11

Contents

Contents 1

Papers included in this thesis 2

General introduction 3

Historical background 3

Structure and reactivity of indoles 5

2,2’-Biindolyls 9

Introduction 9

Reactions of 2,2’-biindolyl 13

Acid induced dimerization of 3-substituted indoles 17

Introduction 17

Results and discussion 18

Indolo[2,3-a]carbazoles 20

Structure and physiological activities 20

Protein kinase C 21

Biogenesis of indolocarbazoles 22

Synthesis 23

Results and discussion 26

Acid induced cyclization 31

Indolo[3,2-a]pyrrolo[3,4-c]carbazoles 34

Introduction 34

Reactions of indole with maleimides 34

Autoxidation of indolines 39

Additional acid induced cyclizations 42

Dimerization of indole-3-maleimides 42

Acknowledgements 44

References and notes 45

22

Papers included in this thesis

This thesis is based on the following papers, referred to in the text by roman numerals I-VI:

I. 2,2’-Biindolyl Revisited. Synthesis and Reactions.

Bergman, J.; Koch, E.; Pelcman, B. Tetrahedron, 1995, 51, 5631

II. Reactions of Indole-3-acetic Acid Derivatives in Trifluoroacetic Acid.

Bergman, J.; Koch, E.; Pelcman, B. Tetrahedron Lett., 1995, 36, 3945

III. 2,2’-Biindolyl. Reactions with Aldehydes.

Bergman, J.; Desarbre, E.; Koch, E., Manuscript

IV. Synthesis of Arcyriaflavin A.

Bergman, J.; Koch, E.; Pelcman, B., Manuscript

V. Synthesis of Indolo[3,2-a]carbazole in one Step from Indole and

Maleimide.

Bergman, J; Desarbre, E. ; Koch, E. Tetrahedron, 1999, 55, 2363

VI. Acid Induced Dimerization of 3-Indolylmaleimides. Formation of

Cyclopentindole Derivatives.

Bergman, J.; Janosik, T.; Koch, E., Manuscript

33

General introduction

Historical background

The chemistry1-3 of indole (1) began in the midst of the 19th century with

extensive research on the natural dye indigo (2), a violet-blue dye, imported to

Europe mainly from India since the 16th century. It is more well known as the

colour of blue jeans. This research resulted in the early development of the German

organic chemical industry, culminating in the development of a viable industrial

process for indigo, as well as the first preparation of indole in 18664 by zinc dust

distillation of oxindole.

N H

H N

O

O

N H

1 2

During the 1930’s it was discovered that a number of important natural products

contained indole moieties. The potent physiological properties of many of these

alkaloids have been utilized in traditional medicine, but now it added stimulus to

research, and many important indole syntheses were developed. During this

period, the essential amino acid tryptophan (3)5 was discovered, as well as the

plant growth hormone indole-3-acetic acid (4)6.

44

N H

CO2HH

NH2

N H

CO2H

3 4

Tryptophan is a constituent of most proteins, and serves in man and animals as a

biosynthetic precursor for a wide variety of tryptamine and other indole containing

metabolites; several of them of paramount physiological importance. Thus, the

hormone serotonin (5)7 is an important neurotransmitter, and melatonin (6) is

thought to control the day and night rhythms.

N H

NH2HO

N H

NHCOMeMeO

5 6

Serotonin is widely distributed in nature, but occurs only in low concentrations,

therefore, the laboratory syntheses of serotonin made it possible to study and

classify the family of serotonin receptors. These findings resulted in the design and

syntheses of useful pharmaceuticals, e.g. the highly selective sumatriptan (7) for

treatment of migraine, but also more notorious compounds, such as lysergic acid

diethylamide (LSD) (8).

N H

NMe2MeHNO2S

NMe

N H

H

Et2NOC

7 8

55

Structure and reactivity of indoles

Indole is a planar heteroaromatic molecule, with a benzene ring fused to the

b-face of the pyrrole ring. The numbering of the atoms of indole starts at the

nitrogen as shown in Figure 1.

N H

1

2

45

67

3

¨

Figure 1

Due to the delocalization of the nitrogen lone-pair into the �-system, indole is a

very weak base with a pKa value of -3.5. This means, for example, that you need a

strongly acidic solution (12 M H2SO4) to completely protonate indole.

Of the three possible cations, the 3-protonated 1b, is the thermodynamically

most stable, since it retains full benzene aromaticity (in contrast to the 2-protonated

cation 1c) with delocalisation over the nitrogen and the 2-carbon (in contrast to the

N-protonated cation 1a). Kinetically, however, the 1H-indolium cation is favoured.

N N N

H H

H

H

HH

H H

+ ++

1H -Indolium cation 3H -Indolium cation 2H -Indolium cation (formed fastest) (most stable)

1a 1b 1c

Due to the electron rich character of the heterocyclic ring, the indole chemistry is

dominated by electrophilic substitution in the 3-position, for the same reasons as

66

discussed above. However, all reactions of indoles do not lead simply to

substitution, which can be illustrated by considering the protonation of indoles.

3H-Indolium cations (1b) are electrophilic species and will react as such under

favourable conditions. In a reaction medium with partial protonation of the indole,

a 3H-indolium cation will be attacked by an unprotonated indole leading to

dimerization as well as trimerization, see scheme 1.

N H

HH

N H

N H N

H

H

N H N

H

NN HH H

NH

NH2

N H

H N

N H

NH2

¨

Indole-dimer

¨

-H+

Indole-trimer

H+

¨

¨

1b

Scheme 1

The indole dimerization is an example of a Mannich reaction, where the

protonated indole (1b) is the Mannich reagent, an immonium ion, which is a fairly

reactive electrophile.

77

The Mannich reaction has long been utilized in biomimetic natural product

synthesis8, particularly as the key step in domino reaction sequences. A classical

example is the synthesis of tropinone (9) by Robinson9 in 1917 by a Mannich

reaction involving succinaldehyde (10), methylamine (11) and acetone (12) (see

Scheme 2).

N

O

MeH

O

O

H

HNMe

MeNH2

HO

+ OH+ ¨¨

OH

10 11 12 9

Scheme 2

This thesis primarily deals with reactions where various indoles act as Mannich

reagents.

These studies have led to the synthesis of some naturally occurring

indolocarbazoles (13), as well as some 3,3’-disubstituted 2,2’-biindolyls (14) (paper

III and II).

H N

N H

N H

O O

N H

ORO

H N

ORO

13 14

88

In paper V and VI, intermolecular and intramolecular Mannich reactions have

been applied in a tandem fashion in the syntheses of indolo[3,2-a]carbazoles (15)

and the cyclopentindole (16).

N H

NH

H NO O

NH

NH

O

O

Ph

N

O

OPh

N H

15 16

The reactions involving 2,2’-biindolyl have led to several 3,3’-disubstituted

derivatives as well as more complicated compounds, e.g. 17 (paper I-II).

N

N

N

N

O

O

17

The aims which were defined when this work began were to finish a project

concerning formation of the naturally occurring indolocarbazoles (paper III-IV)

and to continue an ongoing investigation of the reactivity of 2,2’-biindolyls towards

different electrophiles (paper I-II).

99

2,2’-Biindolyls

Introduction

In our group we have developed a convenient synthesis of 2,2’-biindolyl (18) by

a modification of the original synthesis of Madelung10,11 (scheme 3).

N H

O

O

H N

C5H11ONa/ 360ÞC(26%)

or t-BuOK/ 300ÞC(80%)

N H

H N

18

Scheme 3

2,2’-Biindolyl (18) as a structural element is present in some natural products like

the indolocarbazole arcyriaflavin A (13), isolated from the slime mold Arcyria

nutans, and the sponge pigment fascaplycin (19). The two nitrogens in 2,2’-biindolyl

have also been exploited in the construction of various ligand systems,12 e.g. 20.

N N

N N

OMeMeO

MeOOMe

N H

N H

H N OO

N

N H

O

Ni

Cl-

13 19 20

Our primary interest in making 2,2’-biindolyl was based on a desire to

investigate its reactions with various dienophiles. On the basis of a simple

retrosynthetic analysis, arcyriaflavin A (13) should be directly accessible from

1010

2,2’-biindolyl and maleimide, by way of a Diels-Alder reaction followed by a

dehydrogenation step.

N H

N H

H N OO

N H

N H

H N OO

+

13

Scheme 4

This strategy has however in our hand proved to be full of difficulties, and

several research workers13-15 have reported modest yields, harsh reaction

conditions and/or tedious work up procedures. The loss of resonance energy on

formation of the [4+2] transition state should considerably increase the activation

energy, and the indolocarbazoles obtained by this route are generally believed to

be the result of a stepwise, rather than concerted, process.

A successful total synthesis of K-252a (21) due to Wood16 et al., is based on an

initial coupling of the diazolactam (22) with 2,2’-biindolyl which is followed by a

cyclization to the aglycon staurosporinone (23) (scheme 5). Wood has suggested

that the reaction pathway includes a thermal electrocyclization of an intermediate

3-vinyl biindolyl (24).

1111

N H

N H

R N

O N2

O

N H

N H

R N

O

O

R N

N H

N H

O

H N

NN

O

O

H3C

OHMeO2C

+

N H

N H

R N

O

HO

21

23

22

2425

Scheme 5

Later work by Pindur et al.17 has however, shown that high temperature

(o-dichloro benzene, 230�C) is necessary for this type of electrocyclization. In a

paper by Hudkins18, a lactam (27) regio-isomer is produced, as seen in scheme 6,

by condensation to the intermediate Michael adduct (26) using the same

temperature range which produced Wood’s staurosporinone (pinacolone, 120�C).

N H

N H

H N OO

N H

N H

NH

O

O

NH

N H

N H

O

TFA, 24h 120-125ÞC

26 27

Scheme 6

1212

In the original papers by Madelung11,19 several reactions of 2,2’-biindolyl (18)

with various electrophiles were investigated. However, as 18 is susceptible to

electrophilic substitution both at the nitrogen and at the 3-position, mixtures of

products were often encountered. Thus, treatment of 18 with acetic acid anhydride

gave mixtures of 1-acetyl- and 1,1’-diacetyl- 2,2’-biindolyl as well as 3-acetylated

products. Clean 3,3’-disubstitution could, in contrast, be achieved, when 18 was

reacted in hot benzoyl chloride giving 28. The magnesium-salt of indole are known

to give C-substitution in reactions with electrophiles, and this was found to be true

also for the magnesium-salt of 18. In this way 28 and 29 could be obtained from

benzoyl chloride and acetyl chloride, respectively, although the yield of the latter

was low.

N H

H N

N H

H N

O

O O

O

28 29

Madelung11 also claimed, that the condensation products typical for indoles

could be obtained when 18 was reacted with formaldehyde, acetaldehyde as well

as benzaldehyde, but no details were given.

1313

Reactions of 2,2’-biindolyl

When 2,2’-biindolyl was reacted with an excess (4 equivalents) of formaldehyde

in refluxing acetic acid a slightly yellow precipitate was collected after 15 minutes.

Mass spectral data featured a molecular ion (m/z=572 (70%)) and a fragmentation

pattern where two formaldehyde units are successively disconnected. The 1H-NMR

spectrum exhibited three different, unconnected, gem-coupling methylenes, no NH

signals and eight different signals from the indole rings. Based on these findings

we propose the following structure.

N

N

N

N

O

O

17

Due to the axis of symmetry the NMR-spectrum of 17 is simplified. The

formation of such a compound can be envisaged as a consequence of two

equilibrium reactions: substitution at the 3- and the 1-positions of the indole (see

Scheme 7).

1414

N H

H N

N H

H N

N H

N

HO

N H

N

O

N H

N

O

N

N

N

N

O

O

CH2O

+ H2O

CH2O

30

CH2O

+ H2O

CH2O

Dimerization

31

33

17

32

Scheme 7

The 3-position is favoured and 2,2’-biindolyl will react readily with one

formaldehyde unit. In the acidic solution, the intermediate carbinol will easily lose

water which will create a stabilized carbocation 30. The free 3-position of 30 is,

however, deactivated due to the conjugation with the positive charge on the

nitrogen, so an attack by the indole nitrogen on a second unit of formaldehyde is

favoured leading to 31. An intramolecular Michael-addition will then create the

seven-membered ring 32. Substitution in the free 3-position will form the

intermediate 33 which can dimerize to 17. The driving force in these equilibrium

reactions is the insolubility of the product.

In a publication by Pindur,20 2,2’-biindolyl was reported to form a five-

membered ring (34) when reacted with dimethyl acetylenedicarboxylate under the

influence of a Lewis acid catalyst. The structure was confirmed by X-ray

crystallography. In the light of this work, we reacted 2,2’-biindolyl with the

relatively electron rich anisaldehyde. The intermediate should be less deactivated

in the 3-position due to the resonance structure 35a, and in contrast to the

1515

resonance structure 35b a cyclization of 35a would be favoured according to the

Baldwin rules.21

OMe

N H N

H

OMe

N H N

H

N H

N H

MeO2C

CO2Me

+

+

34 35a 35b

In acetic acid, the reaction was sluggish and TLC-analysis of the reaction mixture

revealed a large number of products, but in acetonitrile with p-toluenesulfonic acid

in catalytic amount the reaction went smoothly and a white solid precipitated.

Mass spectral data featured a molecular ion (m/z=700), so the anticipated

product with a five membered ring could be ruled out .

The 1H-NMR-spectrum showed two different NH-signals and 13 signals in the

aromatic region as well as 12 aromatic CH and one aliphatic in 13C-NMR. Based on

these findings we propose the following structure (36).

H N

H N

N H

N H

ArHArH Ar = OMe

36

There are two possible isomers of this compound, but 36 can adopt a tub-shaped

conformation of the ten-membered ring which will have a good overlap within the

1616

bis-indolyl framework and the unsymmetry of the compound will give two sets of

indole signals in NMR. There is however, only one set of signals from the

p-methoxybenzene rings and the methines, but this can be explained by ring

inversion of the rather flexible ten-membered ring.

When p-nitrobenzaldehyde is reacted with 2,2’-biindolyl in acetic acid, an orange

precipitate can be collected in quantative yield. The product shows 8 signals from

the benzo ring of the indoles, a rather low singlet at 5.8 ppm, 1 singlet at 12.6 ppm

and a broad signal with an integral value of approximately 2, just below 12 ppm

and 4 different signals from the nitrobenzene ring with dd-couplings in 1H-NMR.

The 13C-NMR showed 2 quarternary carbons at 169.3 and 68.7 ppm respectively,

and a CH at 47.4 ppm. We propose the following structure (37).

N

N

H N

N H

NNOH

O

O

HO

37

The low shift of the unsubstituted 3-position can be explained by a

delocalization of the charge on oxygen and the broad signal at ~12 ppm would

arise from the exchangable nitronic acid protons. The relatively high quarternary

carbons would then be a result of the electron withdrawing character of the nitronic

acid group.

In contrast to these findings, several 3,3’-disubstituted 2,2’-biindolyls could be

obtained, by methods that are known to introduce substituents in the 3-position of

1717

an indole. Thus, the Vilsmeier reaction cleanly afforded 38 and the Mannich

reaction gave 39.

N H

H N

N H

H N

N H

H N

HO

HO

Me2N

NMe2

EtO2C

CO2Et

38 39 14c

To make 3,3’-diacetic acid derivatives of 2,2’-biindolyl we used a carbenoid

approach. Thus, 2,2’-biindolyl was reacted with 3 equivalents of ethyl diazoacetate

in refluxing xylene in the presence of copper, yielding the dimer (14c). In the light

of the work by Wood,16 it is noteworthy, that in our hands, rhodium catalysis gave

inferiour results.

Some of these 3,3’-substituted 2,2’-biindolyls can also be synthezised, by acid

induced dimerization of 3-substituted indoles, as will be discussed in the following

section.

Acid induced dimerization of 3-substituted indoles

Introduction

3-Substituted indoles will also dimerize readily when subjected to acidic

conditions, but the resulting dimer 40 is joined in the 2-positions. There is still a

controversy as to whether such substitutions proceed by direct electrophilic attack

at the 2-position, or by an indirect route involving an initial attack at the 3-position

followed by a rearrangement to the 2-position (see scheme 8).

1818

N H

RH

N H

R

N H

N H

R R

N H N

H

RR

N H

N H

R R

H

+ ¨

+

+

-H+

Rearrangement

-H+

40

Scheme 8

A driving force for such migration clearly exists as the aromaticity of the

heterocyclic ring thereby is restored (the final step) and the high migratory

aptitude of the indoline (in practice an α-amino alkyl group) towards electron

deficient centers greatly facilitates the rearrangement.

Results and discussion

When indole-3-acetic acid 41a was dissolved in trifluoroacetic acid (TFA) for 3h

at room temperature, we obtained the expected dimer 42a in 90% yield. The

corresponding diester 42b in the same manner formed from 41b could be isolated

in 95% yield using careful work-up procedures. The diester was formed as a single

diastereomer and later work by other groups22,23 has shown that the trans-isomer is

formed. Dehydrogenation of 42a-c with DDQ gave the 2,2’-biindolyls 14a-c.

1919

N H

ORO

N H

ORO

H

N H

ORO

H N

ORO

N H

ORO

H N

ORO

DDQ

dioxane

TFA, rt

a, R = Hb, R = Mec, R = Et

42a-c 14a-c41a-c

Scheme 9

The diacid 14a has been suggested to be the biologically active principle formed

from the plant hormone auxine, i.e. indole-3-acetic acid (41a).24 Of interest, in this

context, is the easy photo oxidation of 42b25 to the dimer 14b, which occurs at room

temperature under ambient light.

The amino diesters 42b-c showed a strong propensity to undergo lactamization

to 43b-c. Heating pure 14b above its melting point or gentle heating of 14b in

slightly acidified 2-propanol, completely converted 42b to 43b. This lactam shows

strong structural resemblance to the cytotoxic and anti-microbal fascaplycins,

especially homofascaplysin B (44).

N

OOR

H N

O

N

OOMe

H N

O

43b-c 44

2020

Indolo[2,3-a]carbazoles

Structure and physiological activities

N H

N H

H N

N H

N H

45 46

Indolo[2,3-a]carbazole 45 is a symmetrical ring system with an indole fused to

the a-face of a carbazole. Almost all of the known natural products of this class

possess an additional pyrrole ring annulated to the c-face of the carbazole ring

system and have the systematical name 1H-indolo[2,3-a]pyrrolo[3,4-c]carbazole 46,

but for simplicity, all of these ring systems will throughout this thesis be referred to

as indolo[2,3-a]carbazoles.

H N

NN

O

O

NHCH3

MeH

OCH3

H N

N H

N

O

Cl Cl

O

OOH

OCH3

OHHO

47 48

2121

Staurosporinone 47, the first indolo[2,3-a]carbazole to be isolated from Nature,

was initially obtained from26 Streptomyces staurosporeus and was found to exhibit a

wide range of extraordinary and in some cases, unique biological activities.27 Most

noteworthy is the fact that it is up to date the most potent inhibitor of protein

kinase C (PKC) which has been discovered. The structurally related antibiotic

rebeccamycin (48), isolated from the microbe Saccharothrix aerocolonigeneses, has

shown antitumor properties in vitro, but this antiproliferative activity can be linked

to topoisomerase I inhibition.

Protein kinase C

The enzyme system PKC is widely distributed in the tissues and organs of

mammals and other organisms, where it is involved in the transmission of external

signals to the interior of the cells, and thereby in the regulation of many cellular

processes by phosphorylation of a range of cellular proteins, some of them critical

for cell growth and differentiation (e.g. topoisomerase I and II). It has therefore, not

surprisingly, been suggested that PKC should be targeted for anticancer drug

design. Further studies have, however, shown that PKC is involved in many basic

cell processes beyond cell proliferation. Up to date, 12 different isoenzymes have

been identified and it has been shown that in a wide range of tissue diseases,

specific isoforms of PKC are overexpressed or subexpressed. This means that PKC

modulators, like the indolo[2,3-a]carbazoles are potential drugs against a wide

range of diseases, but the lack of selectivity towards different families of kinases as

well as towards the isoforms could induce severe side effects and have led to

caution in their therapeutic use. Many derivatives and synthetic analogues have

been prepared in order to studie the structure-activity reationship, to determine the

different parameters necessary to make more specific PKC inhibitors.

2222

Biogenesis of indolocarbazoles

Some work have been reported on the biogenesis of indolo[2,3-a]carbazole

natural products. Feeding experiments have indicated that staurosporine (47)28 is

produced from two intact tryptophan units. Labelling experiments29 have likewise

shown that two tryptophan units are involved in the biosynthesis of rebeccamycin

(48), but a feeding experiment with (15NH4)2SO4 indicates that the imide nitrogen in

rebeccamycin is not obtained from tryptophan. The authors suggest that

tryptophan may be converted to indole-3-pyruvic acid (49a), IPA, since precedents

exist for this biotransformation. IPA occurs predominantly in the corresponding

enol form 49b.

N H

O

OHO

N H

O

OHO

H

49a 49b

The co-isolation of several bisindolyl maleimides, such as arcyriarubin A (50),

and indolo[2,3-a]carbazoles, such as arcyriaflavins A (13) from the brightly

coloured slime mold Arcyria nutans, has led Steglich30,31 to propose that they are

biogenetically related according to scheme 10.

2323

H N

N H

N H

O OH N

N H

N H

O O

H N

N H

N H

O O

N H

H N

NH

OO

H NO O

O ON H

N H

H NO O

ON H

NH

dihydroarcyriarubins

[O][O]

[O]

[O][O] arcyriarubins (50)

arcyriacyaninsarcyriaflavins (13)

arcyroxocins arcyriaverdins

Scheme 10

Several of these steps have been duplicated in the laboratory.

Synthesis

There are several excellent reviews27,32,33 covering the synthetic efforts towards

indolo[2,3-a]carbazoles. Therefore, I will only give a brief survey of some of these

syntheses.

2424

The synthetic approach of Winterfeldt and Sarstedt34 is interesting as it

illustrates a biosynthetic model reaction. The amide 51, synthesized from

tryptamine and indol-3-yl acetyl chloride, was eventually transformed by an

intramolecular reductive coupling to the bisindole pyrrole 52. After deacetylation,

a final photocyclization yielded the staurosporine aglycon (53) (Scheme 11).

H N

N H

N H

OH N

N H

N H

O

OO

H N

N H

N H

OH N

N H

N H

O

H N

N H

N H

O

OHO

Ac N

N Ac

N Ac

O

OAcAcO

DDQ NaBH4

Ac2O DMAP

TiCl3

2. hυ

1. NaHCO3

51

53 52

Scheme 11

Bergman and Pelcman35 have developed a synthesis where the indoles are

assembled by a double Fischer indolization of the Diels-Alder cycloadduct 54. The

cyclization requires PPSE as the cyclizing agent, since the conventional methods

failed. A mixture of dihydroarcyriaflavin A (55) and arcyriaflavin A (13) was

formed and dehydrogenation of this mixture with Pd/C afforded pure

2525

arcyriaflavin A (13). In this fashion, about a dozen indolo[2,3-a]carbazoles have

been prepared.

TMSO

TMSO

NH

O

O

NH

O

O

TMSO

TMSO

H N

NN N H

N H

OO

H N

N H

N H

OOH N

N H

N H

OO

+

HH

tol ²

24h

3 eq PhNHNH2

MeOH HOAc ² 6h

Pd/C

diglyme ² 24h

68%

91%90%

+

PPSE MeNO2

54

13 55

Scheme 12

In a synthesis by Moody et al., 36 the indolo[2,3-a]carbazole is constructed via an

intramolecular Diels-Alder reaction followed by a nitrene-mediated ring closure.

2626

H N

N H

N H

N H

CO2Et

N H

CO2Et

H N

NO2

O

O

N H

O

H N

O

O

H N

N H O2N

O O

NO2

1. (COCl)2

2. ArCH=CHCH2NH21. KOH, MeOH 2. Ac2O

heat

(EtO)3P

76%

80%

42%37%

Results and discussion

As already discussed, the indolo[2,3-a]carbazole skeleton is derived from

tryptophan moieties and it is reasonable to assume that the a or the b bond are the

first to be formed biosynthetically.

H N

N H

N H

O Oa

b

c

In our biomimetic synthesis of arcyriaflavin A (13), the b bond is formed first by

oxidative coupling of the trianions of indole-3-acetic acid (41a) or the dianions of

the methyl ester (41b), as shown in scheme 13.

2727

N H

OOH

N

OO

N H

N H

OHO

OOH

N H

OOMe

N

OOMe

N H

N H

OO

N H

N H

O OO

1. 2 eq. n-BuLi

MeO

2 eq. LDA

1. 0.5 eq. I2

2. 1 eq. t-BuLi

1. 0.5 eq. I2

2. NaHSO3

41a 56a

Ac2O

57

2. NaHSO3

CH2N2

41b 56b

59

58

OMe

Scheme 13

The trianion (56a) was formed by sequential addition of 2 eq. n-BuLi and 1 eq. t-

BuLi to indole-3-acetic acid (41a). Addition of 0.5 eq. iodine to a solution of this

trianion in THF at -70�C, followed by acidic work-up gave the bisindole succinic

acid (57). The reaction mixture was treated, without any attempts to purify the

diacid, with diazomethane or acetic anhydride to give the diester 58, as a mixture

of diastereomers, or the anhydride 59, as a single diastereomer. However, the

2828

yields of 58 and 59 were not satisfactory (38% and 32% respectively). Fortunately,

the diester 58 could be obtained in a much higher yield (85%) by the iodine

promoted coupling of the dianion 56b, prepared from indole-3-acetic acid methyl

ester (41b) and lithium diisopropylamide (LDA).

Heating the diester 58 or the anhydride 59 with benzylamine gave the

succinimide 60 together with small amounts of the bisamide 61 (Scheme 14). The

imide was readily dehydrogenated with DDQ at room temperature to give the

bisindole maleimide 62. The indolo[2,3-a]pyrrolo[3,4-c]carbazole 63 was obtained

from 60, using two equivalents of DDQ and a catalytic amount of p-TsOH in

refluxing benzene.

N H

N H

N OO

Ph

N H

N H

NHHNOO

PhPh

N H

N H

N OO

Ph

N H

N H

N OO

Ph

+

76-80%

DDQ/ cat TsOH benzene/ rx

DDQ benzene/ rt

94% 90%

58 or 59

60 61

62 63

Scheme 14

2929

The dimer 58 was formed as a mixture of the dl-pair (58a) and the meso (58b)

form. On trituration with dioxane one of them crystallizes in pure form. When the

mixture was reacted with ammonium formate in refluxing triglyme, only one of

the two diastereomers cyclized to the succinimide. The other diastereomer,

identical with the one obtained from trituration with dioxane, could be regained

from the reaction mixture. For sterical reason we hypothesized that this would be

the less crowded imide (64a), formed from the dl-pair (58a) of the diester.

N HN

H

H

MeO OMeOO

N HN

H

HH

MeO OMeOO

H N

N HN

H

OO

H

58a 58b 64a

This was confirmed by an independent synthesis of the succinimide 64b from

arcyriarubin A (50) (see scheme 15).

N H

N H

H N OO

N H

N H

H N OO

HHH2/Pd/C, r.t.

DMA

50 64b

Scheme 15

The succinimide 64b was not identical with our sample. The 1H-NMR spectrum

exhibited a singlet from the succinimide 3- and 4-position at 4.89 ppm (64a, 4.56) as

3030

well as considerable differences in the aromatic region. In a publication by Davis et

al.37 the imide 64a was claimed to be formed by reaction of the indole Grignard

reagent (65) with 3-bromo maleimide (66) (scheme 16).

H N

N HN

H

OO

N MgI

H N OO

Br

+

65 66 64a

Scheme 16

The spectral data they reported for 64a did not match ours. However, no 13C-

NMR spectrum was given and the singlet for the 3- and 4-H of the succinimide was

surprisingly low (3.52 ppm). We propose that the compound Davis obtained

actually is 67a. We have prepared the N-benzyl derivative of this compound (67b)

from 68 and the methylene of this succinimide displays a singlet at 3.63 in the 1H-

NMR spectrum.

R N

N H

OO

N H

N H

N H

O

OEtO

OEt

a, R = H

b, R = Bn

67a-b 68

Thereby we have conclusively ascertained the stereochemistry of the imide 64a

and from that deduced that the pure dimer obtained from dioxane crystallization

of 58 is the meso-form (58b).

3131

N

N HN

H

OO

Ph

HH

N HN

H

HH

MeO OMeOO

O

N HN

H

OO

HH

58b 60b 59

Heating the pure diastereomer 58b or the anhydride 59 with benzylamine gave

the same diastereomerically pure succinimide 60b, indicating that the anhydride

formed has meso-structure.

H N

N H

N H

OO

HH

H N

N H

N H

OOH N

N H

N H

OO

64a 50 13

The imide 64a could be dehydrogenated by DDQ to arcyriarubin A (50), but we

could not obtain arcyriaflavin A (13) with the same methodology which gave the

indolocarbazole 63.

Acid induced cyclization

As the dimer 58 has two unsubstituted 2-positions it should be able to undergo

intramolecular acid promoted ring closure. Indeed, treatment with TFA of the

diastereomerically pure 58b, yields the tetrahydro-indolocarbazole (69). Gentle

heating of the TFA-solution containing 69, furthermore gave the known38 diester

70, which might be attributed to the oxidizing ability of TFA.39

3232

N HN

H

HH

MeO OMeOO

N H

N H

MeO OMeOO

N H

N H

MeO OMeOO

TFA TFA

r.t. ²

58b 69 70

The TFA-treatment gave at room temperature a single product and analysis of

the NMR-spectrum, including NOE, indicates the structure 69b. In particular the

large coupling constant between H2 and H3, and the absence of NOE enhancement

between these hydrogens, suggest a trans-configuration. The NOE enhancement

between H3 and H4 strongly implies that the cis-configuration is not lost during the

cyclization. So does the fact that the diastereomeric mixture of 58 gave three

different cyclized products when treated with TFA.

N H

N H

O

OMeMeO

O

H3H4

H2

H1

11%

14%

HH4%

4% J12= 7.8 Hz J23=11.5 Hz J34= 5.3 Hz

69b

The imide 64b also gave a tetrahydro indolocarbazole (71b), as a single

diastereomer, when treated with TFA. Arcyriaflavin A (13) was obtained by

refluxing the TFA-solution of 71b for 8 hours (see scheme 17).

3333

N H

N H

H N OO

N H

N H

H N OO

N H

N H

H N OO

HHTFA, rt ²

64b 71b 13

Scheme 17

Van Vranken40 utilized the difference in reactivity of the indoline linked to the

indole in the syntheses of the unsymmetrical indolo[2,3-a]carbazole tjipanazole I

(72) (see scheme 18).

N H

N H

N H

N H

HH

N H

N H

Br

NN H

Br

TFA

rt (97%)

NBS

DMF (73%)

O

HO

OH

OH

3 eq. D-Xylose

MeOH, rx (82%)

1. DDQ, dioxane (73%)

2. CuCl, DMF (83%)

NN H

Cl

O

HO

OH

OH

72

Scheme 18

3434

Indolo[3,2-a]pyrrolo[3,4-c]carbazoles

Introduction

For the arcyriaflavin A (13) several syntheses have been developed27 but

surprisingly little has been done to synthesize isomeric structures. Bergman and

Desarbre have prepared indolo[2,3-c]carbazoles (73) from 3,3’-biindolyl41 and the

alkaloid arcyriacyanin A (74) has been synthesized by Steglich et al.42

H N

NHHN

OO

N H

H N

NH

OO

N H

NH

H NO O

73 74 15a

We have prepared 15a and a few derivatives, in a one step reaction starting from

indole and various maleimides.

Reactions of indole with maleimides

Although a few scattered examples of acid induced additions of indoles to

maleimides yielding (indol-3-yl)-3-succinimide 75 had been published from 1962

and onwards, it was not until 1997, when Macor43 published a study of this

Michael type addition, that the generality of this reaction was recognized. The

conditions used were refluxing glacial acetic acid, with maleimide in excess, which

afforded Michael adducts such as 75 in high yields.

3535

However, when we increased the ratio of indole to maleimide another product

with the composition C27H17N3O2, not observed by Macor, eventually became

predominant.

Thus when two equivalents of indole 1 and one equivalent of e.g. N-

benzylmaleimide 76c, were reacted in glacial acetic acid at 100�C, a yellow

precipitate was collected after 72 h. Its structure 15c was assigned based on the

following data. The mass spectrum featured the molecular ion (m/z=415) as the

base peak. The 1H-NMR spectrum exhibited two different indolic NH signals and

the 13C-NMR data featured two carbonyl signals at 168.4 and 169.2. The previously

described Michael adduct 75c was present in the mother liquor (Scheme 19).

NH

N H

R N OO

N H

R N OO

N H

R N OO

HOAc ++a, b, c, d,

R = H R = Me R = Bn R = Et

1 76a-d 15a-d 75a-d

Scheme 19

The structure of the indolo[3,2-a]pyrrolo[3,4-c]carbazoles 15a-d were finally

confirmed by two independent syntheses, both starting with 2,3’-biindolyl as

outlined in scheme 20.

3636

N H

N H

N H

NH

CO2MeMeO2C

NH

N H

R N OOR

N OO

NH2RDMAD

Scheme 20

Further studies showed that the ratio between the indolocarbazole 15 and the

Michael adduct 75 was dependent on the temperature and could also be regulated

by the ratio between indole and maleimide (see Table 1).

Indole

(mmol)

Maleimide

76b (mmol)

Temperatur

e

Indolocarbazole

15b (ratio)

Michael adduct

75b(ratio)

1 3 90�C 1 9

1 3 117�C 1 43

2 1 90�C 1 2

2 1 117�C 1 13

3 1 100�C 1 1.6

3 1 95�C 1 0.9

Table 1

The ratios were taken from the NMR spectra of the crude reaction mixtures. The

first 4 entries illustrate the effect of the temperature on the outcome of the

3737

reactions. However at 90�C the reaction became inconveniently slow, and an

optimum, both in yield and ratio of indolocarbazole, was found at 95�C.

Scheme 21 is a rationalisation of the observed chemistry, in which the two

products are formed in competing reactions from a common intermediate, 78,

which can either deprotonate to the Michael adduct or react with a second

molecule of indole to form the crucial 2,3’-bond in 79, which eventually proceeds to

the hypothetical intermediate 80. The Michael adduct 75b failed to react with

indole under acidic conditions (HOAc, 100�C) indicating that the reaction is under

kinetic control, i.e. the Michael adduct does not equilibrate and it is not an

intermediate. The dehydrogenated maleimide 81b also failed to give the

indolocarbazole (15).

N H

R N OO

H

NH

N H

R N OO

N H

R N OO

Indole

NH

N H

R N OO

NH

N H

R N OO

H HH

H

15

75b

78

8079

1. dehydrogenation

2. cyclization

dehydrogenation

N H

R N OO

81b

IndoleIndole

Scheme 21

3838

Attempts to facilitate the dehydrogenation steps by adding Pd/C to the reaction

mixture were unsuccessful; no indolocarbazole was formed.

Here it might be added that it is known that maleimide can yield adducts (with

e.g. 1,5-dihydroflavin) that subsequently disintegrate to succinimide and

dehydrogenated products. (see Scheme 22)

N

N

N

N

CH3

H

O

O

N R

X

OO

N

N

N

N

CH3

H

O

O

X

N R

OO

N

N

N

N

CH3

O

O

X

R NO O

H+

--+

Scheme 22

It is, however, clear that the dehydrogenation of the presumed intermediates 79

and 80 is not effected by the maleimide as neither succinimide, nor any other

reasonable hydrogenated species thereof were found in the reaction mixture and

the mass balance did not show any lack of maleimide. We propose that the

oxidation is effected by air.

Attempts to isolate any of the postulated hydrogenated intermediates failed but

in the much faster reaction, of 2,3’-biindolyl with N-ethyl maleimide, the

tetrahydro-indolocarbazole, 80d, as well as the Michael adduct 82d, could easily be

obtained (see Scheme 20 and Scheme 23).

3939

N H N

H

N H

N H

Et N

N H

NH

OOEt N OOEt

N OO

Et N

N H

NH

OO

HOAc 100ÞC 5 min

+

80d82d

15d

Scheme 23

The fact that both 80d and 82d yielded the indolocarbazole 15d (100�C, HOAc)

is in harmony with our presumed reaction mechanism in scheme 21 and in these

reactions there is no other oxidizing agent present, except air.

Autoxidation of indolines

In a paper by Van Vranken et al.25, the photo oxidation of 42b was investigated.

The photo oxidation is sluggish under ambient light, affording 50% conversion

after 11 days. However, irradiation with a 150 W lamp effects complete conversion

after 20 h. Solvent is an important parameter in the reaction and chlorinated

solvents are most effective. Peroxides are observed in increasing amounts as the

reaction progressed and in the absence of air, no reaction was observed.

4040

N H

H N

MeO2C

CO2Me

N H

H N

MeO2C

CO2Me

ambient light

CHCl3

42b 14b

Scheme 24

We suggest that the apparent ease of oxidation can be explained by the

stability of an intermediate radical as indicated in figure 2.

N H

H N

MeO2C

CO2Me

N H

H N

MeO2C

CO2Me

N H

H N

MeO2C

CO2Me

Figure 2

The free radicals proceed by direct combination with oxygen, thereby producing

peroxides which by themselves will promote the reaction. Autoxidations are

known to be initiated by ultraviolet light due to the absorbation of enough energy

to effect the necessary homolysis and chlorinated solvents are also effective in

generating radicals.44 This is in harmony with our observation that 69 is easily

oxidized in TFA (scheme 25), which is also an excellent solvent for the generation

of radical cations45,46 and is itself an effective one-electron oxidant39.

4141

N H

N H

CO2MeMeO2C

N H

N H

CO2MeMeO2C

40-50ÞC

TFA

69 70

Scheme 25

We suggest that the same mechanism is in action in the formation of the

intermediate 82d as well as in the final oxidation of 80d to form the

indolo[3,2-a]carbazole 15d.

N H

N H

Et N

N H

NH

OOEt N OO

N H

N H

Et N OO

[O] [O]15d

H+

79d 82d 80d

4242

Additional acid induced cyclizations

Dimerization of indole-3-maleimides

A long time ago, Bergman47 prepared the bisindole 68 by simply refluxing

(HOAc) the commercially available sodium salt of diethyl oxaloacetate with 2

equivalents of indole. When the diester 68 was refluxed in benzylamine for 12 h the

succinimide 67b was produced.

HNN H

EtO2C CO2Et

HNN H

N OO

Ph

68 67b

Our intention now was to provoke acid induced cleavage yielding indole and

81c, which might either dimerize or possibly add indole again forming a desired

60, which in turn might cyclize under the acidic conditions to a tetrahydro

indolocarbazole.

N H

NO O

Ph

N H

NO O

Ph

N H

81c 60

4343

However, the outcome of the treatment of 67b with the strong acid TFA, was an

unexpected product with the composition C38H28N4O4, which is composed of two

different indole units and two different benzyl groups, and featured 2 quarternary

signals in the aliphatic region of the 13C-NMR spectrum, as well as 3 CH2-groups.

We propose the structure 16 and the rationalization of the events leading to this

structure is outlined in scheme 26.

NH

NH

O

O

Ph

N

O

OPh

N H

N H

NO O

Ph

N H

HNN H

N OO

Ph

N H

NO O

Ph

N H N

H

H

H+

+

H+

83

81c

16

67b

Scheme 26

In harmony with this presumed reaction pathway is the fact that the indole

maleimide 81c, when treated in TFA at 25�C, yielded within 2 minutes a

quantitative yield of 16. Interestingly enough, compound 16 can be considered as

an analogue of the bisindole alkaloid yuehchukene 83.48

4444

Acknowledgements

First of all I would like to express my gratitude to my supervisor for introducing

me to this field of research. Your friendly guidance and deep knowledge of

chemistry has been truly appreciated.

I also wish to thank the former head of the department of organic chemistry at

KTH, professor Torbjörn Norin.

Daniel, Tomasz, Thomas, JB and my Father are gratefully acknowledged for proof

reading and comments on the contents of this thesis.

A special thank to my co-authors, Benjamin Pelcman, Eric Desarbre and Tomasz

Janosik.

Mamma, you are an inspiration! Thanks for all the late Fridays at the lab.

A lot of people have made these last months a lot easier by taking so good care of

Emil. Uwe, Auli, Pappa, Fredrik, Maria, Johanna, Mats, Ylva. Tack!

Daniel, for all the support (=huggings) and for taking care of everything at home.

Thank you! I owe you one.

All friends and collegues at the department and at Novum, for making all of this

fun! Especially: Peo, the best AK-assistent, ever! Hans V, for all the advices I never

followed. You were right! Ingvor and Lena, for taking care of me, Solveig for the

Black coffe, Daniel and Anette for being such good hood-mates, Dr(?!) Jocke,

Thomas, Göran and Ulf B, for all the non-chemistry talk, Pelle for the Baden-baden,

Tomasz for being Tomasz, Kerstin N and Magnus C for keeping me informed and

Nathalie for her big smile!

Financial support from the foundation Bengt Lundqvist minne is gratefully

acknowledged.

4545

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