20
Biogenesis of limonoids:
The main features of triterpene biogenesis are
now v;ell established and are summarised in scheme "1, The
limonoids are def raded triterpenes and constitute a group
which derives its name from limonin, the bitter principle
3 9 present m most citrus species '•". Determination of the
structure'of this principle was a magor problem in the mid
fifties v/hich v;as solved through the efforts of several
10-12 internationally reno\ med chemists , Since NPIR
spectroscopy had not yet arrived on the scene, the structure
was deduced largely from chemical transformations of the
molecule. X-ray analysis clarified such doubts as still
existed and established besides the relative stereochemistry,
The absolute configuration of limonin and other members
of the group was assumed on the basis of biogenetic
relationship to the tetracyclic triteri ene tirucallol/euphol
which had been previously related to compounds of known
absolute configuration. Further confirmation v as soon
1^ available through apolication of ORD -''.
Evolution of the limonin carbcai skeleton from
euDhol (3) involves elimin&tion of four carbon atoms of the
21
side chain, cleavage of Gp-C., bond of ring A, migration of
C^^ methyl to G^ and modification of ring D. The limonin
nucleus emerges through oxidative cyclisation leading to
tv;o 5 membered and tv/o 5 membered heterocyclic rings. The
changes can be represented diagrammatically as shovm in
scheme - I.
[ SCHB\/!E I ]
Scission of r ing A as "oostulated in the 'blorcr.etlc
schene v/as o r i g i n a l l y observed in dammarenolic acid and
• 1 5 i s of v;ide occurrence among t r i t e r p e n e s e .g . shoreic acio - ,
22
. . 16 putranjivic acid and migration of the methyl group occurs
with introduction of double bond between C^^ and C. - during
oxidation of butyrospermyl acetate (5) to the 7-keto
compound (5) , Conversion of ring D to the 6 membered lactone
can occur through an in vivo enuivalent of the Baeyer -
Villiger oxidation. Many compounds have been isolated in the
intervening years which lie either on the pathv/ay to limonin
from euphol, or have resulted through scission and modifi
cation of other ring. '\
NZO'^'
H
ACO
(5) (6)
In the scheme that follows limonoids or their
immediate precursors have been so arranged as to place the
more primitive, and therefore siiapler ones, before the
23
structurally more complex. Protolimoaoids are compounds in
which structural changes of the precursor triterpene moiety
are confined to the side chain. Thus flindissol (7)
turraeanthin (8) aphanamixin (9) melianone (10) contain all
the carbon atoms of tirucallol/euphoi but cyclisation of
AcO
R = H r OH
(9)
do;
Lhe side chain to the Cdiurabcd furan has already occurred.
24
Cedrelone (11) and grandifolone (12) are representative of
compounds in v/hich methyl migration is accompanied by loss
of 4-carbon atoms from the side chain. Gedunine (13) type
t<=A
Kry o
(II)
ACO-
;i2)
R = H
of compounds are readily derivable from these through
Baeyer-Villiger rearrangement v/hile nomilin (I' O and
obacunone (15) are more obviously related to limonin (l&)
and ichangin (17) must be its immediate precursor.
25
(14) (15)
O^^'
;i6} (17
Also regarded as members of the limonoid group
arc andirobin (18) and methyl angolensate (19) in v/tiich
26
ring B undergoes scission and ring A remains intact, v/hereas
in nimbin (20) both rings A and B survi\-e and it is ring C
that is modified. Though more complex structurally
swietenolide (21) and mexicanolide (22) are readily accommo
dated in the scheme as products formed through cyclisation of
ring A with the methylene resulting fron scission of ring C,
^ COOCH
US) [19)
CH OC 2 II O
20) 21
COXI^
^
27
2 2 )
28
Structure elucidation of Limonoids;
NMR opectroscQ-py:
The structural problem of liraonin, as already
pointed out, v/as resolved largely through chemical trans
formations and PMR spectroscopy, applied at a later stage
of the work, served only to confirm the number of methyls.
The published data was meagre and only resonances of the
furanoid protons v;ere assigned. The first comprehensive
18 study of PI'IE spectra of limonoids was made by D,L.Dryer ,
vjho also employed it in confirming the structures assigned
to products resulting from deep-seated rearrangement of the
molecule.
In spite of the comparatively poor resolving
pov;er of the 60 KHz instruments, the spectra of limonin
and its derivatives reported by him are fa.irly clear, a
consequence of the wide difference in chemical shifts of
structurally significant protons and presence of but fev;
contiguous methylonos. It was thus possible to make
assignments with some certainty, the important ones being
of (a) methyl resonances (b) epoxidic protons (c) the C-1
and C-19 methine and methylene (d) furfuryl proton and (e)
90
the furanoid protons.
Methyl Resonance:
The C-8 methyl bei.ng farthest av;ay from deshielding
influences resonates at highest field followed by the
gem-dimethyls which are B to the ether ox;}'-gen and the / • — • ' '
18-methyl v/hich is in proximity to the luran ring. That the
resonance of the C-8 methyl is correctly identified follows
from the fact that it is the one most affected by reduction
01 carbonyl or its conversion to oxime. Similarly in obacunone
methyl assignments could be made on the basis of shifts
30
produced by opening of the 7-niembered lactone ring.
Protons under oxygen:
Of these there are five and of the five only the
one at C-1 is further coupled. The 0-19 methylene gives
rise to a singlet In some compounds and in others to AB
doublets. The singlet of the epoxide profcons v;as found to
overlap in some limonoids v/ith the mulfciplet of the C-1
proton but there is no ambig\,iity about the furfuryl proton
as, apart from its being on a carbon under oxygen, it is
further deshielded by the adjacent furan ring.
The epoxide proton appears at much lov;er field
than in similar compounds, because of uhe 0-7 carbonyl group.
The effect is similar to that observed, in case of 0-1
hydrogen in 11-ketosteroids ' nd 0-7 hydrogen in some
diterpenes e.g. fibleucin (23). Reduction of the 0-7 carbonyl
23)
31
gives two compounds, liraonol and epilimonol v/hich are
epimeric at C-7. The position of the epo;.dde proton
differs by about 40 Hz in the two compounds, Spilimonol,
the reduction product in which the epoxide hydrogen appears
at lower field, has been assigned equatorial [ OH as in
this case the hydroxy group ojid epoxide proton are in
proximity and the latter is thus exposed to the deshielding
influence of C-0 bond. An interesting fact that emerges
from a study of different limonoids is that the epoxidic
proton in compounds in which ring A is 7-Kiembored e.g.
obacunone (15) is at higher field than in limonin. Besides
serving to distinguish compounds of the two tj'~oes this
provides evidence of conformational changes in ring B on
expansion of ring A. Further the epoxidic ring also
influences the environment of the C-7 hydrogen in C-7
alcohol causing it to resonate at lovier field when it is
equatorial, as in limonolo Models shov/ that the epoxide
ring is also close tc the fu.riuryl proton and excercises
a deshielding influence on it too, for in dooxylimonin the
proton is shifted upfield hy about 20-30 Hz.
The stereochemistry at C-I7 was established
through recourse to Klyne's modification of Hudson's
32
19 lactone rule , The axial i.e. orientation of the
-fupfuryl proton is clearly discernible in columbin by the
larger coupling constant with the axial proton of the
adjacent methylene in columbin (24-) 20
24
33
Sfcructure of Janfpraolide:
The defatted bark of Jlacourtia cataphracta v;as
extracted v ich ethariol and acetone :..nd from the extract after
the usual purification a v.'hite cry£i:alline solid v/as
isolated. It had sharp melting poini; and initially appeared
pure on tic but the mass spectrum vras not very clear about
the position of M ' and the HMR too seemed to belong to a
mixture. The NHR spectrum shov/ed 8 t art-methyls atj high
field which suggested that the compound was a triterpene but
the mass spectrum had its most prominent ion at 5'J-8 with
peaks of very lov/ intensity only at higher mass values. It
was therefore presumed that the isolated solid was a mixture
of tv/o diterpenes. The IR spectrum (?ig-I) shov/ed a broad
-1 C^O band betv/een 17^0-. 1690 cm vi^n fine structure indicating
the presence of an ester or lactone grouping. Further
information about the structural features of vhc constituent
compounds was available from the ?'70 MHz HMR spectrum
(J ig-I- ). The presence of epoxide appeared likely but
integration over this region and the .uode of coupling v;as
not such as could be accorimodated within the fraraev/ork of
a di-or triterpene molecule and presence of other impurities
was susnccted„ The signals at lo'.'er field wore, ho\/ever,
34
35
f^^-^^ ^V^t j,'VS?"S>)^?a^t^1^:5trfliA'v?-'"^i^^J 'fe'iaf' e*'S2^^ ?(Kw*9Enaitei* RJ^^»_I 'ks?''MS' C£i-5iacL^-*v ^C£i-5iacL *v - i —-tvJ^ i'*
36
clearly resolved and left no doubt of the presence of a
j^-substituted furan and c<. p -unsaturated carbonyl moities.
Especially to be noted are the doublets at 6.6 and 6."15
(^ v/hich were assigned to a CHp=CH-C- grouping and the
doublets at 4.45 and 4.75 v/hich suggested a -CHp~0~C-
grouping« The integration of these doublets is different,
from that of the doublets at 5*5 and of the doublets of
furanoid protons at 6c35« These features of the HMS spectrum
directed attention tov/ards either a colu&bin or a limonin
type of molecule v;ith the mass spectrum favouring the latter
possibility. In any case, no progress could be made v/ithout
resolution of the mixture and to this end extensive tic on
various types of plates v;as attempted. Separation was
ultimately achieved through preparative tic and supplied.
tvjo compounds one of \/hich was identical v/ith limonin. The
position, and multiplicity of the signals, in the off
resonance C spectrum of the isolated limonin agreed
21 closely v;ith values i eported_ m literatu- e " and the tv/o
are compared in the table - 1 .
All chemical shifts are reported in 6 values.
24 2
37
( 16 )
Found
" 0
21
20
1?
141
143
110
120.
11
16v6,
53.
65.
.2
A
»7
.7
.2
.3
>1
A
.8
6
9
8
T a b l e - 1
ReDorted
29.6
21.3
19.6
17.1
I 4 l e 8
1''i3.2
1 1 W ct I
120.2
77.^
167.2
53.7
66.7
Assip;nments
quar te ts of t e r t i a r y meth3/
doublets of furanoid carbo
s ing le t of furanoid carbon
17
16
15
14
38
Zound Re-Qorted Assir^nments
38.0 57 ,6 15
30 .8 29 .6 12
-18.9 17-6 11
5 1 . ^ 45 .2 10
^5 .1 ^ -6.2 9
^-6.0 50o3 8
206.1 207.8 7
35.7 36.0 6 ^
60 ,6 58.0 5
8 0 . 3 7 9 . ^ 4
169.7 170.0 3
36 .4 55.6 2
79o2 78 .4 1
6 5 . 4 19
1x
Comparison of t h e - C s p e c t r a of t h e new compound
d e s i g n a t e d as .iangomolide (PLp-TIl) ( l^lacourt ia Ca taphrac ta
i s s^/nonyaiOLis v/ith F l a c o u r t i a J&ngomoceG'S) and l imonin
(Tal)le-<'5) b r i n g s out c l e a r l v bliat d i f f e rence bet\'Gen t h e
two r e s i d a s only in t h e s t r u c t u r a l f e a t u r e s of . r i n g A.
39
'aJptS^ i^rfc? '^— •
LL
> f -C
40
. > o
o o
o in
JD
o LL
O O CO
rj
41
{ 2 )
Table - 2
Limonin
"50,?„
21.4
20.7
17.7
141,2
145.3
110«1
120.1
77c8
166.6
53«9
65.8
33cO
cra.np;onolide Assip;nments
31.5
24.7 quartets oT t e r t i a r y methyl;
20.0
15.5
150.5
143.;
1^1,1
119e3
77-5
166 c 3
54.1
67.2
38»4
doublets of furanoivl carbons
singlet of furanoid carbon
17
16
15
14
13
42
Jane:omolide Assignment;
12
11
10
9
8
7
6
5
3
2
1
19
Thus -resonance of the recons t i tu ted r ing A
carbons occur at 79.2, 36.4, 169.7, 80.0 axnd 65.4
(carbons 1 ,2 ,3 ,4 , of modified r ing A and 19 methyl carbon)
in the limonin spectrum. .Signals close to these values in 15
the -C NMR s'pectrum of the jangoraolide are more appropriate.
assigned to other carbons of the limonoid nucleus and in
any case one has to find a viable explanation of the
signals at 150,8, 118.3, 166.8, 36,3 a?id 104.8 which, except
Li i
30
18
31
45 .
4 6 .
nonin
. 8
.9
. 4
.1
.0
206 .1
35.
6 0 .
8 0 .
16S
3 6 .
79.
6 5 .
.7
,6
• 3
K7
,4
2
4
28
18
49.
53.
48 .
209.
37.
4 1 .
36 .
166.
118.
150,
104.
. 0
. 0
. 2
. 2
.3
,1
.5
.6
3
.8
3
,8
0
43
for the signal at I6608 have no counterpart in
the limonin spectrum. The NI-IR spectrum 01 jangomolide
is critical in establishing the nature of the ring
system in jangomlide arid is discussed below.
The PKR spectrum of jangomolide (Fig, IV) shows
4 tertiary methyls at 1.3, 1.28, 1,16, od ,p"Substituted
furan at 7.41 (m,H-21 and H-23) and 6,33 (m~H-22), epoxy
lactone at 5.99 (3H-15) & 3.53 (S-H-l?). The presence
of an i: ,|3~unsaturated lactone grouping is evident from
signals at 6,5 and 6,14 "(both doublets, J=9.6 Hz). The
most significant feature of the FI-iR spectrum is the
singlet at 6.06 Since the -C Mi;R allows only one C-C
double bond in the molecule the singlet must arise from
protons en a carbon under two oxygens i.e., an acetal
carbon, ^uch a linkage can come into existence if the
C°19 methyl gets oxidised to an aldehyde group which then
from a haniacetal, vath the C-4 hydroxy group and the newly
formed hydroxyl is inolved in esterification of the
carbonyl groiio as illustrated belo ;*
44
•ffiss^ - N f
m
o
=5
IN
;'Mm*lO<SS»B3»aiVTr5CC3!*'1S'«a!JF£'^
45
HON
The most logica l str^ic-fcure on th i s basis i s (2)
which i s i n complete accord with a l l spec t r a l features of
the compound. Thus the band at 1700 in the i r , spectrum
of the mixture resolves in to bands at 1773, 1750 and 1706
and the high resolut ion mass spectrum shows N"^'- 15 a t
453-15^0 correspondingly Cpj-Hpc-O'*'*o and the molecular
formula C^ H^ Oo 25 2o 8.
The above correlation with limonin fixes up the
stereochemistry of jangomolide at all centres except C~19
i^ei the Junction of the 5 and 6 membered heterocyclic
ringsc The 3-orientation of the hydrogen is based on IWR
evidence of slight coupling between the C-19 and the 0-1
46
hydrogens0 Such long range coupling is a feature of
compounds e.ge steroids (25) in v/hich the concerned
(25)
hydrogens form an M arrangement, Models shov/ that
in jangoraolide such an arrangement is possible only if
the hydrogens is |i , hence it must-have the stereo
chemistry shown as belov; i.26)6
261
47
The family Gnctaceae of the plant kinp;dom is
comparatively unexplored and comprises of but two genera,
Ephedra and Gnetum. While Ephedra vulp;aris is a commercial
22 source of ephedrine , isolation of the alkaloid from Gnetun
2-5 species has also been reported ^, The work on the chemical
constituents of Gnetum ula was taken up in this laboratory
to find out if it also contained phenylethylamines, which
are invariably of therapeutic value. It showed that the
plant did not contain alkaloids but was rich in phenolic
secondary metabolites. One of the 3 6 V/ SL S Q.^3 signed the
stilbcne structure (2?) which was later revised to (28) ""'c
The remainin g three v/ere unambiguously characterised as the
iJesacetophenon benzyl ether (29) and diraer (30) and the
isocoumarin C-glycoside bergenin (3' )»
( 27 )
48
HOx^.-'C^-v^OH
( 2 9 )
o
! 3! )
x u i U ; . . c x V. l ix u i c i o u [ , ^ c j i i i C ciiicL-L^'^-'-'^ ^-^ C/iit C27UU.9
ext rac t led to the i s o l a t i o n of a ncv product uhich was
df r ""nat }d as gneT:ol ard idoncified as the sbiloene (32) on
the basjs of evidence preseatod below*
49
(32 )
M " at m/z 24^ (100:0 positive ferric chloride
reaction, and absence of OMe signal in the NHR spectrum
sugj-ested a tetrahydroxy stilbene structure for ^netol, a
conclusion supported by UV absorption at 309, 318 and 33' nni
(32) and strong hydroxylic absorption v/ithout any indication
of a carbonyl band in the IR spectrum.
Bue to its poor solubility in chloroform the PHR
soectrum of gnetol had to be run in DM50-d^. The 60 MHz
spectrum (ii'ig-V) initially obtained was too poorly resolved
to allow any inferrence of the aromatic substitution«
Since subsequent v/orli shov/ed that gnetol forms a
tetraacetate the rise of the integral over signals in the
50
;> en
51
offset must be assignoci to four protons and on this basis
bhe rise over the aromatic region, as expected for tetra-'
substituted stilbene; accounts for eight protons. Though
the J'IIR spectrum of gnetol and the stilbene isolated earlier
differ in detail the presence in both of a 111 triplet at 6.03
is notev/orthy and structure assigned to the stilbene isolated
earlier 'ind gnetol must provide for a proton absorbing at
this value and having the required multiplicity. It v/as
natural to suspect at this stage that gnetol is the
deraethylation product of (2?) but this possibility was
eliminated when permethylation products of the two v/ere
empaced. It v;as also noted that whereas methylation of (27)
proceeded smoothly and was over in a few hours that of gnetol
took several days. The acetate, ho\/ever, formed readily but
the PlUl spectra of both methyl ether and acetates are
singularly uninformative as multix)lets of the arom.atic
protons merge to a multiplet between 3 and 4 . The acetate
methyls give rise to two singlets at 2.31 and 2.30 each
integrating for 6 protons.
•'.'he mass s ooctrum of gnetol is also of not much
value, Tbis is in line with the behaviour of stilbenes in
v/hich, because of extended, conjugation, the molecular ion
52
is quite stable. The only breakdovnis besides random
cleavages can be attributed to loss of water and,CO from
the molecular ion. In contrast the spectra of dihydrostil-
benes are extremely useful in 'deciding about the distribution
of substituents in the two benzene rings as cleavage of the
central bond gives rise to benzylic (tropylium) cations.
The mass spectrum of dihydrognetol shows M ' at 245 amu and
base peak at 125 ^smi indicating clearly that each ring
carries tv;o hydroxyls. Several possibilities exist for the
structure of gnetol on the basis of data recorded above.
The PHR spectra in hand did not permit a clear cut choice to
be made, the general difficulty ivith stilbenes as opposed
to the flavonoids, v;hich also contain tv/o benzene rings. In
flavonoids, however, the chemical shift of benzenoid".
protons are not uniformly efiecocd by the carbonyl group
and since the chemical shift difference is large compared
to J values the spectra are well resolved and approximately
first order» This mad e it necessary to obtain from abroad
a 560 Mtlz spectrum of gnetol in K-'ISO-d and DMSO-d^ benzene.
The signals of the aromatic protons vvere clearly resolved
in these tv/o spectra and the substitution pattern could now
be inferred.
53
The spectrum (l ig-VI) in DKSO-d^ shov/s two doublets
at 7»37 and 7o25. Because of the large coupling constants
(J:= 17 Hz) it can only be assigned to olefinic protons. The
signals at 6.6 and 6.2 are, however, not as easily assignable
in this spectrum. Addition of benzene leads to further
resolution, the spectrum now showing a partially merged
double doublet at 6.7 and a clear 2H doublet at 6.88. This
means that the two nrotons must have identical chemical
shifts and are coupled to the same protono These tv;o signals
can therefore only be assigned to the contiguous protons of
a 1,2,3 trisubstituted benzene ring. The hydroxyls in one
ring are therefore located ortho to the stilbene double bond.
The 2H doublet at 5.4 must similar ly be assigned to
identical nrotons which are coupled to a proton in meta
position to both. The other beazene ring must ha.ve a
symmetrical substitution and structure (52) follows logically
from this. The triplet at highest field is nov; understandatble.
It must arise from a uroton which is betv/Gen two oxygens
and it must be meta coupled to t\;o identical protons. This
assignment was further confirmed through double resonance
experiments. Irradiation of the triplet at highest field led
to collapse of the doublet at 5«5 to a singlet» The hydrox:vlic
54
protons appear in t h i s spectrum as tv;o sharp s i n g l e t s in
the of f se t not l i ke the broad mul t ip le t s observed ear l ie r ' .
F ina l ly , the s t ruc tu re (32) , assigned to gnetol
on the bas i s of data presented above, vras confirmed through
26 synthesis. Condensation of 2,6-dimethoxybenzaldehyde with 5,5-dimethoxyphcnylacetic acid (prepared by Arndt-Eistert
on OQ
reaction) ' • in the presence of piperidine gave the stilbene
p-carboxylic acid (55) I'/hich on decarboxylation v/ith copper
bronze and quinoline afforded 2,6,11,13-tetramethoxystilbene
(5- ) identical in all respects v/ith gnetol tetramethyl ether.
Attempted demethylation of synthetic (3^) with BCl^ at 0° led
only to partially demethylated product 2,6,11,15~tetraraethoxy-
stilbene which on heating with pyridine hydrochloride at 200
for four hours gave a compound which was found to be identical
with the product obtained when gnetol (32) v;as subjected to
similar reaction conditions. The >mass soectrum of this compouD:
indicated it to be dincric in nature but its structure could
nob be established with certainty.
55
,OMe iCOOH
OMe
OMe
OMe OMe
33) 34!
Bej?ore s t ruc tu re (32) v/as conclusively es tabl i shed
for g;netol some spec t ra l evidence pointed tov;ards (35) as
a possible s t ruc tu re for gneto l . Compound (36) ' ^ as therefore
b j i L u n e S i S y u u j ^JuiiO tJij toc!.i/j-uii UJ. C: ^^^ J~ KyV^xaSifiiO.^j Duli/'cii-ueiij'u.c:
v/ith n~'aetboxy phenyl acoLic acid . The s t i lbene (36) thus
obtained along;vifch i t s corresponding s t i lbene B -carboxyl ic
acid (37) was shown to be d i f fe ren t froia gnetol tetracaeth^/l
e ther (34)„
56
HO
( 35 (36)
MeO,
MeO OMe
37 1
As noted at; the begining structure (27) assigned
to one stilbene had to be revised to (28) in the light oi
results obtained with gnetol» It is necessary in this
coiitort to go briefly into reasons for allotting initially
structure (27).
57
The 90 MHz spectrum of stilbene (27) in DM30-dg
is reproduced in (Fig~VI). The spectrum shows one methoxyl
and the signal in the offset integrates roughly also for
three protons. The number of OH groups in any case is
settled through methylation and acetylation. The spectrum
shows two clear doublets at 7* 0 and 6.78 without any sign
of further coupling. This requires that one of the tv;o
benzene rings must be tetrasubstituted and therefore the
other ring can have only one hydroxyl group. The other
hydroxyl could either be at 0-10, 11, 12 positions but if
it were to occupy position 12, the spectrum v/ould have an
AA'BB' pattern which is nowhere evident. The hydroxyl was
not put in the ortho T30sition to the double bond because
under strongly acidic condition or with sercuric acetate
there is no cyclisation to a dihydrobcnzofuran. This i?
also the reason for olacing the methoxyl ortho to the 0=0 in
the other benzene ring. This structure, however fails to
explain the triplet at 6.;5 v/nich '/as tentatively assigned
to 'che G-10 proton. The assigniient agrees vjith the observed
multiplicity but protons adjacent to only one hydroxyl
normally resonate at lower fields. It seeaed at that time
that either interaction with double bond or the solvent
i -
in
>
- ^ " / ^ S v ^ , "
•(]• •r'-^'i-
t> ^
.1-
X
> > „
X o.
X
J
y " " " • ' " **lftJL ?!?*• f
s ( l
i ' <w
rrva
X
• ' " 1 t
E a a
<iJ
t-.
tG}
o
o
c o
<5
o to a
Q
o
ft
•a"
59
induced shift was responsible for it but vjith the elucidation
of structure (32) for gnetol it became evident that this
proton was wrongly assigned and the structure had to be
revised,
60
The flavonoids form an important group of natural
products and their occurrence, structure, chemistry and
29
pharmacology has been the subject of many publications ^
since the initial work of Robinson on anthocyanins. A
rearrangement of the flavonoid carbon skeleton (38) leads
to isoflavonoids of \;hich by far the most abundant group is
that of isoflavones (39). Derivatives of isoflavanone (40)
i.Cc. compounds in uhich the double bond in the pyrone ring
is missing are far less common in nature but rotenoids (4-1)
which have an extra methylene group and a tetracyclic
system possess basically the isoflavanone nucleus. Another
( 39)
61
class of compounds akin to isoflavanones is represented by
pterocarpanes (42). If both the double bond and. the carbohyl
group of the pyrone ring are completely reduced, isoflavans
(45) result. Untill recently the only representative of this
group v/as isolated from mares urine and appropriately named
equol- vestitol'' , laxifloran , mucronulatol lonchocarpai
came to light after 1950 but the number of isoflavonoid
isolated to date does not exceed forty three «
(42 431
The only direct method for synthesis of isoflavanone 25/4.
v/as introduced by Wanslick v/hich makes use of the base
catalysed coupling of benzopyran -4—one (4- 0 with nascent
quinones (45) but this method, though of much theoretical
62
interest, is limited in scope and the usual procedure is to
BASE
44' (45)
prepare tlie isoflavono having bhe required substitution and
reduce it to the desi3?ed levcl»
The isolation in this laboratory of an isoflav^n
and isoflavanone- - directed attention towards the search for
a. more conv anient s/nthetic method for these two types and
in the follov/ing section the possible approaches and some
preliminai'y vjork is dibcubsed =
The startinr; ooint of -311 isofljvone snytheses is
a deoxybonzoin. Several roooents have been employed for
introduction of C~2 of the isoflavone nucleus, the latest to
63
make the grade 3,s dLniethyl amino formamide dimethyl acetal .
Reduction of isoflavones to isoflavanones and isoflavans is
usually effected over catalysts, Pd/C or PtO, but NaBH^ has
also been employed" . Since the isoflavanone carbonyl is
more easily reducec than the isoflavone carbonyl it is
difficult to stop the reduction at the isoflavanone stage
and yields are poor. Better yields have been reported for
isoflavans but in practice, using samples available in the
laboratory, variable results were obtained. li? therefore,
seemed v/orthwhile to try other ap-oroaches to isoflavanone
and isoflavone synthesis. One method, the reaction of 3-phenyl
4-hydroxycoumarin v/ith DHSO/AcpO (Scheme-Il), discussed
elsev/here'' , would have been ideal but it gave poor results
30 with substituted coumarins' '',
DMSO Ac 2 O. >
H^-S-CK
(I) MeL
(ID RefluxecJ over
K^CO,
in acetone
- >
4 0 )
SCHEME I J l
Si
It v/as sought to be extended bj employing chloro-
methyl, methyl sulphide (;dcheme-IIl) but the results were
ambiguous and are s'sil under investigation,
.fC£CH S~CH 2 3
->
o. 0 "S-CH
(i) Mel (ii) OH
^ -> 40
ISCHEWEIII ]
Attempts -pre a lso iiao'e ro reverse the r e a c t i v i t y
of ortho Mcthoxv bcnz^ldehyde carbonyl bv converting i t to
1,3 d i t ' l l an e (^6) Tiirt a l ] owing th i s to react '-libh
-bromoacctoDhenonD. The reac t ion could take t\/o courses
65
leading to products which could serve as starting material
for either flavan or isoflavan (Scheme-IV) but this approach
to proved sterile and the reaction gave rise to a mixture.
U-. H
SH SH I
CH a: ( 4 6 )
CH ^
S s
o li
PhC-CH2"Br >
S ^ ^
Reduction >
OR
: SCI lEWE I T •
66
Since much syntheijic work has been done on rotenoid;
the literature on rotcnoid syntheois v;as surveyed to check if
any of the methods employed can be modified for isoflavanone
or isoflavan synthesis. The search focussed attention on a
paper published by Ollis "" as a short communication in 1960
in which the synthesis of the rotenoid aunduserone (4-8)
through condensation of the enol acetate (4?) with salicylal-
dehyde is reported. Piperidine was used as the basic catalyst
•OH AcO
H + ilMzi
ii] Reduction iiOOxidation
(47) (48)
and the reaction was conducted in EtOH at room temperature.
The paper gave a yield of SOfs in this reaction but the
details were never published. The reaction must proceed
through initial base catalysed hydrolysis of the enol acetate,
67
the enolate anion attacking the aldehyde group follovjed by
addition of the phenolic hydroxyl across the carbonyl
generated in the reaction. Since reduction involves hydro-
genolysis and saturation of double bond it should proceed in
good yield. This route appeared extermely interesting and
it was hoped that a similar condensation of the enol acetate
of phenyl acetaldehyde with salicylaldehyde can be affected.
Since details v./ere not available, efforts were made initially
to work oub the appropriate reaction conditions by condensing
the enol acetate of cyclohexanone, v/hich resembles (4-7) more
closely, with o-hydroxybonzaldehyde. It night be noted here
that if the cnamine replaces the enol acetate in this
41 reaction, the condensation has been used by Paquette m
the synthesis of xanthones. All possible variation were
tried but only traces of some products could be detected in
uAu xeauuxuii luixouLi:; axL-cr wvxiv up anu. mosuxy SLaruxng
materials v;ere recovered. As against this the corresponding
renction of enamine of cyclohexanone gave very good yields
A'-? _ of tne xanfchone , i-'aouetre nas also used enamines of
aldehydes and obtained good yields of isoflavone by this
method but this does not offer any marked advantage over
existing methods for isoflavone synthesis and has not been
68
explored from thin view point. Recently a novel s.ynthesis
of isoflavylium salt from pbenylacetaldehyde dimethyl acetal
and o-hydroxy benzaldehyde has been reported ^. The conden
sation v/as brought about v;ith anhydrous HOlO^ obtained
through adding 609i aq. HCIO^ to acetic anhydride. The reactioi
for which the adduced mechanism must be assumed was tried and
the reported yield was found to be reproducable. If the
Ph-CH -CH
^0CH3 H
\ ) CH3
Ph-CH~ CH z O-CH 2 3
,0H
\
O
/]
H CH-0 = CH-CH-Ph
3 2
f
5^
\.r \ H 19
Pn V-
•f
.0
H 0 -€ H
Vh
C£Q
69
reduction of isoriavjlium salt to isol'lavan could be afiected
in good yields this method could be developed as a general
isoflavan synthesis. The reduction was attempted in various
solvents e.g» methanol, diglyrae, acetic acid but led to
mixtures v/hich could not be satisfactorily resolved. In
tertiary butanol, hov/ever, the reduction gave only one major
product which had the same melting point as that of isoflavan.
Phenylacetaldehyde dimethylacetals are not readily accessible
compounds and if the procedure v/as to have general utility
an equivalent starting material would have to be employed.
It appeared of intrest to check if styrene oxide could be
substituted with phen3/lacetaldehyde dimethyl acebal. Ring
opening under acidic conditions should give the enol acetate
which can be expected to react in the same way as the enol
methyl ether interm.ediate in the original reaction. I'ais
turnedout to be the case and exposure of the styrene oxide
to the samic reaction conditions also afforded the isoflavyliun
salt in comparable ajiiour.ts. The poor result in hydrogenation
stop, however, did not encourage further investigation of
this approach for the desired G;,~nthe3is has to be superior
to method involving reduction of isoflavones.
70
The melting points were taken on Kofler block and
are uncorrected. Infrared spectra ;vere generally recorded on
'a Perkin Elmer 521 spectrometer. Ultraviolet spectra in 95/'
methanol solutions were measured on a Beckmann DK~2 spectro
meter. The solvent used for NMH spectra is indicated. The
spectra v/ere recorded on different instranents and the field
strength is indicated, THS was employed as internal reference.
•^Q> spectra were measured on Brucker \'ffl-90 spectrometer at
22.63 WHz in the pulsed mode. Mass spectra were taken on an
Atlas instrument.
Solvents used for chromatography v/ere purified by
standard methods wherever necessary.