Aromatization of 3,4-dihydro-β-carboline-3-qarboxylic acid and its derivatives through a carbanion intermediate : mechanistic study and use in chemical synthesis

Abstract

3,4-dlhydro-β-carboline-3-carboxylic acid , its esters and amide derivatives (AH 2) undergo complete aromatization into corresponding β-carboline derivatives (A) by basic treatment under mild conditions. This, together with the easy obtention of 3,4-dihydro-β-carboline from the parent N αformyltryptophan derivatives constitutes an attractive possibility to obtain complex molecules containing the β-carboline ring. The mechanism of the reaction was investigated by polarography, U.V.spectrometry and polarimetry. In alcaline media, (AH 2) undergoes two successive equilibria : the first yielding a C 3 carbanion occurs with suppression of the chirality while the second yields a thermodynamically preferred C 4 carbanion which undergoes aromatization through an oxidative pathway (K' a = 2×10 -14 at 45°C).