Constituents of Geranium thunbergii Sieb. et Zucc. IV. Ellagitannins. (2). Structure of Geraniin

Abstract

The main tannin of Geranium thunbergii has been isolated as yellow crystals, and named geraniin. Geraniin gave upon hydrolysis in boiling water, gallic acid, hexahydroxy-diphenic acid, ellagic acid, and corilagin, and is shown by the proton nuclear magnetic resonance spectra to be a corilagin derivative esterified at 0–2 and 0–4 of D-glucopyranose in the molecule. Upon condensation with o-phenylenediamine, geraniin yielded a phenazine derivative, named phenazine A (X), which was transformed into phenazine B (XI) upon prolonged reaction. These two phenazine derivatives gave phenazine C (XII) by further prolonged reaction, or by hydrolysis of X or XI in boiling water. Corilagin was also isolated from the hydrolysis products mixture. The structure of phenazine C was proved by identification of its hydrolyzed product XV with synthetic specimen which was prepared via hydrogenolysis of dimethyl dimethoxytetrabenzyloxydiphenoate. These data along with the PMR spectra show that the structure of geraniin to be I. The carbon nuclear magnetic resonance spectra of geraniin indicate partial hydration to form geminal diol at the cyclohexenetrione moiety. © 1977, The Pharmaceutical Society of Japan. All rights reserved.