Transformation of Indole Alkaloids. VI. A Novel Conversion of Oxindole Alkaloids into Indole Alkaloids via Indoline Derivatives

Abstract

Oxindole alkaloids chosen as starting materials were isopteropodine (1a) pteropodine (1b) isoformosanine (7a) and formosanine (7b). Reduction of the iminoethers prepared from the oxindole alkaloids with Et₃ O^⁺B^^-F₄, with NaBH₃ (OAc) or with NaBH₄/SnCl₄·2Et₂O afforded the indoline derivatives (3a, 3b, 9a, 9b). The configuration at C₇ of 3a, b was determined by NMR analysis of the N (a)-acetyl-N (b)-oxides (6a, b). The signals of H-14β were observed at high field (δ0.5-1.0) owing to the shielding effect of the aromatic ring in all A type compounds studied. Oxidative conversion of the indolines to indole alkaloids was achieved by using MnO₂ or Me₂SO/ (COCl)₂/Et₂N as oxidizing agents, and the latter reagent gave better results than the former. Tetrahydroalstonine (11) and akuammigine (12) were obtained from 3a, b, and 19-epiajmalicine (14) and 3-iso-19-epiajmalicine (15) from 9a, b. The A type compounds were more susceptible than the B type compounds to the present reduction and oxidation sequence.