From iridoids to dyes: a theoretical study on genipin reactivity

Abstract

A mechanistic theoretical study of the reaction between methylamine and genipin, an iridoid obtained from some natural sources, is here presented. Starting from the opening of the genipin six-membered ring experimentally hypothesized and proved in our previous theoretical work, methylamine reaction with the three different forms of genipin in equilibrium has been studied in terms of activation enthalpies and product stabilization. The kinetically favored reaction pathways, initiated by the attack of the amine on the carbonyl groups of the dialdehydic open-ring form of genipin, have been then studied until the formation of the first orange/red product. The catalytic role of a few water molecules has been clearly shown for many reactions in the process and a fundamental redox autocatalytic cycle has been proposed for the formation of the orange/red product. This mechanism explains well the experimental evidence previously reported in literature. ¹H-NMR and UV-visible spectra have been reproduced for each stable intermediate of the process.