From DEPARTMENT OF BIOSCIENCES AT NOVUM

Karolinska Institutet, Stockholm, Sweden

SYNTHESIS OF INDOLE AND OXINDOLE DERIVATIVES

INCORPORATING PYRROLIDINO, PYRROLO OR IMIDAZOLO

MOIETIES

Stanley Rehn

Stockholm 2004

All previously published papers have been reproduced with permission from the publishers. Published and printed by Karolinska University Press Box 200, SE-171 77 Stockholm, Sweden © Stanley Rehn, 2004 ISBN 91-7140-169-5

Till Amanda

iv

Abstract

The focus of this thesis is on the synthesis of oxindole- and indole-derivatives incorporating pyrrolidins, pyrroles or imidazoles moieties.

Pyrrolidino-2-spiro-3�-oxindole derivatives have been prepared in high yielding three-component reactions between isatin, α-amino acid derivatives, and suitable dipolarophiles. Condensation between isatin and an α-amino acid yielded a cyclic intermediate, an oxazolidinone, which decarboxylate to give a 1,3-dipolar species, an azomethine ylide, which have been reacted with several dipolarophiles such as N-benzylmaleimide and methyl acrylate. Both N-substituted and N-unsubstituted α-amino acids have been used as the amine component.

3-Methyleneoxindole acetic acid ethyl ester was reacted with p-toluenesulfonylmethyl isocyanide (TosMIC) under basic conditions which gave (in a high yield) a colourless product. Two possible structures could be deduced from the analytical data, a pyrroloquinolone and an isomeric ß-carboline. To clarify which one of the alternatives that was actually formed from the TosMIC reaction both the ß-carboline and the pyrroloquinolone were synthesised. The ß-carboline was obtained when 3-ethoxycarbonylmethyl-1H-indole-2-carboxylic acid ethyl ester was treated with a tosylimine. An alternative synthesis of the pyrroloquinolone was performed via a reduction of a 2,3,4-trisubstituted pyrrole obtained in turn by treatment of a vinyl sulfone with ethyl isocyanoacetate under basic conditions. This molecule (the pyrroloquinolone), obtained in a low yield by a multistep procedure, proved to be identical with the product obtained easily via the TosMIC route.

The reaction between 3-aminocrotonates and 3-acetonylideneoxindole in refluxing toluene resulted in 2-pyrrolo-3�-yloxindoles in high yields, (around 90 %). At room temperature the 2-pyrrolo-3�-yloxindoles exist as a mixture of keto-enol tautomers. Treatment with POCl3 yielded the corresponding 2-chloro-3-pyrrolyl indole, which gave a pyrrolo annulated indolopyrane upon basic hydrolysis of the ester function of the methyl ester.

3-Imidazolylindoles were synthesised in good yields from the corresponding benzylimine and TosMIC. Treatment of cyclohexanone benzylimine with α-chloroacrylonitrile yielded, after expulsion of HCN by refluxing in ethanol, 1-benzyl-4,5,6,7-tetrahydroindole. Formylation and benzylimine formation followed by treatment with TosMIC furnished the desired 2-imidazolyltetrahydroindole.

Keywords: isatin, three-component reaction, α-amino acid, azomethine ylide, pyrrolidino-3-spiro-3�-oxindole derivatives, 3-methyleneoxindole derivatives, pyrroloquinolone, TosMIC, β-carboline, tosylimine, pyrrole, keto-enol tautomerism, indolopyran-2-one, imidazole, benzylimine, tetrahydroindole.

v

List of publications The thesis is based on the following papers, referred to in the text by the Roman numerals I-IV:

I. The Three-Component Reaction between Isatin, α-Amino Acids, and Dipolarophiles. Rehn, S.; Bergman, J.; Stensland, B. Eur. J. Org. Chem, 2004, 413-418.

II. Synthesis of 4-oxo-4,5-dihydro-3H-pyrrolo[2,3-c]quinoline-1-carboxylic acid ethyl ester and its isomer 1-oxo-2,9-dihydro-1H-β-carboline-4-carboxylic acid ethyl ester. Bergman, J.; Rehn, S. Tetrahedron, 2002, 45, 9179-9185.

III. The reaction between 3-aminocrotonates and oxindole 3-ylidene derivatives: synthesis of highly substituted pyrroles. Rehn, S.; Bergman, J. Tetrahedron, accepted.

IV. Synthetic studies towards the alkaloid granulatimide: synthesis of 3-imidazolylindole and 2-imidazolyltetrahydroindole. Rehn, S.; Bergman, J. Manuscript

vi

Contents Abstract��.������...���������������������.iv List of papers����������������������������.v Contents���������.���������������������vi

1 Introduction to isatin chemistry .............................................................................1

1.1 Synthesis of isatin ..............................................................................................1 1.2 Fundamental reactivity of isatins.......................................................................2

1.2.1 Aromatic substitution.....................................................................................2 1.2.2 N-Alkylation and N-acylation .......................................................................3 1.2.3 Carbonyl reactions .........................................................................................3

2 Pyrrolidino-2-spiro-3�-oxindole..............................................................................9

2.1 Naturally occuring 3-spiro-oxindoles................................................................9 2.2 Ninhydrin and the Strecker degradation............................................................9 2.3 Azomethine ylides ...........................................................................................10

2.3.1 1,2-prototropic shift .....................................................................................10 2.3.2 Decarboxylative condensation ....................................................................11 2.3.3 Three-component reactions (paper I) ..........................................................12

3 Reactions on 3-methyleneoxindole derivatives ...................................................15

3.1 3-Methyleneoxindole acetic acid ethyl ester...................................................15 3.1.1 Isocyanides...................................................................................................16

3.2 Addition of TosMIC to 3-methyleneoxindole derivatives (paper II) .............16 3.2.1 Mechanistic aspects .....................................................................................17 3.2.2 β-Carbolines.................................................................................................18 3.2.3 Pyrroloquinolones........................................................................................20

3.3 3-(Pyrrol-4-yl)-oxindole (paper III) ................................................................22 3.3.1 Introduction..................................................................................................22 3.3.2 3-Aminocrotonates and 3-methyleneoxindole acetic acid ethyl ester........23 3.3.3 3-Aminocrotonates and 3-acetonylideneoxindole ......................................24 3.3.4 Chlorination of pyrrolo-oxindoles with POCl3 ...........................................26

4 Synthetic studies towards the alkaloid granulatimide (paper IV) ...................28

4.1 Introduction to imidazolyl indoles...................................................................28 4.2 3-(Imidazolyl)-indoles .....................................................................................29 4.3 2-(Imidazolyl)-tetrahydroindoles ....................................................................29

4.3.1 Published procedures to granulatimide .......................................................29 4.3.2 Retrosynthesis of granulatimide..................................................................30 4.3.3 Synthesis of 2-imidazolyltetrahydroindole .................................................31

5 Acknowledgements.................................................................................................33

6 Appendix: supplementary material .....................................................................34

6.1 Experimental part to section 4 .........................................................................34

7 Abbreviations..........................................................................................................38

1

1 Introduction to isatin chemistry

Isatin 1 (indole-2,3-dione)1 has been known since 1841 when Erdmann and Laurent prepared it by oxidation of indigo 2 by nitric and chromic acids. Although known as a synthetic molecule for almost 140 years, isatin was later found in nature, for instance in the fruits of the cannon ball tree, Couroupita quianensis Aubl.2 In man, isatin has been found to function as an endogenous monoamine oxidase inhibitor.3

NH

O

ONH

O HN

O1

234

5

67

1 2

Figure 1. Isatin 1 and indigo 2.

1.1 Synthesis of isatin

The importance of indigo as a possible synthetic dyestuff whithin the textile industry led to intense research in the area of indigo chemistry. As an offspring to the efforts in indigo research, the chemistry of isatin was explored, and several synthetic pathways to isatin were developed. The oldest and the most important method of synthesising isatin is the Sandmeyer methodology that starts from an aniline 3, which reacts with chloral hydrate and hydroxylamine hydrochloride in water containing sodium sulfate to form an isonitrosoacetanilide 4. The isolated isonitrosoanilide 4 is then treated with concentrated sulfuric acid to yield the isatin 5.

NH2 NH

O

NOHa b

NH

O

Oc

3 4 5

R R R

Scheme 1. The Sandmeyer synthesis. a) Cl3CCH(OH)2, H2NOH·HCl, Na2SO4. b) H2SO4. c) H2O.

Second to Sandmeyer�s procedure of isatin synthesis is the method of Stollé whereby the aniline (usually as its hydrochloride) is reacted with oxalyl chloride to form an intermediate, chlorooxalyl anilide, which in turn can be cyclised in the presence of a Lewis acid such as aluminium chloride to the isatin. A recent (but rarely used) aproach (Scheme 2) was described by Gassman et al.4 in the late seventies. From a suitable substituted aniline 3, a 3-methylthio-2-oxindole 8 was synthesised and was subsequently 3-chlorinated with NCS and finally hydrolysed to the isatin 5. The reaction proceeds via an azasulfonium salt followed by an abstraction of a 1 (a) Sumpter, W. C. Chem. Rev. 1944, 34, 393-434. (b) Popp, F. D. Adv. Heterocycl. Chem. 1975, 18,

1-58. (c) da Silva, J. F. M., Garden, S. J., Pinto, A. C. J. Braz. Chem. Soc. 2001, 12, 273-324. 2 Bergman, J., Lindström, J-O., Tilstam, U. Tetrahedron 1985, 41, 2879-2881. 3 Hamaue, N. Yakugaku Zasshi 2000, 120, 352-362. 4 Gassman, P. G.; Cue Jr, B. W.; Luh, T-Y. J. Org. Chem. 1977, 42, 1344-1348.

2

proton, to give a sulfur ylide 7, which underwent a Sommelet-Hauser type rearrangement, and after ring closure yields the 3-methylthio-2-oxindole 8.5

NH2

R

SMe

CO2EtCl

Cl

NH

XSMe

CO2Et

Et3N

RNH

O

SMe

RNH

O

O(1) NCS(2) HgO/BF3

5

3

6

7

8

Scheme 2. The Gassman synthesis.

1.2 Fundamental reactivity of isatins

Isatin will mainly react at three different sites, namely aromatic substitution at C-5, N-alkylation, and carbonyl reactions at C-3. If the system carry electron-withdrawing groups in the benzene ring or at the nitrogen attack at C-2 can also occur.1c

NO

OE

R X

Nuc

H

Figure 2. Reactivity of isatin.

1.2.1 Aromatic substitution

Nitration of isatin yields 5-nitroisatin where the reaction proceeds smoothly at 0 ûC6 but the temperature needs to be controlled precisely7 or else the nitration will give rise to several nitrated products8. Nevertheless, 5,7-dinitroisatin can be synthesised by merely heating 3,3,5,7-tetranitrooxindole, which in turn can be obtained from the nitration of oxindole.9 When bromine was added to a solution of

5 (a) Gassman, P. G.; van Bergen, T. J. J. Am. Chem. Soc. 1974, 96, 5508-5512. (b) Gassman, P. G.;

Gruetzmacher, G.; van Bergen, T. J. J. Am. Chem. Soc. 1974, 96, 5512-5517. 6 Calvery, H. O.; Noller, C. R.; Adams, R. J. Am. Chem. Soc. 1925, 47, 3058-3060. 7 Daisley, R. W.; Shah, V. K. J. Pharm. Sci. 1984, 73, 407-408. 8 Mazhilis, L. I.; Terent�ev, P. B.; Boltin, V.A. Chem. Heterocycl. Compd. (Engl. Transl.) 1989, 25,

50-54. 9 Bergman, J.; Bergman, S.; Brimert, T. Tetrahedron 1999, 55, 10447-10466.

3

isatin in ethanol dibromination occurred to yield 5,7-dibromoisatin in good yield.10 Nevertheless mono bromination at C-5 has been achieved by treatment of isatin with N-bromosuccinimide, while 5-chloroisatin was obtained using N-chlorosuccinimide.11 Recently, mono iodination of isatin yielding 5-iodoisatin was achieved by using an aqueous potassium dichloroiodate (KICl2) as the iodinating agent.12

1.2.2 N-Alkylation and N-acylation

Commonly, N-alkylation of isatin proceeds via the sodium salt of isatin, which is reacted with appropriate alkyl halide or alkyl sulfonate. Methylation can, nevertheless, be achieved with other reagents, for example potassium tert-butoxide and dimethyl oxalate (60% yield)13. However, normal alkylation conditions such as dimethyl sulfate in ethanolic potassium hydroxide (80% yield)14 or benzylation with sodium hydride and benzyl bromide (99% yield)15 give better yields.

N-Acetylation was performed by heating isatin in acetic anhydride for a couple of hours,16 although a more recent procedure (sodium acetate and isatin were heated shortly in acetic anhydride) has been published.17 Protecting amines as N-carbamates is a commonly adopted strategy, which has also been applied to isatin. N-Boc isatin18 and N-CBz isatin19 have been synthesized in 89% yield and 85% yield, respectively.

1.2.3 Carbonyl reactions

All ketones, as well as, the C-3 carbonyl of isatin are susceptible towards nucleophiles. Ketalisation serves this perfectly, as a good example of nucleophilic attack on the carbonyl functionality. Thus employing ethylene glycol20, 1,2-ethanedithiol21 or 2-mercapto ethanol20a on isatin yields different spiro ketals of oxindole. Though the example above considers heteroatom dinucleophiles, carbon nucleophiles do also react at C-3. Grignard reagents also attack at C-3 and yield the 3-hydroxy-3-substituted oxindoles, which readily can be reduced to 3-substituted indoles.22

When isatins are exposed to a weak base and a reagent with an active methylene, 3-substituted-3-hydroxy oxindoles are formed. The tertiary alcohol can easily be dehydrated under acidic conditions to yield 3-methyleneoxindole derivatives. One

10 Lindwall, H. G.; Bandes, J.; Weinberg, L. J. Am. Chem. Soc. 1931, 53, 317-318. 11 Buu-Hoi, N. P. Rec. Trav. Chim. 1954, 73, 197-202. 12 Garden, S. J.; Torres, J. C.; de Souza Melo, S. C.; Lima, A. S.; Pinto, A. C.; Lima, E. L. S.

Tetrahedron Lett. 2001, 42, 2089�2092. 13 Bergman. J.; Norrby, P-O.; Sand, P. Tetrahedron 1990, 46, 6113-6124. 14 Harley-Mason, J.; Ingleby, R. F. J. J. Chem. Soc. 1958, 3639-3642. 15 Overman, L. E.; Peterson, E. A. Tetrahedron 2003, 59, 6905-6619. 16 Suida, W. Chem. Ber. 1878, 11, 584-590. 17 Somogyi, L. Bull. Chem. Soc. Jpn. 2001, 74, 873-881. 18 Wille, G.; Steglich, W. Synthesis 2001, 759-762. 19 Yamagishi, M.; Yamada, Y.; Ozaki, K.; Tani, J.; Suzuki, M. Chem. Pharm. Bull. 1991, 39, 626-629. 20 (a) Rajopadhye, M.; Popp, F. D. J. Med. Chem. 1988, 31, 1001-1005. (b) Cliffe, I. A.; Lien, E. L.;

Mansell, H. L.; Steiner, K. E.; Todd, R. S.; White, A. C.; Black, R. M. J. Med. Chem. 1992, 35, 1169-1175.

21 (a ) Baker, J. T.; Duke, C. C. Aust. J. Chem. 1972, 25, 2467-2475. (b) Wenkert. E.; Bringi, N. V.; Choulett, H. E. Acta Chem. Scand. 1982, B36, 348-350.

22 Bergman, J. Acta Chem. Scand. 1971, B25, 1277-1280.

4

example of this reaction is described in Scheme 3. Condensation between isatin 1 and acetone 9 yields initially 3-acetonyl-3-hydroxy oxindole 10, which can eventually be dehydrated to 3-acetonylideneoxindole 11.23 Utilising this Knoevenagel condensation strategy other products between isatin and ketones were obtained and these condensation products (e.g. 3-hydroxy-3-nitromethyloxindole)24b were further manipulated to yield different oxindole derivatives.24 One other example is the preparation of the α, β unsaturated ketone 3-phenacylideneoxindole from isatin and acetophenone via 3-hydroxy-3-phenacyloxindole.25

NH

O

O

NH

O

HOO

Me+Et2NH

MeO

Me NH

O

MeO

H

1 9 10 11

Scheme 3. Condensation between isatin and acetone.

The preparation of 3-methyleneoxindole acetic acid ethyl ester 12 was first described in 1953 in a three step procedure involving a condensation between oxindole 13 and diethyl oxalate to yield the stable enol 14 (Scheme 4).26 Catalytic hydrogenation gave oxindole 15, which were followed by dehydration to yield the α, β unsaturated ethyl ester 12. However α, β unsaturated esters are more readily available via a Wittig type of reaction involving a phosphorus ylide. Thus isatin was heated together with ethoxycarbonylmethylenetriphenylphosphorane in glacial acetic acid and after a couple of hours the product, 3-methyleneoxindole acetic acid ethyl ester, was obtained in 69 % yield.27 A couple of years later Franke published a synthesis wherein he used a modified Wittig reagent (triethyl phosphonoacetate 16) together with isatin 1. This readily performed Horner-Wadsworth-Emmons reaction gave 3-methyleneoxindole acetic acid ethyl ester 12 in 65 % yield.28

23 (a) Braude, F.; Lindwall, H. G. J. Am. Chem. Soc. 1933, 55, 327-327. (b) Garden. S. J.; da Silva, R.

B.; Pinto, A. C. Tetrahedron, 2002, 58, 8399-8412. 24 (a) Pietra, S.; Tacconi, G. Farmaco 1958, 13, 893-910. (b) Tacconi, G.; Pietra, S. Farmaco 1963, 18,

409-423. (c) Tacconi, G. Gazz. Chim. Ital. 1968, 98, 344-357. 25 Lindwall, H. G.; Maclennan, J. S. J. Am. Chem. Soc. 1932, 54, 4739-4744. 26 Julian, P. L.; Printy, H. C.; Ketcham, R.; Doone, R. J. Am. Chem. Soc. 1953, 75, 5305-5309. 27 Brandman, H. A. J. Heterocyclic Chem., 1973, 10, 383-384. 28 Franke A. Liebigs Ann. Chem. 1978, 717-725.

5

NH

O

O PO

OEtOEt

EtO2C+d

NH

O

EtO2C

NH

Oa

NH

O

HOCO2Et

b

NH

O

HOCO2Et

c

1 16

13 14 15

12

Scheme 4. Synthesis of 3-methyleneoxindole acetic acid ethyl ester. a) (CO2Et)2, NaOEt, EtOH. b) H2, 10 % Pd-C, EtOH. c) H2SO4 (cat), AcOH. d) NaOEt, DMF.

Reacting isatin with hydroxylamine or hydrazine derivatives give rise to the expected condensed products. However when utilising amines a variety of products can be obtained.29 Treatment of isatin with ammonia as shown in Scheme 5, gave rise to a mixture of products, there among isamic acid 19 and isamide 20. The structures of these two products were elucidated in 1976.30 Some years earlier a short comunication31 presented the structure of isamic acid 19 and a year later the crystal structure of the p-bromophenacyl isamate32 was published. It is believed that the 3-imino isatin attacks a second molecule of isatin to yield a dimeric structure 17 whereupon the second isatin moiety is cleaved and further, via lactamization, gives rise to a spiro oxazolidinone oxindole 18. Opening and re-closure of the spiro oxindole forms the isamic acid 19. Reaction with a second equivalent of ammonia yields the isamide 20.

29 Bergman, J.; Stålhandske, C.; Vallberg, H. Acta Chem. Scand. 1997, 51, 753-759. 30 Cornforth, J. W. J. Chem. Soc. Perkin Trans. 1 1976, 2004-2009. 31 Field, G. F. Chem. Commun. 1969, 886. 32 Blount, J. F. Chem. Commun. 1970, 432.

6

NH

NHN

O

ORO

NH

O

ONH3

NH

ON

HO

HN

O

NH

NO

O

NH2

O

1 17

18 19 : R=H20 : R=NH2

Scheme 5. R = H, Isamic acid. R = NH2, isamide.

A more straightforward reaction is the condensation between isatin and aromatic amines that usually yield the anils, as outlined in Scheme 6, where isatin 1 is condensed with p-anisidine to yield the anil 21. These anils can be used in synthesis of other oxindole containing systems. A recent example is the synthesis of 3-spiro β-lactam oxindole 23 via the 3,3-disubstituted oxindole 22.33 Compound 22 was obtained when anil 21 was treated with ketene silyl acetal of ethyl acetate in the presence of a Lewis acid. A diverse pool of primary amines such as n-butylamine will also give rise to imines together with isatin, and this particular imine was one part in a recent Staudinger ketene-imine cycloaddition yielding a 3-spiro β-lactam oxindole.34

NH

O

Oa

NH

N

O

OMe

b

NH

O

NHPMP

CO2Et

NH

O

NH

O1 21

22 23

Scheme 6. a) p-anisidine, EtOH. b) CH2=COEt(OTMS), BF3·OEt2, DCM.

33 Nishikawa, T.; Kajii, S.; Isobe, M. Chem. Lett. 2004, 33, 440-441. 34 Lin, X.; Weinreb, S. M. Tetrahedron Lett. 2001, 42, 2631-3633.

7

Introduction of a second nucleophilic group besides the amino functionallity, offers the possibility to form several products. Depicted in Scheme 7 is the reaction between an isatin 24 and 3-amino propanol. Depending on the reaction conditions, three different products can be obtained: the ring annulated product 25, the 3-spiro oxindole 26 or the condensation product 27.35 Compounds like o-phenylenediamine can also give rise to three different products depending on the reaction conditions, thus for example 2-aminobenzylamine (the higher homologue of o-phenylene-diamine) yielded only the spiro compound 28 when stirred at room temperature in methanol as shown in Scheme 7.36

NH

O

OX

NH

NHO

OX

NH

O

NX

(CH2)2OH

N

XO

Nb a

c

d

NH

NHHN

OX

24 2526

27

28

Scheme 7. a) X = F. 2 eq. H2N(CH2)3OH, EtOH, cat AcOH, 40-45 ºC, 3h (40%). b) X = F. 2 eq. H2N(CH2)3OH, EtOH, cat AcOH, reflux temperature, 6h (70%). c) X = F. 2 eq. H2N(CH2)3OH, EtOH,

cat AcOH, reflux temperature, 25 h (70%). d) X = H. 1 eq. 2-aminobenzylamine, methanol, rt., 48h (67%).

35 Dandia, A.; Sati, M.; Sanan, S.; Joshi, R. Org. Prep. Proced. Int. 2003, 35, 433-438. 36 Bergman, J.; Engqvist, R.; Stålhandske, C.; Wallberg, H. Tetrahedron 2003, 59, 1033-1048.

8

A good example of the reactivity of isatin can be found in the synthesis37 of the mitosane core (-) 29 (Scheme 8), which forms the core of the bifunctional DNA alkylating mitomycin C.38

N

O

NH

OMe

(-) 29

N NH

OMe

O

OH2N

Me

ONH2

O

mitomycin C

Figure 3. Mitosane core (-) 29 and mitomycin C.

The reaction sequence started with the protection of the benzylic carbonyl of isatin 1 as a ketal to yield the spirodioxolane oxindole 30, which is a well-established reaction . Secondly, protection of the nitrogen with (Boc)2O (DMAP, Et3N/DCM), yielded compound 31. Saponification of the Boc-protected isatin ketal 31 gave the acid 32, which was treated with allyl bromide to yield the diene 33.

NH

O

O a

NH

OO

O

NO

OO

Boc

b

NH

OOCO2H

Boc

c

N

OO

Boc

O

O

N

OO

Boc

O

N

OO

O

Boc

N

OOMe

NH

d

RCM

29

1 30 31

32 33

34 35

Scheme 8. a) ethylene glycol, p-TsOH, PhH, reflux; b) (Boc)2O, DMAP, Et3N, DCM, rt; c) NaOH, THF/H2O, reflux; d) allyl bromide, NaH, DMF, 0 °C to rt.

Further transformations gave the diene 34, which was subjected to a ring closing metathesis yielding the benzoazocin-5-one 35. Additional chemical transformations led to the tricyclic mitosane core 29 via a transannular cyclization.

37 (a) Papaioannou, N.; Evans, C. A.; Blank, J. T.; Miller, S. J. Org. Lett. 2001, 3, 2879-2882. (b)

Papaioannou, N.; Blank, J. T.; Miller, S. J. J. Org. Chem. 2003, 68, 2728-2754. 38 Tomasz, M.; Palom, Y. Pharmacol. Therapeut. 1997, 76, 73-87

9

2 Pyrrolidino-2-spiro-3�-oxindole

2.1 Naturally occuring 3-spiro-oxindoles

A number of naturally occurring pharmacologically active alkaloids have been recognised to include the structural motif of pyrrolidino-3-spiro-3�-oxindoles.39 Examples (Figure 4) include the fairly simple (-)-horsfiline 3640 which has been isolated from H. superba (a small tree indigenous to Malaysia) and the more structurally complex spirotryprostatin B 3741 which exhibits cell cycle inhibition. In addition to the naturally occurring pyrrolidino-3-spiro-3�-oxindoles synthetic pyrrolidino- and piperidino-spiro-3�-oxindoles have been shown to exhibit local anaesthetic properties42. Just recently it was also shown that 3,3-diaryloxindoles could act as Ca2+-depleting translation initiation inhibitors.43

NH

N

MeOO

NHN N

O

O

O

3736

Me

MeMe

H

Figure 4. (-)-Horsfiline 36 and spirotryprostatin B 37.

2.2 Ninhydrin and the Strecker degradation

Ninhydrin 38 is utilised for the chemical development of latent fingerprints, since it works as a powerful indicator for α-amino acids.44 Fewer are aware of the chemistry that is taking place when α-amino acids are detected by ninhydrin 38.45 The chemistry of ninhydrin 38 and related compounds such as alloxan 39 (Figure 5) and isatin can be dated back to 1862 when Strecker observed that alloxan 39 reacts with alanine to give carbon dioxide and acetaldehyde.46 Hence the Strecker degradation was discovered and was further investigated in 1948 by Schönberg et al.47

39 (a) Marti. C.; Carreira, E. M. Eur. J. Org. Chem. 2003, 2209-2219. (b) Joshi, K. C.; Jain, R.; Ghand,

P. Heterocycles 1985, 23, 957-996. 40 Jossang, A.; Jossang, P.; Hadi, H. A.; Sevenet, T.; Bodo, B. J. Org. Chem. 1991, 56, 6527-6530. 41 (a) Cui, C. B.; Kakeya, H.; Osada, H. J Antibiot. 1996, 49, 832-835. (b) Meyers, C.; Carreira, E. M.

Angew. Chem. Int. Ed. 2003, 42, 694-696, and references 2-6 therein. 42 Kornet, M. J.; Thio, A. P. J. Med. Chem. 1976, 19, 892-898. 43 Natarajan, A.; Fan, Y-H.; Chen, H.; Guo, Y.; Iysere, J.; Harbinski, F.; Christ, W. J.; Aktas, H.;

Halperin, J. A. J. Med. Chem. 2004, 47, 1882-1885. 44 Joullié, M. M.; Thompson, T. R.; Nemeroff, N. Tetrahedron 1991, 47, 8791-8830. 45 Petrovskaia, O.; Taylor, B. M.; Hauze, D. B.; Carroll, P.; Joullié, M. M. J. Org. Chem. 2001, 66,

7666-7675. 46 Schönberg, A.; Moubasher, R. Chem. Rev. 1952, 50, 261-277. 47 (a) Schönberg, A.; Moubasher, R.; Mostafa, A. J. Chem. Soc. 1948, 176-182. (b) Schönberg, A.;

Moubasher, R. J. Chem. Soc. 1950, 1422.

10

HN

HNO O

OO

O

O

OHOH

3938 Figure 5. Ninhydrin 38 and alloxan 39.

The mechanism of the Strecker degradation is exemplified in Scheme 9 whereby alanine 40 is condensed with ninhydrin 38 to yield an azomethine carboxylic acid 41, which will undergo decarboxylation. Hydrolysis of the imine 43 yields the amine 44 which is condensed with an additional ninhydrin molecule and finally, the end product Ruhemann´s purple 45 containing the nitrogen atom from the alanine 40, is formed. The remnant of the α-amino acid, the aldehyde, contains one carbon and one nitrogen less as compared with to the initial α-amino acid.

H2NO

MeO

-CO2

Me CHO

-H2O

H2O, H

O

+

O

OHOH

O

O

N

Me

O

O

O

O

NMe

O

O

N

O

O

O

O

H2O

HN

Me

O

O

HNH2

+ 38

H

4138 40

42 43

44 45 Scheme 9. Strecker degradation between ninhydrin and alanine.

2.3 Azomethine ylides

2.3.1 1,2-prototropic shift

When ninhydrin was treated with various α-amino acids together with a dipolarophile (e.g. N-phenylmaleimide) formation of cycloadducts was observed.48 Thus, there exists a relationship between the Strecker degradation and the formation of azomethine ylides. Schiff bases (imines from aldehydes) bearing an electron withdrawing group at the α position will generate a stabilised azomethine ylide by a prototropic shift when heated in toluene.49 These stabilised azomethine ylides can 48 Grigg, R.; Malone, J. F.; Mongkolaussavaratana, T.; Thianpatanagul, S. Tetrahedron 1989, 45,

3849-3862. 49 (a) Amornraksa, K.; Grigg, R. Gunaratne, H. Q. N.; Kemp, J.; Sridharan, V. J. Chem. Soc. Perkin

Trans. 1 1987, 2285- 2296. (b) Tsuge. O.; Kanemasa, S.; Ohe, M.; Yorozu, K.; Takenaka, S.; Ueno, K. Bull. Chem. Soc. Jpn. 1987, 60, 4067-4078.

11

then undergo cycloaddition reactions to yield pyrrolidines. Although there are several routes to azomethine ylides such as the prototopic generation (discussed above) or desilylation of N-silylmethyl amine reagents,50 I prefer to use the decarboxylative condensation route due to: 1) The readily accessible pole of α-amino acids which are cheap and available in large quantities. 2) The reaction can be performed by a simple procedure and under neutral conditions. 3) The cycloaddition takes place with a variety of dipolarophiles.

2.3.2 Decarboxylative condensation

In 1970 Rizzi reported evidence for a thermally generated nonstabilised azomethine ylide51 intermediate from the decarboxylative condensation between sarcosine and benzophenone.52 This way of generating the 1,3-dipolar azomethine ylide is believed to proceed via initial formation of an oxazolidinone, which upon heating will eliminate carbon dioxide. Tsuge et al.53 reacted N-trityl glycine 46 with formaldehyde 47 to yield the precursor to the N-triphenylmethyl azomethine ylide 49, i.e. the 3-(triphenylmethyl)-5-oxazolidinone 48 in almost quantatively yield. Heating this trityloxazolidinon 48 resulted in expulsion of carbon dioxide and formation of an azomethine ylide 49, which underwent a cycloaddition reaction with the N-(p-tolyl)-maleimide 50 present to yield the bicyclic compound 51.

NH

TrtCO2H + HCHO(aq)

O

NTrt

O

heat

-CO2

TrtN N

O

O

p-Tol N

O

O

p-TolNTrt+

46 47 48

49 50 51 Scheme 10. Cycloaddition reaction between trityl-5-oxazolidinone 48 and N-(p-tolyl)-maleimide 50.

Since the creation of pyrrolidines is readily achieved via the azomethine ylide and a dipolarophile, the three-component reaction between α-amino acids, isatin and dipolarophile would be expected to lead to 3-spiro-(pyrrolidino)-oxindoles. In 1990 Grigg et al.54 showed that when isatin was condensed with benzylamine, a 1,2-prototropy resulted in an azomethine ylide which was further reacted with methyl acrylate in a cycloaddition reaction. A year later the decarboxylative condensation

50 (a) Padwa, A.; Chen, Y-Y. Tetrahedron Lett. 1983, 24, 3447-3450. (b) Vedejs, E.; West, F, G.

Chem. Rev. 1986, 86, 941-955. 51 Vedejs, E. In Nonstabilized azomethine ylides; Curran, D. P., Ed.; Advances in cycloaddition; Vol 1.

Jai; London, 1988; pp 33-51. 52 Rizzi, G. P. J. Org. Chem. 1970, 35, 2069-2072. 53 Tsuge, O.; Kanemasa, S.; Ohe, M.; Takenaka, S. Bull. Chem. Soc. Jpn. 1987, 60, 4079-4089. 54 Ardill, H.; Dorrity, M. J. R.; Grigg, R.; Leon-Ling, M�S.; Malone, J. F.; Sridharan, V.;

Thianpatanagul, S. Tetrahedron 1990, 46, 6433-6488.

12

between isatin and secondary α-amino acids to give the azomethine ylide, which in turn was reacted with methyl acrylate, was published.55

2.3.3 Three-component reactions (paper I)

Since the major part of the decarboxylative condensation route to pyrrolidino-2-spiro-3�-oxindole derivatives involves cyclic α-amino acids, a study involving acyclic α-amino acids was initiated. This study included three acyclic N-substituted α-amino acids, three N-unsubstituted α-amino acids and two cyclic α-amino acids. N-Benzylmaleimide was chosen as the dipolarophile, both N-phenylmaleimide as well as unsymmetrical dipolarophiles were also used.

Isatin 1 was reacted with the N-substituted acyclic α-amino acids sarcosine (N-methyl glycine) 53a , N-benzylglycine 53b and N-methylalanine 53c in a methanol/water medium at 90 ºC in the presence of N-benzylmaleimide 53a. The tetracyclic spiro compounds 55 were obtained in good yields.

NH

O

O NH

R2

CO2H+

NH

N

R4

R3 R2

R1

O

a H

H

R1 + R3R4

1 53 54 55 Scheme 11. a) MeOH:H2O (3:1), 90 ºC.

Obviously the amide hydrogen present in isatin did not disturb the reaction since the use of N-methylated isatin did not alter the course of the reaction.

Table 1. Products obtained by the three-component reaction between α-amino acids 53, isatin 1 and dipolarophiles 54.

Entry Compound R1 R2 R3 R4 Time (h) Yield (%)

1 55aa Me H -C(O)NBnC(O)- 18 92

2 N-Me 55aa Me H -C(O)NBnC(O)- 18 82

3 55ba Bn H -C(O)NBnC(O)- 18 77

4 55ca Me Me -C(O)NBnC(O)- 18 79

5 55da H CH(CH3)2 -C(O)NBnC(O)- 2 94

6 55ea H Me -C(O)NBnC(O)- 18 95

7 55fa H H -C(O)NBnC(O)- 2 39

8 55ga -CH2SCH2- -C(O)NBnC(O)- 2 95

9 55ha -CH2CH2CH2- -C(O)NPhC(O)- 0.5 87

10 55bc Bn H CO2Me H 18 38

11 55gc -CH2SCH2- CO2Me H 2 69

Good single crystals were obtained when adduct 55ba was recrystallized from acetonitrile, and an X-ray structure (Figure 6) could be obtained from these crystals.

55 Coulter, T.; Grigg, R.; Malone, J. F.; Sridharan, V. Tetrahedron Lett. 1991, 32, 5417-5420.

13

The stereochemical outcome of the cycloaddition was apparent from the X-ray structure. Also observable is the aromatic T-stacking involving the phenyl ring from the former N-benzylmaleimide and H15 on carbon 4 in the oxindole ring (i.e., C15 in Figure 6). This T-stacking results in a shift upfield (δ = 6.17 ppm for H15 in compound 55ba) compared to compound 55bc (δ = 6.91 ppm) obtained from isatin 1, N-benzylglycine 53b and methyl acrylate 54c. On the other hand when N-phenylmaleimide 54b was incorporated into the structure, an upfield shift was also noticed (δ = 6.73 ppm for compound 55hb) but due to the lacking methylene the influence is smaller.

Figure 6. Molecular structure of 55ba showing the atom numbering scheme. An acetonitrile solvate

molecule is excluded.

As discussed earlier, azomethine ylides have been prepared by condensation between isatin and an unsubstituted amine followed by a 1,2-prototropy.54 Nevertheless, the use of an N-unsubstituted α-amino acid accompanied by isatin with the intention to prepare an azomethine ylide via the decarboxylative condensation route has gained scarce attention.56 The tetracyclic pyrrolidino-3-spiro-3�-oxindole derivatives 55da and 55ea were prepared from the decarboxylative condensation between isatin 1 and the α-amino acids valine 53d and alanine 53e together with and N-benzylmaleimide 54a. The yield of these reactions were within the same range of yields as for the N-substituted α-amino acids. Even glycine 53f yielded a pyrrolidino-3-spiro-3�-oxindole derivative 55fa albeit in a low yield (39 %). A competing reaction path involving 1,2-prototropy may be the reason.

In 2001 Azizian et al.57 published a study, wherein the reaction between isatin 1, proline 53h and N-arylmaleimides in refluxing ethanol yielded pentacyclic pyrrolidino-3-spiro-3�-oxindoles derivatives 55. Utilising the conditions in Scheme 11 with isatin 1, proline 53h and N-phenylmaleimide 54b gave the pyrrolidino-3-spiro-3�-oxindole derivative 55hb in 87% yield. Also thiazolidine-4-carboxylic acid 56 (a) Fokas, D.; Ryan .W. J.; Casebier, D. S.; Coffen, D. L. Tetrahedron Lett. 1998, 39, 2235-2238. (b)

El-Ahl, A�A. S. Heteroatom Chem. 2002, 13, 324-329. 57 Azizian, J.; Asadi, A.; Jadidi, K. Synth. Commun. 2001, 31, 2727-2733.

14

53g will give rise to pentacyclic oxindole derivatives of the type 55ga (Scheme 11) when the decarboxylative condensation taken place between isatin 1 and the imino acid 53g, followed by a reaction with N-benzylmaleimide 54a as the dipolarophile. The decarboxylative condensation between thiazolidine-4-carboxylic acid 53g and proline 53h together with isatin 1 to yield the azomethine ylide have been subjected to profound studies, both synthetically and theoretically58, therefore there is good evidence for the mechanistic course and the stereochemical outcome (Scheme 12).

NH

O

O + -H2O

NH

N

O

SO

O

NH

ON

S O

O

-CO2

NH

O

N

SNO O

Bn

NH

NS

NBnO

O

O

H

H

S

HNCO2HH

H H

H

1 53g 56 57

58

54a

55ga Scheme 12. Decarboxylative condensation path to azomethine ylide.

Interestingly, the reactions involving proline 53h, thiazolidine-4-carboxylic acid 53g and valine 53d proceeded approximately 10 times faster, as compared with e.g. sarcosine 53a. The reason for this difference is perhaps due to the fact that the ring or the extra bulkiness pushes the equilibrium towards the spiro oxazolidinone 57, and that these features also will stabilise the azomethine ylide 58. This geometric feature will induce the reactions to proceed faster by promoting the formation of the spiro oxazolidinone 57, which I think is the species that will eliminate carbon dioxide to give the reactive azomethine ylide 58. There is, nevertheless, also a possibility that compound 56 is decarboxylated to yield the azomethine ylide 58.

58 (a) Pardasani, R. T.; Pardasani, P.; Ghosh, R.; Sherry, D.; Mukherjee, T. Heteroatom Chem. 1999,

10, 381-384. (b) Pardasani, P.; Pardasani, R. T.; Sherry, D.; Chaturvedi, V. Synth. Commun. 2002, 32, 435-441. (c) Pardasani, R. T.; Pardasani, P.; Yadav, S. K.; Bharatam, P. V. J. Heterocyclic Chem., 2003, 40, 557-563. (d) Pardasani, R. T.; Pardasani, P.; Chaturvedi. V.; Yadav. S. K.; Saxena, A.; Sharma, I. Heteroatom Chem. 2003, 14, 36-41.

15

3 Reactions on 3-methyleneoxindole derivatives

3.1 3-Methyleneoxindole acetic acid ethyl ester

In the pursue of the synthesis of pyrrolidino-3-spiro-3�-oxindoles I wanted to explore the use of the easily available 3-methyleneoxindole derivatives as starting materials. Spiro-oxindoles have already been prepared from 3-methyleneoxindoles. Franke treated the 3-methyleneoxindole acetic acid ethyl ester 12 with diazomethane and isolated the 3-spiro-pyrazolyl-oxindole 59, which upon heating expelled nitrogen to yield 3-spiro-cyclopropano-oxindole 60 (Figure 7).28 Diels-Alder reactions between 3-methyleneoxindole derivatives and dienes yielded as expected 3-spiro-(cyclohexene-oxindoles) 61.59

NH

NNEtO2C

ONH

EtO2C

ONH

O

X

59 60 61

Y R2

R1

Figure 7. Spiro adducts from 3-methyleneoxindole acetic acid ethyl ester.

Worth mentioning in this context is the first step in an exquisite asymmetric total synthesis of spirotryprostatin B wherein Williams and co-workers treated 3-methyleneoxindole acetic acid ethyl ester 12 with an azomethine ylide generated from aldehyde 63 and morpholine derivative 62.60 A similar type of 1,3-dipolar cycloaddition to the oxindole ethyl ester 12 was performed with the azomethine ylide generated from N-benzylglycine and isatin via the decarboxylative condensation route.61

NH

O

EtO2C HNO

O

PhPh

OHCMe

MeOMe

toluene,mol. sieves

N

HN CO2Et

O

H O

PhPh

OMe

MeMeO

12

62

6364

Scheme 13. The first 1,3-dipolar step in the synthesis towards spirotryprostatin B.

59 (a) Kato, T.; Yamanaka, H.; Ichikawa, H. Chem. Pharm. Bull. 1969, 17(b) Richards, C. G.;

Thurston, D. E. Tetrahedron 1983, 39, 1817-1821. (b) Wenkert, E.; Liu, S. Synthesis 1992, 323-327.

60 Sebahar. P. R.; Williams, R. M. J. Am. Chem. Soc. 2000, 122, 5666-5667. 61 Rehn, S.; Bergman, J.; Stensland, B. Eur. J. Org. Chem. 2004, 413-418.

16

3.1.1 Isocyanides

Isocyanides have received a lot of attention, especially in the synthesis of various nitrogen containing heterocycles.62 One of the isocyanide reagents, TosMIC (p-tosylmethylisocyanide), was �discovered� in 1972 by van Leusen et al.63 In the years that followed he showed that TosMIC is a versatile reagent for preparations of 5-membered nitrogen containing heterocycles such as oxazoles64, imidazoles65, thiazoles66 and pyrroles67.

3.2 Addition of TosMIC to 3-methyleneoxindole derivatives (paper II)

A reaction whereby 3-methyleneoxindole acetic acid ethyl ester 12 was treated with TosMIC 65 under basic conditions was performed. The idea was to synthesise a 3-spiro-oxindole derivative. From this reaction a high melting (over 300 °C) white solid was obtained with the elemental composition C14H12N2O3 in 74% yield. Analysis of the 1H NMR spectrum revealed apart from a 1,2-disubstituted benzene ring and an ethoxycarbonyl group three signals (two NH and one aromatic CH). From this information, together with the information obtained from the 13C NMR and IR, two possible structures were suggested, namely the β-carboline 66 and the pyrroloquinolone 67.

NH

O

12

OR

a

NH

NH

OR

O

orNH

NH

OR

O

66 67

R Yield (%)OEt 74OMe 52Ph 79

N

Tos

C

65 Scheme 14. a) KOtBu, THF, reflux 0.5h.

When the same conditions were applied to the corresponding methyl ester, a compound of the same kind was obtained although in lower yield (52%). Also 3-phenacylideneoxindole25 yielded a product (79%) where a benzoyl group replaced the ester functionality.

62 Marcaccini, S.; Torroba, T. Org. Prep. Proced. Int. 1993, 25, 141-208. 63 van Leusen, A. M.; Boerma, G. J. M.; Helmholdt, R. B.; Siderius, H.; Strating, J. Tetrahedron Lett.

1972, 13, 2367-2368. 64 van Leusen, A. M.; Hoogenboom, B. E.; Siderius, H. Tetrahedron Lett. 1972, 13, 2369-2372. 65 van Leusen, A. M.; Wildeman, J.; Oldenziel, O. H. J. Org. Chem. 1977, 42, 1153-1159. 66 van Leusen, A. M.; Wildeman, J. Synthesis 1977, 501-503. 67 van Leusen, A. M.; Siderius, H. Hoogenboom, B. E.; van Leusen, D. Tetrahedron Lett. 1972, 13,

5337-5340.

17

3.2.1 Mechanistic aspects

Two tentative rationalisations are suggested in Scheme 15, one leading to the β-carboline 66 while the other ends up giving the pyrroloquinolone 67. The initial step involves a Michael addition to the exocyclic double bond leading to the intermediate 68. Michael adducts from 3-methylenoxindole derivatives have been reported e.g. the reaction between some 3-methylenoxindole derivatives and malononitrile.68 To form the β-carboline 66 intermediate 69 would have to ring-close onto isocyanide carbon, a hydrogen shift and after elimination of p-toluensulfinate yield the indolo annulated oxazepine 70. Valence tautomerism ought to yield the epoxide 71 which thereafter opens, and finally after tautomerisation gives the β-carboline 66. The valence tautomerisation between 70 and 71 can be compared with the corresponding oxepin equilibrium. On the other hand, if TosMIC works as it normally does, a spiro pyrrolo oxindole 72 should to be formed via ring closure and loss of p-touluensulfinate and a hydrogen shift. Cleavage of the oxindole moiety, promoted by the basic conditions, results in compound 73, which undergoes recyclisation to the pyrroloquinolone 67 via isocyanate 74. This type of rearrangement of isatins to quinolones has precedent in the literature.69 It seems that the very spiro structure of 72 render it sensitive to secondary reactions such as ring expansion to quinolones. Cleavage of 3-spiro oxindole derivatives was also observed when isatin was treated with ammonia (Scheme 5).

68 Higashiyam, K.; Nagase, H.; Yamaguchi, R.; Kawai, K-I.; Otomasu, H. Chem. Pharm. Bull. 1985,

33, 544-550. 69 (a) Bennet, G. B; Mason, R. B.; Shapiro, M. J. J. Org. Chem. 1978, 43, 4383-4385. (b) Eistert, B.;

Selzer, H. Chem. Ber. 1963, 96, 1234-1255.

18

NH

N

ONH

O

NEtO2C

NC

O

N

EtO2C

NC

O

N

EtO2C

- H+

+ H+

NH

NH

EtO2C

O

NH

N

O

NH

NH

EtO2C

O

NH

O

EtO2C

a

NH

O

EtO2CN

C

Tos

NH

O

EtO2CN

C

Tos

12 68 69

EtO2C

pyrroloquinolone pathway

ß-carbolinepathway

EtO2C

1 23

4

567

89

1

23

456

78

9

70

71

66

67

7273

74

Scheme 15. a) TosMIC, KOtBu, THF.

In spite of the efforts to analyse the spectroscopic data available no enlightenment to the true structure of the TosMIC adduct (66 or 67) could be achieved. Two-dimensional NMR studies did not exclude or point towards any of the two candidate stuctures. The task was then clear, alternative synthesis of both the β-carboline 66 and the pyrroloquinolone 67 had to be performed.

3.2.2 β-Carbolines

There are four different carbolines 75, α-, β-, γ- and δ-carbolines,which differs in the position of the nitrogen in the annulated pyridine ring. β-Carbolines are the most abundant of the carbolines.

19

NH

N

αβ

γδ

NH

NHN

NH

OCl

Cl

Me75 76 77 Figure 8. Carbolines, tetrahydro β-carboline and baurine C.

Nature has been endowed with many different β-carbolines due to their accessibility from the amino acid tryptophan, which serves as the starting material in the biosynthetic pathway of β-carbolines. Both the non-aromatic tetrahydro-β-carboline 76 and the fully aromatic β-carboline have been isolated from various sources. Tetrahydro β-carbolines 76 have for instance been identified in several commercial sausages such as salami and Spanish chorizo.70 Baurine C 77, a chlorine containing β-carboline which has been isolated from the terrestrial blue-green alga Dichothrix baueriana, shows activity against herpes simplex virus type 2.71 Since β-carbolines are well-documented biologically active compounds, they are interesting goals in synthetic chemistry.72 The most common way to prepare β-carbolines is the Pictet�Spengler type of reaction whereby tryptamines and aldehydes are condensed to give the tetrahydro-β-carbolines followed by oxidation to the fully aromatised β-carbolines.73 Nevertheless, this strategy cannot be applied to the synthesis of β-carboline 66, hence a new route had to be developed. Retrosynthesis of compound 66, shown in Scheme 16, begun with breaking the amide bond which ought to be formed easily by introducing an ammonia source to enol 78. Formylation to the 2,3-disubstituted indole 79, available by Fischer indolisation according to Robinson et al.74, should yield the enol 78.

NH

NH

EtO2C

O NH

EtO2C

O

OH

OEt

NH

EtO2C

O

OEt

66 78 79 Scheme 16. Retrosynthesis of β-carboline 66.

The 3-ethoxycarbonylmethyl-indole-2-carboxylic acid ethyl ester 79 was treated with NaH and ethyl formate to yield the formylated species 78. Ring closure was attempted with ammonium acetate in refluxing DMF but the annulated pyridone I sought after could not be isolated. Instead lactonization to the indolopyrone 80 occurred. Maybe an attack by the ammonia was prevented by an initial deprotonation of the enol and therefore no lactamisation occurred.

70 Herraiz, T.; Papavergou, E. J. Agric. Food Chem. 2004, 52, 2652-2658. 71 Larsen, L. K.; Moore, R. E.; Patterson, G. M. L. J. Nat. Prod., 1994, 57, 419-421. 72 Donova, A. K.; Statkova-Abeghe, S. S.; Venkov, A. P.; Ivanov, I. I. Synth.. Commun. 2004, 34,

2813-2821, and references cited therein. 73 Love, B. E. Org. Prep. Proced. Int. 1996, 28, 1-64, and references cited therein. 74 Robinson, J. R.; Good, N. E. Can. J. Chem. 1975, 35, 1578-1581.

20

NH

EtO2C

CO2Eta

NH

EtO2C

CO2Et

78 (87%)

b

NH

O

EtO2C

O

80 (27%)

N

EtO2C

CO2Et

Me81 (56%)

NMe2

b

N

NH

EtO2C

O

82 (42%)Me

NH

NH

O

66 (20%)

d

c

NTos

OEt

EtO2C

79

83

OH

Scheme 17. a) NaH, ethyl formate, Et2O. b) NH4OAc, p-TosOH(cat), DMF, reflux. c) DMFDMA,

DMF, heat. d) NaH, Et2O.

To circumvent the possible acidity problem with the enol, a dimethylvinylamino group was introduced by treating the indole 79 with DMFDMA. However, in addition to the introduction of the dimethylvinylamino group this reagent also N-methylated, perhaps not unexpectedly,75 the indole nitrogen. Together with the known N-methylated dimethylvinyl indole 8176 the N-methylated diethyl ester 97 was obtained in 23% yield. Now, treatment with an ammonia source yielded the mono N-methylated β-carboline 82 in 42%. Finally a strategy involving the tosylimine 8377 was adopted whereby the tosylimine 83 served as a synthetic equivalent to a C-N fragment. In this fashion the previously unknown β-carboline 66 was prepared in a modest yield (20%). The NMR data of the TosMIC adduct and the β-carboline 66 did not match. Consequently, the TosMIC adduct was not the β-carboline 66.

3.2.3 Pyrroloquinolones

A survey of the literature revealed very few reports leading to pyrrolo[2,3-c]quinolones 84, even though structures of this type had been shown to inhibit proliferation of tumour cells.78 German researchers have synthesised pyrrolo[2,3-c]quinolone 86 by an intramolecular amide bond formation via reduction of a 3-(2-nitropheny)-5-methyl-2-ethoxycarbonyl pyrrole 85 (Figure 9).79 The chemistry of the parent compound of 84, the non-oxidised pyrrolo[2,3-c]quinoline 87, has been reviewed and does also occur as a moiety in certain alkaloids80.

75 (a) Stanovinik, B.; Ti�ler, M.; Hribar, A.; Barlin, G. B.; Brown, D, J. Aust. J. Chem. 1981, 34, 1729-

1738. (b) Middleton, R. W.; Monney. H.; Parrik, J. Synthesis 1991, 740-743. 76 Gray, N. M.; Dappen, M. S.; Cheng, B. K.; Cordi, A. A.; Biesterfeldt, J. P. J. Med. Chem.1991, 34,

1283-1292. 77 Anglada, L.; Marquez, M.; Sacristan, A.; Ortiz, J. A. Eur. J. Med. Chem. Chim. Ther. 1988, 23, 97-

100. 78 Thal, C.; Boye, O.; Guenard, D.; Potier, P. WO 9 002 733, 1990. 79 Görlitzer, K.; Fabian, J.; Frohberg, P.; Drutkowski, G. Pharmazie, 2002, 57, 243-247. 80 Khan, M. A.;.da Rocha, J. F. Heterocycles 1978, 9, 1617-1629.

21

N

NH

NH

NH

O

Me

NO2

NH

CO2Et

Me

NH

NH

O

84 85 86 87

Figure 9. Pyrrolo[2,3-c]quinoline derivatives.

The retrosynthesis starts with breakage of the amide bond that then will lead to the trisubstituted pyrrole 88. This pyrrole, which ought to be acylated in the 2-position, should be available from the cinnamic acid derivative 89 and TosMIC

NH

NH

O

EtO2C

NH

CO2EtO2N

EtO2C

NO2

+ TosMIC

67 88 89

R

Scheme 18. Retrosyntheis of pyrroloquinolone 67.

The cinnamic acid derivative 8981 could indeed be reacted with TosMIC to yield the adduct 4-(o-nitrophenyl)-pyrrole-3-carboxylic acid ethyl ester 88 (R=H, Scheme 18) which in turn was subjected to several acylating agents such as trichloroacetyl chloride, oxalyl chloride and TFAA. None of these reagents were successful in their acylation task, most likely due to the two electron withdrawing groups present in the pyrrole.

Changing the starting material to the α, β-unsaturated sulfone 90 obtained from a condensation between o-nitrobenzaldehyde and tosylacetonitrile. Treating the α, β-unsaturated sulfone 90 with ethyl isocyanoacetate yielded the 2,3,4-trisubstituted pyrrole 91 in 72% yield in a modified Barton-Zard reaction.82 Ring closure to the relatively unusual pyrroloquinolone 92 (65% yield) was achieved by reduction of the nitro functionality and the formation of an intramolecular amide bond. Once the skeleton was correctly assembled, transformation to the desired carbethoxy group was achieved by hydrolysis in a sulfuric acid/ethanol medium. The hydrolysis yielded a 4:1 mixture of the acid 93 and the ethyl ester 67. The proton NMR spectrum of the pyrroloquinolone ethyl ester 67 obtained via the lengthy route (11% total yield) in Scheme 19 matched perfectly with the product obtained from the high yielding (74%) reaction wherein TosMIC was added to 3-methyleneoxindole acetic acid ethyl ester.

81 Sinisterra, J. V.; Mouloungui, Z.; Dekmas, M.; Gaser, A. Synthesis 1985, 1097-1100. 82 (a) Barton, D. H.; Kervagoret, J.; Zard, S. Z. Tetrahedron 1990, 46, 7587-7598. (b) Fumoto, Y. Uno,

H.; Murashima, T. Ono, N. Heterocycles, 2001, 54, 705-720.

22

NO2

TosNC

90 (71%)

a b cNH

NO2CO2Et

NC

NH

NH

O

NC

NH

NH

O

EtO2C

NH

NH

O

HO2C

+

6793

91 (72%) 92 (65%)

Scheme 19. a) ethyl isocyanoacetate, DBU, THF, 0 °C. b) Fe, AcOH, reflux. c) EtOH/ conc. H2SO4

(1:1), reflux.

3.3 3-(Pyrrol-4-yl)-oxindole (paper III)

3.3.1 Introduction

Enaminones (e.g. 3-aminocrotonates) are versatile reagents that have been used in the synthesis of a multitude of heterocycles such as pyridines, pyrroles, imidazoles and pyrimidines.83 There are two electrophilic centers namely C-1 and C-4 (cf. α, β-unsaturated esters) and two nucleophilic centers C-2 and the amino functionallity (cf. enamines). Thus enaminones are suitable for reactions with polydentate reagents, a reaction that usually will give heterocycles. One example is the reaction between methyl propiolate 94 and the enaminone 3-amino-cyclohex-2-enone 95 gives the tetrahydroquinolone derivative 96.84 Another well-known example is the Nenitzescu indole synthesis wherein an enaminone is reacted with a quinone to yield an indole derivative.85

CO2Me

O

H2N NH

O

O

94 95 96

+

H

Scheme 20. Reaction of an enaminone to give tetrahydroquinolone derivative 96.

83 (a) Elassar, A-Z. A.; El-Khair, A. A. Tetrahedron 2003, 59, 8463-8480. (b) Stanovnik, B.; Svete, J.

Chem. Rev. 2004, 104, 2433-2480. (c) Negri, G.; Kascheres, C.; Kascheres, A. J. J. Heterocycl. Chem. 2004, 41, 461-491.

84 (a) Sluyter, M. A. T.; Pandit, U. K.; Speckamp, W. N.; Huisman, H. O. Tetrahedron Lett. 1966, 7, 87-90. (b) Ruda, M.; Bergman, J.; Koehler, K.; Ye, L. Heterocycl. Commun. 2003, 9, 571-574.

85 Allen, G. R. Org. Reactions 1973, 20, 337-454.

23

As seen previously 3-methylene oxindole derivatives are attacked by nucleophiles on the exocyclic double bond. That is precisely what Tacconi et al.86 reported in 1976 when N-methyl-3-phenacylideneoxindole was treated with 1-pyrrolidinocyclo-pentene. The reaction proceeded via an initial attack on the exocyclic double bond by the enamine that gave an intermediary zwitterionic compound 97, which later yielded a new enamine 98. Hydrolysis of enamine 98 and treatment with aniline yielded the pyrrolo-oxindole 99.

NMe

O

PhO

N

NMe

O

PhO

N

NMe

O

O Ph

O

NMe

O

NPh

Ph

97 98

99 100

H H

H

Figure 10. Enamine adducts based on 3-phenacylideneoxindole.

A similar reaction between 1-pyrrolidinocyclohexene and N-methyl-3-phenacylideneoxindole gave an adduct in the same way as described above. This adduct was carefully hydrolysed and depending on the conditions during the work-up, two different diastereoisomers were isolated, one of them, compound 100, is shown in Figure 10. The structure of compound 100 was supported by extensive 1H NMR studies and finally confirmed by an X-ray diffraction analysis.87 Since enaminones share the reactivity of enamines, a study on the reactions of enaminones with 3-methylenoxindole derivatives was also initiated.

3.3.2 3-Aminocrotonates and 3-methyleneoxindole acetic acid ethyl ester

The two enaminones 101, methyl- and ethyl- 3-aminocrotonate, were refluxed in ethanol together with 3-methyleneoxindole acetic acid ethyl ester 12. This reaction yielded a colourless solid 103, which was isolated after partial evaporation of the solvent. The formation of 103 proceeds via an initial zwitterionic adduct 102 as discussed above.

86 Tacconi, G.; Invernizzi, A., G.; Desimoni, G. J. Chem. Soc. Perkin Trans. 1 1976, 1872-1879. 87 López-Alvarado, P.; García-Granda, S.; Álvarez-Rúa, C.; Avendaño, C. Eur. J. Org. Chem. 2002,

1702-1707.

24

NH

O H2N Me

O

OR

NH

O

H2NMe

O

OR

+

EtO2C

EtO2C R Yield (%)a: Et 62b: Me 45

Me NH2

CO2REtO2C

NHO

H

ethanol

reflux

10112

102 103 Scheme 21. Addition of 3-aminocrotonates to 3-methyleneoxindole acetic acid ethyl ester.

The 1H NMR spectrum of 103a showed the presence of two isomers, the E, Z pair of the double bond. Recrystallisation from ethyl acetate yielded only one isomer, although isomerisation took place in solution. On the contrary, the methyl ester 103b was collected as one single isomer. To elucidate the stereochemistry of the double bond in the methyl ester 103b, NOE difference spectra were recorded. The methyl signal at 1.94 ppm (the vinylic methyl group) was irradiated and it resulted in an NOE interaction at 3.96 ppm. This signal corresponds to the α-proton of the ethyl ester. On the other hand, when the signal corresponding to the methoxy group (3.22 ppm) was irradiated no NOE interaction could be detected at 1.94 ppm. This piece of information corroborated the double bond to be in Z configuration, as depicted in Scheme 21.

3.3.3 3-Aminocrotonates and 3-acetonylideneoxindole

A literature survey regarding 3-aminocrotonates and α, β-unsaturated carbonyl compounds ended up in the classical Hantzsch pyrrole synthesis dating back to 1890.88 The reaction involves an in situ formation of a 3-aminocrotonate ester, which is alkylated by a halo�ketone or �aldehyde and finally cyclised to the pyrrole. A couple of years ago a one-pot synthesis of pyrrole derivatives was published wherein the pyrrole 105 was prepared from 3-aminocrotonates such as 101b and dibenzoylethylene 104.89 Interestingly, this reaction was performed without solvent and gave the pyrrole in almost quantatively yield.

MeO2C

NH2MePh

Ph

O

O

+-H2O N

HMe

MeO2C

Ph

Ph

O

101b 104 105 Scheme 22. One-pot synthesis of pyrroles in the solid-state.

88 (a) Hantzsch, A. Ber. 1890, 23, 1474-1476. (b) Roomi, M., W.; MacDonald, S. F. Can. J. Chem.

1970, 48, 1689-1697. 89 Kaupp, G.; Schmeyers, J.; Kuse, A.; Atfeh, A. Angew. Chem. Int. Ed. 1999, 38, 2896-2899.

25

The 3-acetonylideneoxindole 11 was refluxed in toluene together with three different 3-aminocrotonates, namely ethyl 3-aminocrotonate 101a, methyl 3-aminocrotonate 101b and methyl 3-(methylamino)crotonate 101c. Solid products were obtained in good yields from all these reactions.

NH

MeO

ONH

Me

O

O

HNMe

CO2R1

NH

N

CO2R1

O

MeMe

11 106

107'

NH

N

CO2R1

OH

MeMe

107''

R2

R2R2

MeHN

CO2R1

R2

+-H2O

101

R1 R2 Yield 107':107''a: Et H 91% 3:2b: Me H 89% 5:3c: Me Me 88% 2:1

a H

H

Scheme 23. Synthesis of pyrrolo oxindole. a) Toluene, reflux.

As seen in Scheme 23 the products obtained were the cyclised pyrrolo-oxindoles 107. Why do the adducts 106 ring close and not the adduct 103 obtained in the reaction described in Scheme 21? The explanation can be found in the different oxidation level in the α, β-unsaturated ester and the α, β-unsaturated ketone. The adduct 103 incorporates an ester but adduct 106 contains a more reactive carbonyl (ketone) which then cyclises more easily. Synthesis of 3-pyrrol-3�-yloxindoles have been achieved, in very low yields (3-5%), by reacting 3-diazooxindoles with pyrroles in the presence of a rhodium(II) acetate catalyst, even though the main product from this reaction were 3-pyrrol-3�yloxindoles obtained in good yields (60-70%).90 The reaction between 3-aminocrotonate esters and 3-acetonylideneoxindole give a pyrrole moiety with much less expensive reagents and with higer yields (around 90%).

In contrast to the non-cyclised adducts 103 the pyrrolo-oxindoles 107 occure as keto-enol tautomers with a slight preference towards the keto form 107�. This keto-enol tautomerism could be observed in the NMR spectra as a doubling of the signals, i.e. the characteristic pattern of an ethyl group was doubled. When the NMR tube was heated to 125 °C both 13C spectra and 1H spectra only showed one set of signals. A plausible explanation for the shift towards the keto form may be found in the increased steric hindrance in the planar enol form.

As good crystals were obtained by recrystallisation of 107c an X-ray structure analysis was conducted. In the solid state the only form present was the racemic keto form. Hence in solution three different species were present, the enol 107c�� and the

90 (a) Muthusamy, S.; Gunanathan, C. Synlett 2002, 1783-1786. (b) Muthusamy, S.; Gunanathan, C.;

Nethaji, M. J. Org. Chem. 2004, 69, 5631-5637.

26

two enantiomers 107c�(S) and 107c�(R). In the solid state the molecules are joined as dimers by hydrogen bonding, the indolic hydrogen is paired with the amide oxygen.

Figure 11. The racemic compound 107c�. The atom-labelling scheme is shown.

Incorporating an α, β-unsaturated ketone, 3-phenacylideneoxindole 108 was also reacted with methyl 3-aminocrotonate 101a to yield the pyrrolo-oxindole 109.

NH

O

PhO

a

NH

O

HN

PhMe

CO2Me

108 109'

NH

OH

109''

NH

Me

MeO2CPh

(92%)

Scheme 24. a) methyl 3-amino crotonate 101a, toluene, reflux, 3h.

Somewhat surprisingly the keto-enol equilibrium was more shifted towards the keto form (90%) compared to the pyrrolo oxindoles obtained from 3-acetonylidene-oxindole. The predominance of the keto form can be explained in terms of π-stacking between the oxindole nucleus and the phenyl ring. When the pyrrolo oxindole 109 is toggled between the keto 109�and the enol form 109�� C-3 is switching between sp3 and sp2 hybridisation. As an sp2 carbon exhibit a planar geometry, this fact will force the indole and the pyrrole ring to be in the same plane. This configuration diminishes the possibility of π-stacking between the phenyl ring and the oxindole. Consequently when C-3 is sp3 hybridised it adopts a tetrahedral geometry and permit the phenyl ring to bend over the oxindole to form π-stacking between the phenyl ring and the indole ring.

3.3.4 Chlorination of pyrrolo-oxindoles with POCl3

To achieve a permanent transformation of the oxindole to an indolic moiety, treatment with phosphorus oxychloride (POCl3) ought to trap the enol form as a 2-chloroindole derivative. Thus the pyrrolo-oxindole 107c was heated in neat POCl3 (and quenched by pouring it onto an ice/water mixture and made alkaline with a 45% KOH(aq) solution). Two products were obtained, the minor one was the expected 2-

27

chloroindole derivative 110 and the major product was identified as the uncommon pyrrolo annulated indolo-2-pyranone 111. The cause of the formation of the pyrrolo annunlated indolo-2-pyranone derivative 111 was attributed to the addition of the very strong alkaline KOH(aq) solution, which probably hydrolysed the methyl ester. Ring closure was then effected by the attack of the carboxylate anion on the 2-chloroindole moiety.

NH

O

N MeMe

CO2Mea

NH

Cl

N MeMe

CO2Me

Me Me

107c 110

(69%) NH

O

NMe

MeMe

Ob

(quant.)

111

Scheme 25. a) POCl3, Et3N, MeCN reflux over night. b) 45% KOH(aq), MeOH, reflux 3h.

In order to gain more information about the formation of the pyrrolo annulated indolo-2-pyranone 111 the chlorination reaction was performed in a two-steps, first the POCl3 chlorination, and then a separate ester hydrolysis. Changing the conditions of the chlorination step to POCl3 together with triethylamine in refluxing acetonitrile gave the 2-chloro-3-pyrrolo-indole derivative 110 in 69% yield. Hydrolysis of the methyl ester was performed and yielded the pyrrolo annulated indolo-2-pyranone 111 in quantitative yield.

28

4 Synthetic studies towards the alkaloid granulatimide (paper IV)

4.1 Introduction to imidazolyl indoles

Among the 20 α amino acids that are usually found in proteins three incorporate nitrogen heterocycles. One of them is the saturated nitrogen heterocycle proline (pyrrolidine-2-carboxylic acid). The other two, histidine and tryptophane, have aromatic nitrogen heterocycles in the side chain. Tryptophan contains an indole and histidine is an imidazole derivative. Noteworthy, both of these α amino acids are essential amino acids for vertebrates, which means they cannot be biosynthesised from other sources but have to be ingested via the food.

Histidine incorporates an imidazole side chine that possesses several properties that have key features in proteins. Having a pKa value near 7, imidazole is well suited to play a crucial role in enzymes where proton transport is required. Decarboxylated histidine, histamine, is intimately connected with inflammatory and allergic reactions. As being important not only in the biosynthesis and functionality of proteins, tryptophan and histidine are present independently or in combination in several alkaloids. Some examples are granulatimide 112,91 isogranulatimide 113,91 didemnimide A 11492 and grossularine 11593.

NH

HN

N

NH

OO

112

NH

N

NHN

Me2N

O

NH

113

115114

NH

HN

N

NH

OO

NN

HN

NH

OO

Figure 12. Alkaloids incorporating imidazolyl indoles.

Granulatimide 112 and isogranulatimide 113 have been isolated from the ascidian Didemnum granulatum and crude methanol extracts from the ascidian showed activity during screening for G2 cell cycle checkpoint inhibitors.91 The synthesis (Scheme 27) of the granulatimides 112 and 113 showed that isogranulatimide 113 91 Berlinck, R. G. S.; Britton, R.; Piers, E.; Lim, L.; Roberge, M.; da Rocha, R. M.; Andersen, R. J. J.

Org. Chem., 1998, 63, 9850-9856. 92 Vervoort, H. C.; Richard-Gross, S. E.; Fenical, W. J. Org.Chem. 1997, 62, 1486-1490. 93 (a) Moquin-Pattey, C.; Guyot, M. Tetrahedron 1989, 45, 3445-3450. (b) Choshi, T.; Yamada, S.;

Sugino, E.; Kuwada, T.; Hibino, S. J. Org. Chem. 1995, 60, 5899-5904.

29

was the natural occurring G2 checkpoint inhibitor but also granulatimide 112 exhibited this pharmacological property.91

4.2 3-(Imidazolyl)-indoles

There are two main ways of synthesising 3-(imidazolyl)-indoles. One starts with an imidazole and then the indole moiety is built from there and one from an existing indole derivative. As TosMIC has been used successfully to synthesise imidazoles, the choice of starting from an indole derivative was easy since 3-formyl indole is a commercial compound and an excellent staring material for an imidazole synthesis. Thus conversion of 3-formyl indole into the benzyl imine 116 proceeded as expected, likewise did the addition of TosMIC 65a to the imine 116, which gave a 3-(imidazolyl)-indole derivative 117a in 79% yield.

NH

NBn

+ N CR

Tos Et3N

THF/MeOH NH

NN

RBn

116 65 117 R (yield)a: H (79%)b: Ph (66%)c: p-methoxphenyl (34%)

Scheme 26. Synthesis of 3-imidazolyl indoles 117.

Aryl substituted TosMIC derivatives (65b and 65c) yielded tri-substituted imidazole derivatives (117b and 117c). When p-methoxyphenyl substituted TosMIC 65c was used a lower yield of the imidazole 117c was obtained. The destabilising effects that the p-methoxyphenyl group exerts on the intermediate anion created can rationalise this decrease of the yield. Sisko et al.94 have published a procedure wherein the imine is formed in situ and thereafter reacted with an aryl substituted TosMIC reagent to yield 1,4,5-trisubstituted imidazoles. By adopting and modify this sequence by first reacting 3-formylindole with benzylamine and then adding TosMIC and triethylamine yielded 3-(3-benzyl-5-phenyl-3H-imidazol-4-yl)-1H-indole 117b in 86% yield. This sequence was also tried with plain TosMIC but this only resulted in a plethora of non-separable compounds. These results are in accordance with what has already been reported.94

4.3 2-(Imidazolyl)-tetrahydroindoles

4.3.1 Published procedures to granulatimide

The first published syntheses of granulatimide 112 and isogranulatimide 113 appeared together with the isolation, and followed a biomimetic pathway via didemnimide A 114.91 The indole part was derived from indole-3-acetamide 118, which was condensed with the imidazole derivative 119 to yield, after reduction and removal of the methoxymethyl protecting group, the alkaloid didemnimide A 114. A

94 Sisko, J.; Kassick, A. J.; Mellinger, M.; Filan, J. J.; Allen, A.; Olsen, M. A. J. Org. Chem. 2000, 65,

1516-1524.

30

photolysis of didemnimide A 114 resulted in formation of granulatimide 112 in 91% yield and isogranulatimide 113 in 8% yield.

NH

N

NH

HNO O

NH

N

NH

HNO O

10% Pd-C,MeCN

NH

NN

OMeO2C

OMe

SPh

+

NH

NN

NH

N

N

HNO O

Br

EtMgBr

R

RNOMe

SnBu3

N

N

I

R+

CONH2

118

119

114 112

121

120

122

123

124

HN OO

Br Br

UV

UV

Scheme 27. Published syntheses of granulatimide.

The second synthesis of granulatimide 112, developed by Japanese researchers, also utilised a photoreaction to obtain the alkaloid.95 The 2-stannylindole 120 was prepared in situ from the 2-lithiated 1-methoxyindole and then utilised in Stille coupling reaction with 4-iodo-imidazole 121 to yield the key intermediate 2-imidazolyl indole derivative 122. The dibromomaleimide 123 was attached to the 3-position of indole via the magnesiumbromide salt of compound 122 to yield the 2,3-disubstituted indole 124. To create the bond between the imidazole and the maleimide ring compound 124 was irradiated with UV-light as outlined above. Granulatimide 112 was then finally obtained after deprotection of the imidazole part.

4.3.2 Retrosynthesis of granulatimide

As described earlier in the synthesis of (3-benzyl-3H-imidazol-4-yl)-1H-indole 117a the construction of an imidazole directly bonded to an indole was easily effected by reacting a benzylimine with the TosMIC reagent. This TosMIC strategy was chosen to build the 2-imidazolyl indole 122 required as the key intermediate for the synthesis of granulatimide 112 according the retrosynthesis in Scheme 28. An

95 Yoshida, T.; Nishiyachi, M.; Nakashima, N.; Murase, M.; Kotani, E. Chem. Pharm. Bull. 2002, 50,

872-876.

31

addition of TosMIC to a Schiff base like 126 ought to yield an 2-imidazolylindole 122. The imine 126 should be obtainable from a 2-formylindole 127. The second important compound is the 2-formyl indole 127, which unfortunately is not as accessible as 3-formylindole. Hence a new route to 2-formylindoles had to be formulated.

NH

N

NH

HNO O

NH

N

NHNO O

N

N R

TosMIC

NCHO

+R

R R

112 122 125

126 127 Scheme 28. Retrosynthesis of granulatimide 112.

4.3.3 Synthesis of 2-imidazolyltetrahydroindole

By using the tetrahydroindole it is possible to introduce the formyl group by the Vilsmeier reagent hence pyrroles do react with electrophiles at the 2-position. This is contrary to indoles where electrophilic reagents end up in the 3-position. A survey of the literature regarding the synthesis of tetrahydroindoles revealed a promising procedure published by a Danish group. Thus Madsen et al96 have prepared tetrahydroindoles by reacting α-chloroacrylonitrile with methyl- and cyclohexan-cyclohexanone imine. In a similar approach, cyclohexanone benzylimine 128 was reacted with α-chloroacrylonitrile to yield the ring-closed compound 129.

NBn

NBn

CNNBn

NHBn

a bR

128 129 130: R=H (90%)131: R=CHO (61%)132: R=CHNBn

cd

Scheme 29. a) α-chloroacrylonitrile, Et3N, MeCN, 0 °C. b)EtOH, reflux. c) POCl3, DMF, 0 °C to rt. d)

BnNH2, THF, rt.

To eliminate HCN Madsen et al.96 performed a pyrolysis. This procedure could be further simplified when it was discovered that mere heating of 129 in ethanol caused elimination of HCN. Purification of tetrahydroindole 130 can be carried out by chromatography but it is not imperative. Now, with the previously known97 tetrahydroindole 130 in hand formylation with the Vilsmeier reagent (DMF/POCl3)

96 Madsen, J. Ø.; Meldal, M.; Mortensen, S.; Olsson, B. Acta Chem. Scand. 1981, B35, 77-81. 97 Trost, B. M.; Keinan, E. J. Org. Chem. 1980, 45, 2741-2746.

32

proceeded smoothly to yield 2-formyltetrahydroindole derivative 131. The synthesis of the Schiff base 132 was executed in almost quantative yield in a THF solution. During the work with the imine 132 we noticed that it was very sensitive towards acids. In an attempt to purify small amounts of the imine 132 by chromatography on silica gel it only resulted in hydrolysis of the imine back to the aldehyde 131.

Formation of the 2-imidazolyltetrahydroindole 133 was observed when the conditions for imidazole synthesis described in Scheme 26 were applied. Better yields was obtained when the imine 132 as refluxed in DCM together with TosMIC and Et3N (Scheme 30). Using these conditions the 2-imidazolyltetrahydroindole 133 was obtained in 72% yield.

NBn

N Bn

NBn

132

N

N

Bn

a

133

(72%)

Scheme 30. Et3N, TosMIC, DCM, reflux, 24h.

To conclude, the synthesis of a potential key intermediate, 2-imidazolyletetrahydroindole 133, in a synthetic route towards the alkaloid granulatimide 112 have been presented. The construction of the imidazole moiety was effected with TosMIC chemistry. Moreover 3-imidazolylindoles were synthesised utilising the same TosMIC strategy.

33

5 Acknowledgements

Min handledare professor Jan Bergman som en gång antog mig som doktorand. Du har lärt mig kemisk �know-how� och visat upp ett brinnande intresse för den organiska kemins underbara värld.

Docent Alf Claesson för ditt engagemang som extern handledare och tack AstraZeneca R&D, Södertälje för det finansiella stödet.

Robert Engqvist för delandet av dragskåpsrad och labutrymme samt diskussioner rörandes musik, sport och där i mellan kemi.

Dr. Tomasz Janosik som alltid har en bestämd åsikt om det mesta, speciellt inom den gastronomiska sfären. Speciellt tack till din �rödfärgning� av avhandlingen.

Dr. Per Wiklund och Dr. Niklas Wahlström som har kamperat i samma lab och bidragit till kemidiskussioner och övrigt stämningshöjande som t.ex. bränder�

Dr. Birgitta Stensland för dina �bilder� av några av mina föreningar.

Malin Björk för att ha stått ut med mig som skrivbordsgranne, nu får du extra utrymme för dina papper.

Ann-Louise Johnson för ditt slit med att hålla köket rent och snyggt. RESPEKT.

Solveig, din närvaro har en uppiggande effekt på alla tröga doktorander.

Kollegorna i skrivrummet. De nuvarande Jealux, Sassa och Birgitta men även de som har passerat revy i skrivrummet.

Medlemmar i JB gruppen; Johnny, Jeff, Hamid, Ivan men även alla andra som har passerat revy under mina år i gruppen.

Vänner i Ödeshög och i Södertälje.

Ett innerligt tack till mina föräldrar och syskon utan stödet från er skulle inte denna avhandling finnas.

Åsa tack för att du sa ja!

34

6 Appendix: supplementary material

6.1 Experimental part to section 4

General Remarks: Solvents (PA grade) were commercial and used without further purification. NMR spectra were recorded in DMSO-d6 solutions at 25 °C, unless otherwise stated, on a Bruker DPX 300 spectrometer, operating at 300 MHz for 1H and 75 MHz for 13C. δ Values are reported in ppm and J values in Herz. IR spectra were recorded on a Thermo Nicolet Avatar 330 FT-IR instrument using single-reflection ATR. Melting points were determined with a Büchi melting point B-545 apparatus and are uncorrected.

3-(3-Benzyl-3H-imidazol-4-yl)-1H-indole, 117a

NH

NN

Triethylamine (0.18 g, 1.73 mmol) was added to a solution of benzyl-[1-(1H-

indole-3-yl)methylidene]-amine 116 (0.27 g, 1.15 mmol) and TosMIC (0.27 g, 1.38 mmol) in methanol (6 ml) and THF (3 ml). After 4 h of reflux the reaction was quenched with 80 ml ice/water and acidified with acetic acid. Extraction with EtOAc (4x20 ml) and washing of the combined organic phases with sat. aq. NaHCO3 and brine yielded after drying with MgSO4 and evaporation of the solvent 0.39 g of a brownish gum. Purification by dry flash cromatography (hexane-EtOAc) yielded 3-(3-benzyl-3H-imidazol-4-yl)-1H-indole 117a (0.25 g, 79 %). Recrystallization from acetonitrile afforded analytically pure 3-(3-benzyl-3H-imidazol-4-yl)-1H-indole 117a. Mp:139-140 ûC; IR (neat) 1450, 1226, 818, 756, 712 cm-1; δH = 5.29 (2H, s), 6.96-6.98 (2H, m), 7.02-7.08 (1H, m), 7.10-7.30 (6H, m), 7.41 (1H, d, J=8.0), 7.53 (1H, d, J=7.8), 7.81 (1H, s), 11.32 (1H ,s); δC = 48.5 (t), 103.8 (s), 111.8 (d), 119.1 (d), 120.1 (d), 122.2 (d), 124.2 (d), 126.6 (d), 127.1 (s), 127.3 (s), 127.7 (d), 128.7 (d), 136.3 (s), 137.0 (s), 137.6 (d)

3-(3-Benzyl-5-phenyl-3H-imidazol-4-yl)-1H-indole, 117b

NH

NN

Sequencial procedure. 3-Formylindole (1.84 g, 13 mmol) and benzylamine (4.35

g , 41 mmol) were stirred together with MgSO4 (6.5 g) in THF (70 ml). After 24 h of stirring α-tosylbenzyl isocyanide 65b (2.76 g, 10 mmol) and Et3N (1.31 g, 13 mmol) were added and the reaction was stirred for 3 days at rt. Removal of all solid material (MgSO4) and evaporation of the solvent gave a crude yellow solid. Trituration with

35

hot DCM (30 ml) gave 2.36 g (66%) of the trisubstituted imidazole 117b as a pale yellow solid.

One pot procedure. Benzylamine (2.23 g, 21 mmol) and Et3N (0.79 g, 8 mmol) were added to a stirred suspension of 3-formylindole (1.14 g, 8 mmol), α-tosylbenzyl isocyanide 65b (1.93 g, 7 mmol) and MgSO4(5 g) in THF (50 ml). After two days of stirring at ambient temperature the reaction mixture was filtered and added to water (300 ml) and acidified with AcOH. The trisubstituted imidazole 117b was collected as a yellow solid. Mp 232-233 ûC; IR (neat 1456, 1444, 939, 779, 731, 717, 696, 653 cm-1; δH = 5.01 (2H, s), 6.89-6.95 (3H, m), 7.01-7.25 (8H, m), 7.31 (1H, d, J = 2.5), 7.45-7.52 (3H, m), 7.96 (1H, s), 11.46 (1H, s); δC = 47.5 (t), 103,4 (s), 112.0 (d), 118.7 (d), 119.5 (d), 121.6 (d), 121.9 (s), 125.3 (d), 125.7 (d), 126.4 (d), 126.6 (d), 126.9 (s), 127.3 (d), 127.9 (d), 128.4 (d), 135.3 (s), 136.1 (s), 137.85 (s), 137.89 (d), 138.0 (s)

3-[3-Benzyl-5-(4-methoxy-phenyl)-3H-imidazol-4-yl]-1H-indole, 117c

NH

NN

OCH3

Triethylamine (1.11 g, 11 mmol) was added to a solution of benzylimine 116 (1.28

g, 5 mmol) and p-methoxyphenylTosMIC 65c (1.65 g, 5 mmol) in a medium consist of MeOH (5 ml) and THF (20 ml). After 24 h of stirring at rt the mixture was added to water (150 ml) and acidified with AcOH, and extracted with EtOAc (4x30 ml). The organic phase (combined) was washed with sat. aq. NaHCO3 (30 ml) and brine (30 ml), dried (MgSO4) and evaporated. Recrystallization from ethanol yielded 0.72 g (34 %) of the trisubstituted imidazole 117c. Mp 223-224 ûC; IR (neat): 1443, 1247, 1176, 836, 756, 725, 692, 661, 610 cm-1; δH = 3.64 (3H, s), 4.99 (2H, s), 6.69 (2H, d, J=8.9), 6.90-6.94 (3H, m), 7.02 (1H, d, J=7.8), 7.09-7.14 (1H, m), 7.17-7.24 (3H, m), 7.40-7.46 (3H, m), 7.91 (1H, s), 11.44 (1H, s); δC = 47.5 (t), 54.9 (q), 103.6 (s), 111.9 (d), 113.4 (d), 118.8 (d), 119.5 (d), 120.7 (s), 121.6 (d), 126.4 (d), 126.6 (d), 126.7 (d), 127.0 (s), 127.3 (d), 128.1 (s), 128.4 (d), 136.1 (s), 137.6 (d), 137.97(s), 138.05 (s), 157.4 (s).

1-N-benzyl-4,5,6,7-tetrahydroindole, 130

NBn

During a period of 15 min α-chloroacrylonitrile (21.1g, 0.24 mol) was added to stirred solution of cyclohexylidenebenzylamine 128 (45.1g, 0.24 mol) and triethylamine (40ml, 0.29 mol) in acetonitrile (200 ml), while the tempertature was kept between 25 °C and 30 °C. After 1h of stirring at rt the reaction mixture was cooled to 5 °C and Et3N·HCl was filtered off and the solvent was evaporated. The residue was treated with ether (100 ml) and the solution was aged over night at 5 °C whereupon more Et3N·HCl was removed and the ether was evaporated to yield crude 129 as a yellow oil. This product was refluxed in ethanol (150 ml) for 2 days and then

36

evaporated to yield 46.64 g (90%) of crude N-benzyl-4,5,6,7-tetrahydroindole 130 as a brownish oil. Purifiction could be achieved by chromatography. The NMR data were in accordance with published data.97 δH (CDCl3) = 1.77-1.84 (4H, m), 2.45-2.49 (2H, m), 2.58-2.61 (2H, m), 5.00 (2H, s), 6.03 (1H, d, J=2.7), 6.61 (1H, d, J=2.7), 7.07-7.39 (5H, m); δC (CDCl3) = 21.9 (t), 23.40 (t), 23.45 (t), 23.7 (t), 50.1 (t), 106.6 (d), 117.8 (s), 119.5 (d), 126.7 (d), 127.3 (d), 128.1 (s), 128.8 (d), 138.7 (s)

1-N-benzyl-2-formyl-4,5,6,7-tetrahydroindole, 131

NBn

CHO

POCl3 (1.87 g, 12 mmol) was added to a stirred solution of the terahydroindole

130 (1.29 g, 6 mmol) in DMF (5 ml) at 0 °C. After 5 minutes at this temperature the reaction mixture was allowed to reach rt and then stirred for 16 h and then quenched by adding it to a suspension of ice/water (100 ml) and made alkaline by adding 2M KOH(aq). When all ice melted the mixture was extracted trice with EtOAc (50 ml) and the organic phase (combined) was washed with water (4x50 ml) and brine (50 ml) finally dried (MgSO4) to yield a brown oily crude product (1.32 g). Purification by dry flash cromatography yielded 0.89 g (61%) of the 2-formylpyrrole derivative as a yellow oil. IR (neat): 2931, 2853, 1650, 729, 695 cm-1; δH (CDCl3) = 1.70-1.85 (4H, m), 2.48-2.59 (4H, m), 5.58 (2H, s), 6.76 (1H, s), 7.03-7.05 (2H, m), 7.23-7.33 (3H, m), 9.45 (1H, s); δC (CDCl3) = 22.1 (t), 22.6 (t), 22.9 (t), 23.2 (t), 48.1 (t), 120.8 (s), 123.5 (d), 126.5 (d), 127.2 (d), 128.7 (d), 130.7 (s), 137.9 (s), 140.7 (s), 178.5 (d)

Benzyl-(1-benzyl-4,5,6,7-tetrahydro-1H-indol-2-ylmethylene)-amine, 132

N

N Bn

Bn Benzylamine (0.73 g, 7 mmol) was added to the 2-formyl pyrrole 131 (1.63 g, 0.73

mmol) in THF (20 ml) and stirred at room temperature for 16 h. The solvent was then evaporated to yield the crude benzyl imine 132 (2.17 g, 96 % yield). This imine was then used without further purification. IR (neat): 2927, 2828, 1632, 726, 694 cm-1; δH (CDCl3) = 1.74-1.84 (4H, m), 2.49-2.60 (4H, m), 4.65 (2H, s), 5.76 (2H, s), 6.42 (1H, s), 7.00-7.39 (11H, m) 8.19 (1H ,s); δC (CDCl3) = 22.1 (t), 23.0 (t), 23.1 (t), 23.5 (t), 48.1 (t), 65.2 (t), 116.0 (d), 118.8 (s), 126.41 (d), 126.43 (d), 126.7 (d), 127.5 (d), 128.2 (d), 128.4 (s), 128.5 (d), 135.3 (s), 139.3 (s), 140.7 (s), 153.5 (d)

1-Benzyl-2-(3-benzyl-3H-imidazol-4-yl)-4,5,6,7-tetrahydro-1H-indole, 133

NBn

N

N

Bn Triethylamine (0.96 g, 9.5 mmol) was added to a solution of benzylimine 132

(1.56 g, 4.7 mmol) and TosMIC (0.93 g, 4.7 mmol) in DCM (30 ml). The mixture was then heated at reflux for one day and then evaporated and purified by dry flash chromatography (hexane/ethyl acetate) to yield 2-imidazolyltetrahydroindole 133 1.27 g (72 % yield) as a yellow oil which solidified upon standing. Mp:97-103 ûC; IR (neat): 2924, 2850, 719, 694, 657 cm-1; δH (CDCl3) = 1.77-1.85 (4H, m), 2.42-2.46

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(2H, m), 2.55-2.60 (2H, m), 4.75 (2H, s), 4.83 (2H ,s), 6.01 (1H, s), 6.81-6.83 (2H, m), 6.92-6.95 (3H, m), 7.22-7.29 (6H, m), 7.50 (1H, s); δH (CDCl3) = 22.2 (t), 23.0 (t), 23.2 (t), 23.5 (t), 46.7 (t), 48.2 (t) 110.4 (d), 117.7 (s), 118.4 (s), 124.8 (s), 125.8 (d), 126.9 (d), 127.2 (d), 127.7 (d), 128.45 (d), 128.53 (d), 129.6 (d), 130.0 (s), 136.6 (s), 137.7 (d), 138.6 (s).

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7 Abbreviations AcOH acetic acid aq aqueous Bn benzyl Boc tert-butyloxycarbonyl (Boc)2O di-tert-butyl dicarbonate Bu butyl cat. catalyst CBz carbobenzyloxy conc. concentrate DBU diazabicycloundecan DCM dichloromethane DMAP 4-dimethylaminopyridine DMF N,N-dimethylformamide DMFDMA dimethylformamide dimethyl acetal DNA deoxyribonucleic acid E electrophile Et ethyl eq. equivalent Me methyl MeCN acetonitrile IR infrared mol. molecular NCS N-chlorosuccinimide NMR nuclear magnetic resonance NOE nuclear Overhauser effect Nuc nucleophile OAc acetate Ph phenyl PhH benzene PMP para-methoxyphenyl quant. quantitative RCM ring closing metathesis rt room temperature THF tetrahydrofuran TFAA trifluoroacetic anhydride Trt triphenylmethyl Tos para-tosyl TosMIC para-tosylmethylisocyanide p-TsOH para-toluensulfonic acid UV ultraviolet