Sulfonation reactions of (β-carbolines

Abstract

The homogeneous sulfonations of (β-carboline derivatives norharmane 2, harmane 3, harmine 4, harmol 5, -N,N′-dimethyl-harmane 6, and harmaline 7 have been studied in 80–90% w/w sulfuric acid solutions at 25 °C. Ultraviolet–visible and 13C nuclear magnetic resonance spectra and chemical analysis of the reaction mixtures show that for all the substrates studied only C-6 sulfonated products are obtained. The sulfonation reactions of the benzene unsubstituted substrates 2, 3, and 6 and the partially aromatic substrate 7 proceed by direct electrophilic attack of the H3SO4+ cation on the benzene ring of the (β-carboline skeleton. Sulfonation of 4 and 5 at the lower sulfuric acid concentrations is effected by the H3SO4+ entity. With increasing sulfuric acid concentration there is a gradual changeover of the sulfonating entity from H3SO4+ to H2S2O7. The different mechanisms of sulfonation are discussed. Keywords: β-carbolines, sulfonation, kinetics.