Synthesis of 5 H-pyrido [ 4 , 3-b ] indole by a modification of Pomeranz-Fritsch isoquinoline synthesis

5H-Pyrido[4,3-b]indole was obtained from 3-formylindole in 16% overall yield by Jackson and Shannon modification of the Pomeranz-Fristch isoquinoline synthesis. The final cyclisation occurred but the removal of the tosyl group and oxidation of the dihydrocompound was not efficient. Changes in the concentration of the acid catalyst gave 29% as the best yield for the last step. An NMR study of the cyclisation is described.


Introduction
Pyrido [4,3-b]indoles, commonly known as γ-carbolines, have been studied for their biological activity, namely as antipsychotic and antitumor agents [1].They may be considered as smaller analogues of the ellipticine /olivacine anticancer agents [2] that contain indole and isoquinoline moieties One classical way of building the pyridine ring consists in starting from an arylaldehyde and apply the Pomeranz-Fritsch conditions.This method (cyclisation of an iminoacetaldehyde diethyl acetal) was improved by Bobbitt (hydrogenation of the imine to the corresponding amine) and later by Jackson and Shannon.These authors cyclised the tosyl derivative of this amine in smooth conditions: 6N HCl in dioxane [3].Although consisting of several steps, in general this method gives high yields and the cyclisation occurs in one pot.This method was applied successfully to synthesise ellipticine and olivacine derivatives [4].In the work described here, starting from 3-formylindole, the modification of Jackson and Shannon of the Pomeranz-Fritsch method was used to build the third ring of the γcarboline (scheme 1).

Results and Discussion
Azeotropic distillation of a mixture of 3-formylindole 1 with aminoacetaldehyde diethyl acetal in toluene gave the imine 2, as an orange oil, in quantitative yield.The 1 H-NMR spectrum showed a singlet at δ 8.45, corresponding to the ArCH=N, together with other signals due to the acetal side chain.Due to the instability of the imine it was used as such for the next step and it was reduced with NaBH 4 in methanol [5], to the amine 3, as colourless oil, in 85% yield.In its proton NMR spectrum, it was observed the replacement of the singlet at δ 8.45 ppm by a two proton signal at δ 4.06, corresponding to ArCH  In the NMR spectrum, which was obtained at room temperature, it was observed that about 12% conversion of the OEt groups, into ethanol, had occurred.A small amount of TosOH 6 was also present.After 15 minutes no signals due the acetal group were seen indicating that cyclisation occurred with the formation of the dihydro compound.An increase on the signals of TosOH (δ 2.2, s, CH 3 ; 7.4, d, and δ 7.6, d) was detected, which indicates the formation of the γ-carboline 8 in the same extent.The signal at δ 9.31 corresponding to H-1 of the γ-carboline is seen from 15 min.on, however it was overlapped by another signal.At 60 minutes both TosOH 6 and the intermediate dihydro 5 reached the maximum and no further changes were detected for compounds 5 and 6, later on.These observations led us to conclude that the dihydro compound 5 (70-75%) was the major product of the reaction and the yield for γ-carboline was assumed to be 25-30%.The results obtained in this study are shown in figure 1.The most polar product, the γ-carboline 8, was characterized by mp and 1 H and 13 C NMR. On the 1 H NMR spectrum a singlet at δ 9.3 and a doublet at 8.4 with coupling constant of 5.6 Hz, due to the pyridine ring were observed.The remaining expected signals were also present in both the proton and the carbon spectra, as it is described in the experimental part.The ESI mass spectrum showed the fragment m/z 169 (M+1) + .

Experimental
All melting points were measured on a Gallenkamp apparatus and are uncorrected. 1H NMR spectra were recorded at 300 or 400 MHz and 13 C NMR spectra were determined at 75.4 MHz on a Varian Unity Plus or on a Bruker Avance II 400 spectrometer.In the NMR spectra was used TMS as internal reference.Low resolution EI mass spectra were determined on a Unicam GC-MS 120.Elemental analysis data were obtained on a Leco CHNS-932.

Microwave heating
To a solution of the sulphonamide 4 (0.403 g, 0.97 mmol) in dioxane (10 mL) was added HCl (6N, 2mL) at 0-5ºC and the mixture was heated in the microwave for 15 min.The work-up was identical to that described above and gave a brown oil (93 mg) which was purified by column chromatography.Elution with mixtures of chloroform:ethanol of increasing polarity gave the sulphinamide 7, as a brown solid (15 mg, 10.0 %), m.p. 101-106ºC, with an NMR identical to the first sample obtained, and the pyridoíndole 8 as a brown solid (36 mg, 22%, m.p. 224-227 ºC).Its 1 H NMR was identical to that described above.

H NMR study of the cyclisation of 3-(2,2-diethoxyethyl-N-p-tosylaminomethyl)indole 4
The sulphonamide 4 (10 mg, 0.024 mmol) was dissolved in DMSO-d 6 (0.5 ml) in an NMR tube, 37% DCl (0.075 ml) was added and the spectrum was obtained.The reaction mixture was heated at 66ºC and the NMR spectra registered at 15, 30, 45, 60, 120, 180, 240 and 300 min.The results obtained in this study are discussed above and they are shown in figure 1. Approximate yields were drawn from data: the major product (70-75%) from the reaction was the dihydrocompound 5 and the minor product (25-30%) was the pyridoindole 8.

Cyclisation of 3-(2,2-diethoxyethyl-N-p-tosylaminomethyl)-indole 4 in the conditions from the 1 H NMR study
To a solution of the sulphonamide 4 (2.0 g, 4.8 mmol) in DMSO (10 mL) was added HCl (12N, 2mL) at 0-5ºC and the mixture was heated at 70ºC for 5 h.Water (100 mL) was added and a solid came out which was filtered and discarded.The aqueous layer was extracted with diethyl ether (4x30 mL), the combined organic extracts were dried (MgSO 4 ) and the solvent was removed to give p-toluenesulphinic acid 6 as a beige solid (43%).The aqueous layer was basified with ammonia (25%) until pH= 8 and extracted with ethyl acetate (4 x 50 mL).Drying (MgSO 4 ) and evaporation of the solvent gave a solid whose NMR spectrum showed to be a mixture of pyridoindole slightly contaminated with p-toluenesulphinic acid.The yield for the pyridoindole 8 was deduced from the spectrum as 29%.The fact that the yield is lower than that for the ptoluenesulphinic acid is possibly due to loss during extraction.

Figure 1 .
Figure 1.Results of concentration versus time, obtained by NMR study.