Studies on the coloration mechanism of furostanol derivatives with Ehrlich reagent. II. On the reaction of furostanol glycoside with Ehrlich reagent.

Abstract

The coloration mechanism in the Ehrlich reaction between furostanol glycoside and p-dimethylaminobenzaldehyde has been studied. From the reddish reaction mixture of protoaspidistrin, one of the furostanol saponins, with Ehrlich reagent, colorless condensation products were isolated and characterized as methyl 23-(p-dimethylaminobenzylidenyl)-proto-aspidistrin (5), 3, 26-dihydroxy-23-(p-dimethylaminobenzylidenyl)-furost-5, 20 (22)-diene 3-O-β-D-galactopyranosido-26-O-β-D-glucopyranoside (9), 23-(p-dimethylaminobenzylidenyl)-diosgenin (6), 3β-hydroxy-26, 7'-epoxy-23-(p-dimethylaminobenzyl)-furost-5, 20 (22)-diene (7) and a stereoisomer of 7 (8). Among these products, compounds 5, 9 and 6 showed red coloration under acidic conditions, while compounds 7 and 8, which should not be able to form iminium cations under acidic conditions, did not show the red coloration. It was concluded that the Ehrlich reaction is initiated by condensation between p-dimethylaminobenzaldehyde and furostanol saponin to form 23-benzylidenyl derivatives, which are protonated to form iminium cations under acidic conditions.