A new synthesis of some 2,4’-biindolyls

New 2,4'-biindolyls have been synthesized using new methodology enabled by the regioselective reactivity of methyl 5,7-dimethoxyindole-2-carboxylate, which undergoes a modified Vilsmeier-Haack reaction at C4 with indolin-2-one in the presence of phosphoryl chloride. Selective formylation and acylation with oxalyl chloride of the resulting 2,4’-biindolyl were achieved at C4, and the resulting glyoxyloyl chloride was converted to related glyoxylic amides. Removal of the ester substituent at C2 of the 2,4’-biindolyl produced a more reactive biindolyl, which underwent sequential mono-formylation at C3' and then at C3. The selective reactivity of the biindolyls is attributed to conjugation between the two indole moieties, and evidence for the degree of conjugation was observed via 1 H NMR spectroscopy.


Introduction
The 2,4'-biindolyls have received considerable attention, predominantly in connection with tryptophan tryptophylquinone (TTQ) 1 (Figure 1).TTQ is a novel o-quinone cofactor found in bacterial methylamine dehydrogenases (MADH) and aromatic amine dehydrogenase (AADH) and is tightly associated in the enzyme matrix through a peptide linkage.TTQ is derived from two tryptophan residues at the enzyme active site and is the redox centre of MADH and AADH, catalysing the oxidation of primary amines to the corresponding aldehydes and ammonia. 1 The 2,4'-biindolyl structure is also present in natural products such as the blue-green pigment arcyriacyanin A 2 (Figure 1) isolated from the slime mould Arcyria obvelata (= A. nutans, Myxomycetes). 4-6 Steglich and co-workers reported three strategies for the synthesis of arcyriacyanin A 2, including one wherein the preparation of the parent 2,4'-biindolyl 3 (Figure 1) was achieved via Stille coupling of a stannylindole with 1-tosyl-4-bromoindole. 6Murase and co-workers 5 also prepared the parent 2,4'-biindolyl 3, via a palladium catalysed cross-coupling reaction of an indolylborate and a 4-iodoindole.Both methods require N-protection and deprotection during the preparation of the 2,4'-biindolyl.In both groups the work was then directed towards the coupling of the Grignard reagent of 2,4'-biindolyl 3 with N-protected dibromomaleimide to give arcyriacyanin A 2. Itoh and co-workers synthesized 2,4'-biindolyl 4 (Figure 1) via a Fischer indolization of the related 4-propionylindole. 7Biindolyl 4 was then converted into the indolequinone derivative 5 (Figure 1), which was studied as a model for the organic cofactor TTQ of bacterial amine dehydrogenases. 1  In the earlier literature 2,4'-biindolyls were reported by Prota and co-workers in the context of melanin and eumelanin structures and biosynthesis.5,7-Dihydroxyindole was found to undergo dimerization to form the related 2,4'-biindolyls via photo-oxidation, autoxidation, and enzymic oxidation. 8-105,5'-Dihydroxy-2,4'-bitryptaminyl 6 (Figure 1) was observed as a minor constituent of four dimeric products formed during the controlled potential electrochemical oxidation of millimolar concentrations of 5-hydroxytryptamine (5-HT) in aqueous solutions at pH 2, 11 while a trimer was isolated from the electrochemical oxidation of 5,6dihydroxytryptamine (5,6-DHT). 12More recently Prota and co-workers examined the oxidative degradation of these 2,4'-biindolyls to form substituted pyrroles, as part of their study of the mechanism of pigment breakdown of mammalian skin, hair, and eyes. 13 The selective electrophilic substitution of methyl 5,7-dimethoxyindole-2-carboxylate 7 14,15 was used to prepare a 2,4'-diindolyl compound and the reactivity of this compound towards electrophilic substitution was investigated.Attempts were also made to bridge the C3 and C3' positions of the 2,4'-biindolyl compound.

Synthesis of Methyl 5,7-dimethoxy-4-(indol-2'-yl)indole-2-carboxylate 9
Numerous imines have been prepared by the modified Vilsmeier-Haack reaction, in which dimethylformamide was replaced by secondary amides such as anilides, pyrrolidinones, piperidinone, and 4,4-dimethyl-2oxazolidinone. 16,17Bergman and Eklund have also reported the preparation of 2,3'-biindolyls by combination of 1-methylindole and 1-methylindolin-2-one in the presence of phosphoryl chloride. 184,6-Dimethoxyindoles with substituents at either or both of C2 and C3 have been reacted with indolin-2-ones in the presence of either phosphoryl chloride or trifluoromethanesulfonic anhydride (triflic anhydride) to give biindolyls, the regiochemistry being determined by the most reactive position available. 19,20Generally the indole and indolin-2-one were refluxed together in chloroform with phosphoryl chloride.A certain level of reactivity is necessary, as dimethyl 4,6-dimethoxyindole-2,3-dicarboxylate (with two deactivating ester groups) failed to react with indolin-2-one 8 in the presence of phosphoryl chloride: however, when triflic anhydride was employed the desired 2,7'-biindolyl was obtained in 70% yield. 19 We have previously reported on the general reactivity of methyl 5,7-dimethoxyindole-2-carboxylate 7 and disclosed that this indole undergoes selective electrophilic substitution at C4. 14,15 Thus a 2,4'-biindolyl would be expected to form from the condensation of indole 7 and indolin-2-one 8.A solution of the indole 7 and indolin-2-one 8 in chloroform was heated under reflux with two equivalents of phosphoryl chloride for days without reaction.The approach using triflic anhydride was also attempted but gave no reaction after several days at room temperature.However, under forcing conditions, indole 7 and indolin-2-one 8 in neat phosphoryl chloride at 60 ˚C for 2.5 hours gave biindolyl 9 as fine yellow needles in quantitative yield (Scheme 1).The 1 H NMR spectrum (CDCl 3 ) of biindolyl 9 included a doublet at 7.63 ppm, corresponding to H3, and showing the expected coupling with the NH proton of the dimethoxyindole nucleus, indicating that substitution had occurred at C4.The singlet resonance at 6.63 ppm, corresponding to H6, also confirmed substitution at C4.It is also interesting to note that of the two indole NH signals, the broader resonance at 9.43 ppm corresponds to the NH of the less substituted indole nucleus and is shifted downfield due to hydrogen bonding to the nearby methoxy group.Another consequence of the hydrogen bonding is broadening and a decreased height of the methoxy singlet at 3.92 ppm.In comparison with the preparation of 2,4'biindolyls 3 and 4 described above, the preparation of biindolyl 9 involved a one-pot procedure and did not require protection of the indole nitrogen.

Electrophilic substitution of methyl 5,7-dimethoxy-4-(indol-2'-yl)indole-2-carboxylate 9
A consideration of the reactivity of methyl 5,7-dimethoxyindole-2-carboxylate 7 shows that C4 is more reactive than C3 towards electrophiles. 14,15This can be accounted for by the activation of C4 by the methoxy groups and the deactivation of C3 by the ester group.In comparison, biindolyl 9 also possesses two competing reactive sites, C3 located on the 5,7-dimethoxyindole nucleus and C3' located on the less substituted indole nucleus.The dimethoxyindole nucleus is electron rich due to the presence of the methoxy groups, however the C3 is deactivated by the ester at C2. On the other hand, C3' of the less substituted indole nucleus is also activated by the methoxy groups and the nitrogen atom of the dimethoxyindole nucleus at C2' and also deactivated by the ester.Formylation of compound 9 was achieved using mild conditions to give only the 3'aldehyde 10 in quantitative yield after careful workup (Scheme 2).Presumably there is also a steric effect supporting this selectivity.The 1 H NMR spectrum (CDCl 3 ) of 3'-aldehyde 10 includes an aldehyde resonance at 9.87 ppm along with two indole NH resonances.In addition, a doublet at 7.10 ppm is assigned to H3, coupled to the dimethoxyindole NH.A comparison of their NMR spectra shows that the 3'-aldehyde 10 shows weaker hydrogen bonding between the less substituted indole NH and the nearby methoxy group than does the biindolyl 9.This indicates that the two indole nuclei of 3'-aldehyde 10 are twisted out of plane with respect to each other, thus decreasing conjugation between the two indoles: consequently the formyl group at C3' would have less of a deactivating influence on the 5,7-dimethoxyindole nucleus of biindolyl 10.During the workup process, after basification, if the mixture was allowed to warm to room temperature, the formylated product 10 was observed to dissolve and upon acidification the hydrolysed free acid product 11 was isolated in 88% yield (Scheme 2).Attempted diformylation of biindolyl 9 either directly or via the formylation of 3'-aldehyde 10 failed, even with the use of phosphoryl chloride as solvent at 60 C, while extended reaction times using these forcing conditions only resulted in decomposition.The biindolyl 9 was converted to the glyoxylic amides 13 and 14, via the glyoxyloyl chloride 12, which resulted from reaction with oxalyl chloride (Scheme 3).Substitution of the biindolyl 9 by oxalyl chloride at C3 was not observed, neither was cyclization of the 3'indolylglyoxyloyl chloride 12 on to the C3 position of the dimethoxyindole ring.

Preparation of 5,7-dimethoxy-4-(indol-2'-yl)indole 16
It was of interest to prepare the less-substituted 5,7-dimethoxy-4-(indol-2'-yl)indole 16 so as to increase reactivity at C3. Thus biindolyl 9 was refluxed in aqueous ethanolic potassium hydroxide to give the carboxylic acid 15 in quantitative yield.A mixture of the carboxylic acid 15 and copper powder in quinoline was then heated at reflux to give the 5,7-dimethoxy-4-(indol-2'-yl)indole 16 in 70% yield (Scheme 4).A comparison of the 1 H NMR spectra (CDCl 3 ) of biindolyl 9 and biindolyl 16 reveals that the dimethoxyindole NH resonance of biindolyl 16 is shifted upfield with respect to that of the dimethoxyindole NH of biindolyl 9 due to the absence of hydrogen bonding with the ester at C2.There is no significant change in the chemical shifts of the less substituted indole NH protons in the two biindolyl compounds.Also, the hydrogen bonding interaction between the less substituted indole NH and the C5 methoxy group of the 5,7dimethoxyindole nucleus is observed in the NMR spectra of both biindolyls 9 and 16, where the C5 methoxy group resonance is approximately 60% of the height of the C7 methoxy group resonance.This indicates that the two indole rings are co-planar and conjugated, thus activating the less substituted indole nucleus towards electrophiles.

Formylation of 5,7-Dimethoxy-4-(indol-2'-yl)indole 16
The Vilsmeier formylation of biindolyl 16 proceeded quickly using mild conditions to give the 3'-aldehyde 17 as a yellow powder in 91% yield (Scheme 5).No other product was observed.This result confirms that the biindolyl 16 is more reactive than biindolyl 9, because of the absence of the ester group.Moreover, C3' located on the less substituted indole nucleus is more reactive than C3 located on the 5,7-dimethoxyindole nucleus.The 3'-aldehyde 17 was found to decompose in chloroform over a short period of time to produce intractable tars.However, a 1 H NMR spectrum in acid-free CDCl 3 showed an aldehyde signal at 9.94 ppm.The dimethoxyindole NH resonance was observed at 8.40 ppm, unchanged from that of the biindolyl 16 from which it was derived.The less substituted indole NH resonance was observed at 8.80, significantly upfield from that of biindolyl 16, suggesting that there is less interaction between the NH and the C5 methoxy group, and therefore less conjugation between the two non-coplanar indole rings.The observation of two methoxy signals of equal height is consistent with this situation.Examination of the 1 H NMR spectrum (DMSO-d 6 ) of 3'aldehyde 17 revealed two multiplets, coupled to each other, corresponding to H2 and H3, and consistent with formylation at C3' rather than C3.
The biindolyl 16 could be diformylated under relatively mild conditions to give the 3,3'-dialdehyde 18 as an orange powder in 59% yield.This compound was also found to decompose quickly in chloroform.The 1 H NMR spectrum in DMSO-d 6 displays two aldehyde resonances and two indole NH resonances, while H2 appears as a doublet at 8.03 ppm, showing coupling to the dimethoxyindole NH.Because of its instability, the 3,3'dialdehyde 18 was fully characterized as its stabilized N,N'-diBoc derivative 19 (Scheme 5).Treatment of the 2,7'-biindolyl 20 with phosphoryl chloride and dimethylformamide has been reported to undergo formylation at C3' to give the indolopyrroloquinoline 22 in 40% yield presumably via the C3'-aldehyde 21 (Scheme 6).Attempts to bridge C3 and C3' of biindolyl 16 in order to achieve structures related to arcyriacyanin were unsuccessful.Sequential treatment of biindolyl 16 with methyl magnesium iodide and 3,4dibromomaleimide 21 left the biindolyl 16 unchanged.The reaction failure could be partly due to the interference from the oxygen atoms of the methoxy groups, which could complex with methyl magnesium iodide.However, attempts to react biindolyl 16 with 3,4-dibromomaleimide, via a Heck reaction were also unsuccessful.Treatment of biindolyl 16 with dimethyl acetylene dicarboxylate 18 at 205 C gave a complex mixture of products and more selective reaction could not be achieved.

Conclusions
In conclusion, the interesting 2,4'-biindolyl system 9 has been synthesized from methyl 5,7-dimethoxyindole-2-carboxylate 7 and was found to undergo regioselective reaction at C3'.The presence of an activating 5,7dimethoxyindole substituent at C2 induced reactivity at the C3' position of the less substituted indole of biindolyl 9, while on the other hand, the presence of the ester at C2 deactivated the C3 position of the dimethoxyindole of biindolyl 9.The degree of coplanarity of the two indole rings is presumably a controlling factor in the stepwise formylation of 2,4'-biindolyl 16.

Experimental Section
General. 1 H and 13 C NMR spectra were recorded on a Bruker AC300F ( 1 H: 300MHz, 13 C: 75.5 MHz) or a Bruker AM500 spectrometer.The chemical shifts (δ) and coupling constants (J) are expressed in ppm and hertz respectively.Carbon attribution C, CH, CH 2 and CH 3 were determined by 13 C, DEPT and HMQC experiments.Infrared (IR) spectra were recorded on a Mattson Genesis Series FTIR spectrometer using potassium bromide disks.Ultraviolet and visible (UV/Vis) spectra were recorded in tetrahydrofuran using a Carey 100 spectrometer.Mass spectra were recorded on a VG Quattro MS (EI) or a Finnigan MAT (MALDI).High resolution mass spectrometry (HRMS) was carried out at the Research School of Chemistry, Australian National University.Melting points were measured using a Mel-Temp melting point apparatus.Microanalyses were performed at the Campbell Microanalytical Laboratory, University of Otago, New Zealand.Column chromatography was carried out using Merck 230-400 mesh silica gel or Merck 70-230 mesh silica gel, whilst preparative TLC was performed using Merck 60GF 254 silica gel.