water (-2 ml) was added slowly to the solution. The mixture was extracted
with diethylether (3 x 2 ml), and washed with brine (5 ml). The combined organic extracts
were dried over MgS04, and the solvent removed in vacrio. Flash chromatography (silica,
gradient 5% -25% EtOAcl hexanes), provided the product (0.159 g, 88%) as an oil.
R f = 0.35 (30% EtOAc/ haxanes); IR (neat) 3374, 2928,2853, 1459, 1417, 1376, 1340, 1310, 1258, 1178, 1148, 1072, 1053,960,872,791,728 cm-'; 1H (400 MHz, CDC13), 6
3.63 (t, J = 6.6H2, 2H,Hs), 3.15 (m, l H , H6), 2.95 (td, J = 13.0,4.S Hz, lH, Hz), 2.86 (m, lH, Hz), 1.86 (m, lH, H7), 1.66- 1.24 (m, 22H, 2(CH2CH2CH2), H7, Hg, H4,
H3, Hz, OH), 0.9 (td, J =7.2, 4 Hz, 6H, 2 C b ) ; 13C (50 MHz, CDC13), 6 62.9, 41.2,
39.4, 37.3, 32.7, 28.3, 28.2, 27.1, 26.8, 25.7, 25.5, 19.32, 19.29, 13.68, 13.65; LRMS,
(EI+) m/., 369, 351,333,311,279, 175, 153, 129, 109, 87, 67; HRMS (EI+) ~ r / z calc.
(MH+) 369.0369, found 369.0373.
Preparation of MPEG-DOX-CI (20)21
Dry monomethylated polyethylene glycol (5 g, 1 rnmol) was dissolved with heat in
anhydrous THF (50 ml) with the exclusion of hurnidity and cooled to room temperature.
Sodium hydride (0.12 g, 3 rnmol) was added with stirring followed after 10 minutes by sodium iodide (0.17 g, 1.15 mmoI) and a,al-dichloro-p-xylene (5.25 g, 30 mmol). The
reaction mixture was stirred for 96 hours. The reaction mixture was filtered washed with a
small amount of dichloromethane (5 ml). The MPEG derivative was precipitated with
anhydrous diethyl ether and cooling to 4 OC the for 20 min. The precipitate was collected and
redisolved in a minimum of hot ethanol, then cooled to 4 OC to reprecipitate the product. The
product was collected and dried in vacuo . The product was obtained in 96% based on the
relative integration of the Hg and the Hl0 peaks in the 400 MHz proton NMR spectrum.
1H NMR (400 MHz, CDC13), 7.321, (dd, 4H, J =8.33, 14.01 Hz, aromatic), 4.55 (s, 2H,
OB2Ph) , - 4.54 (s, 2H, C1CH2Ph), - 3.35 (s, 3H, CH30PEG).
Preparation of MPEG-DOX-yl-lipoic acid (58)
To a solution of MPEG-DOX-CI (0.207 g, -0.04 mmol) in CH3CN (2 ml), was added
lipoic acid (0.025 g, 0.1 17 rnmol) and cesium carbonate (0.021 g, 0.058 mrnol). The
reaction mixture was heated to 82 O C and stirred for 5 hours, during which time a white
precipitate formed . After cooling to room temperature the solution was filtered to remove
the white precipitate. Diethyl ether (15 ml) was added to the filtrate and the mixture was
cooled to 4 OC for 20 min. A white precipitate formed and was collected and redisolved in
hot ethanol. The product was reprecipitated by cooling to 4 OC for 20 minutes. The solid
product was collected and dried Nt vacuo. Functionalization of the MPEG-DOX-CI occurred
in 92% based on the relative integration of H9 and H2 in the 400 MHz 1H NMR spectrum.
The remainder is hydrolyzed MPEG-DOX-OH.
IH NMR (400 MHz, CDC13) 7.29 (br s, 4H arornatic), 5.06 (s, 2H, Hg), 4.53 (s, 2H,
H IO), 3.85-3.40 (m, PEG and HG), 3.35 (s, 3H, CH3), 3.18-3.05 (m, 2H, Hg), 2,46-2.38
(dt,lH, H7), 2.36- 2.30 (t, 2H, Hz), 1.93-1.83 (m, lH, H7), 1.74-1.36 (m, 6H, H3, Hq,
Preparation of MPEG-DOX-yl-6,8-Dimercapto-octanoic acid
To a solution of MPEG-DOX-lipoic acid (58) (0.500 g, -0.1 mmol) in dioxane:water (4: 1,
1 ml), was added triphenyl phosphine (0.027 g, 1.03 mmol). The solution was heated to 40
"C then HCl (2M, 2 drops) was added and the reaction mixture was left stirring at 40 OC over
night. The reaction solvent was rernoved in vacuo and the resulting residue was taken up in
a minimum of dichloromethane. Diethyl ether ( 15 ml ) was added to the solution and the
mixture was cooled to 4 O C for 20 minutes. The resulting precipitate was collected and
dissolved in a minimum of hot ethanol. The solution was again cooled to 4 O C and the
resulting precipitate was collected and dried in vacuo. Overail functionalization of the MPEG
to the title compound occurred in 92% based on the relative integration of OCH3 and Hg in
the 400 MHz IH NMR spectrum. The remainder is starting material.
lH (400 MHz, CDC13), 6 7.29 (2,4H, aromatic), 5.06 (s, 2H, Hg), 4.53 (s, 2H, Hio),
3.85-3.40 (m, PEG), 3.34 (s, 3H, CH3), 2.86 (m, lH, Hg), 2.69 (m, 2H, Hg), 2.35 (t,
2H, Hz), 1.83 (m, IH, H7), 1.76-1.18 (m, 9H, H7, 2H5, 2H4, 2H3, 2SH, OH).
Preparation of MPEG-DOX-phthalamide (65)
To a solution of MPEG-DOX-Cl(0.25 g, -0.05 rnrnol) in DMF (5 ml), was added
potassium phthalamide (0.027 g, 0.146 mmol) and sodium iodide (0.001 g, 0.005 mmol).
The solution was stirred at reflux 120 OC for 3 hours. Once the solution had cooled to roorn
temperature, diethyl ether (15 ml) was added and the mixture was cooled to 4 OC for 20
ininutes. The resulting precipitate was collected and redisolved in a minimum of hot ethanol.
The solution was again cooled to 4 OC. The product precipitated as a white soIid which was
collected and dried in vacuo. Based on relative integrations of OCH3 and H2 in the 400
MHz lH NMR spectrum, functionalization of the MPEG-DOX-Cl occurred in 93% yield. i~ NMR (400 MHz, CDC13), 6 7.80 (dd, 2H, H3, H4), 7.67 (dd, 2H, Hg, H6), 7.22-7.40
(m, 4B, aromatic), 4.80 (s, 2H, H2), 4.48 (s, 2H, H3), 3.85-3.40 (m, PEG) 3.34 (s,
4.6H, OC&).
Preparation of MPEG-DOX-amine (66)
To a solution of MPEG-DOX-phthalamide (65) (1.0 g, -0.2 rnmol) in ethanol (20 ml) was
added hydrazine monohydrate (0.095 ml, 2.0 rnrnol). The solution was stirred for 12 hours
at 80 OC. The solvent was removed in vacuo, and the resulting residue was taken up in a minimum of dichloromethane. Diethyl ether (40 ml) was added to the dichlorornethane
solution and the mixture was cooled to 4 OC for 20 minutes. A white precipitated fonned
which was collected and redisolved in a minimum of hot ethanol. The solution was cooled
to 4 O C and the resulting precipitated was dried irz vacuo. Conversion of the MPEG-DOX-
Phthalamide to the titie cornpound was observed in 75 % yield based on relative integrations
of OCH3 and H2 in the 400 MHz 1~ NMR.
IH NMR (400 MHz, CDC13), 6 7.29 (m, 4H, aromatic), 5.28 (s, lH, NH), 4.53 (s, 2H,
Hl), 3.88 (s, 2H, Hz), 3.81-3.41 (m, PEG), 3.34, (s, 4H, OCH3).
Preparation of MPEG-DOX-Lipoic amide (67)
To a solution of lipoic acid (0.095 g, 0.46 rnmol) in dichloromethane (1 ml) at O OC, was
added a solution of DCC (0.048g, 0.23 mrnol) in dichioromethane (1 ml), also at O OC. The
solution was stirred at O "C for 1 hour and then filtered to remove the white precipitate which
had formed. The filtrate was then added to a solution of the MPEG-DOX-amine (66) (0.60
g, -O. 1 lmrnol) in dichloromethane (3 mi) and stirred at room temperature for 2 hours.
Diethyl ether (15 ml) was added to the solution and the mixture was cooled to 4 OC for 20
minutes. The resulting precipitate was collected and redisolved in a minimum of hot ethanol.
The solution was again cooled to 4 OC. The product precipitated as a white solid which was
collected and dried in vacuo. The overail conversion of MPEG-DOX-Cl to the title
compound occurred in 44% based on the relative integrations of OCH3 and H2 in the 400
MHz 1H NMR spectrum. The remainder was starting materiai. IH (400 MHz, CDC13), 6 7.27 (m, 4H aromatic), 5.79 (m, 1 H, NH), 4.52 (s, 2H, Hi*),
4.40 (d, 2H, Hg), 3.86-3.41 (m, PEG, Hg), 3.36 (s, 3.7H, CH3), 3.10 (m, 2H, Hg), 2.44
(m, lH, H7), 2.19 (t, 2H, H2), 1 .go (m, IH, H7), 1.78- 1.40 (m, 6H, Hs, H4, H3),
Preparation of MPEG-lipoic acid (68)
s-s
8 7
O
To a solution of lipoic acid (0.121g, 0.59 rnrnol) in dichloromethane (2 ml) at O OC, was
added a solution of DCC (0.060 g, 0.29 rnrnol) in dichloromethane (2 ml), also at O OC. The
solution was stirred at O "C for 1 hour and then filtered to remove the solid precipitate which
had formed. The filtrate was then added to a solution of MPEG (0.50 g, -0. lrnrnol), in
dichloromethane (6 ml). The solution was stirred at room temperature for 2 hours. Diethyl
ether (30 ml) was added to the solution and the mixture was cooled to 4 O C . The resulting
precipitate was collected and redisolved in a minimum of hot ethanol. The solution was
again cooled to 4 OC. The product precipitated as a white solid which was collected and dried
h vaciro. Functionalization of the MPEG occurred in 16% yield based on the relative
heights of OCH3, and Hg in the 400 MHz 1H NMR spectrum. The remainder was MPEG. 1H (400 MHz, CDC13), 6 4.19 (m, 2H, Hg), 3.95-3.39 (m, PEG), 3.35 (s, 19H, C&),
2.62, (t, 4H), 2.33 (m, lH), 2.13 (br s, 5H and H20), 1.71-1.32 (m, 3H).
Preparation of MPEG-DOX-cinnamyl ether (57)
To a solution of cinnamyl aicohol(0.017 g, 0.13 rnrnol) in THF (2 ml) was added NaH (0.010 g, 0.24 mmol). The solution was left stirring for 20 minutes at 50 O C . Then MPEG- DOX-Cl (0.200 g, 0.04 mmol), Na1 (0.009 g, 0.06 rnrnol), and 18-crown-6 (0.031 g, 0.1 1
mrnol) were added to the solution. The reaction was stirred at 50 OC for 48 hours. The
solution was filtered and washed with dichloromethane. Diethyl ether was added to the
filtrate and the mixture was cooled to 4 OC for -20 minutes. The precipitate which formed
was collected and redisolved in a minimum of hot ethanol. The mixture was again cooled to
4 OC until the PEG derivative precipitated, The precipitate was collected and dried in vacuo.
The product was obtained in 64% yield based on the relative integration of the C3 protons to
the Cs protons.
lH (400 MHz, CDC13), 6 7.39-7.25 (m, 9H, aromatic), 6.59 (d, J=17.2 Hz, lH, Hi),
6.33-6.04 (m, lH, Hz), 4.54 (s, 2H, H4), 4.52 (s, 2N, Hg), 4.16 (dd, J=1.6, 6 Hz, 2H,
H3), 3.4-3.8 (m, PEG), 3.36 (s, 3H, OC&).
Appendix A: Selected Spectral Data
qj: f.
IV)
\ I I I I I I i n 1
I I I I I I
m N - O h N b N
C
References
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