Chemical Structures of (+)-Catechin-Formaldehyde Reaction Products (Stiasny Precipitates) Under Strong Acid Conditions: Part 1. Solid-State 13C-NMR Analysis

Abstract

Summary To determine more accurately the amount of polyflavanoids in tannin extracts from radiata pine bark, which react with formaldehyde, it is important to elucidate chemical structures of (﹢)-catechin-formaldehyde reaction products (Stiasny precipitates). The Stiasny value of 106.4% for (﹢)-catechin was obtained from the reaction with formaldehyde under strong acid conditions. This value is almost identical to the value calculated from the Stiasny precipitates, which were formed in the ratio of two molecules of (﹢)-catechin to three molecules of formaldehyde. The solid-state 13C NMR spectrum was measured to elucidate the structures of the Stiasny precipitates, and analysed on the basis of the chemical shifts of (﹢)-catechin. The NMR results indicate that C-6 and C-8 in the A-ring are bonded by methylene bridges to form polymers, and also form methylene bridges with C2′, C5′ or C6′ in the B-ring of a catechin unit. Consequently, the reaction products of (﹢)-catechin-formaldehyde under strong acid conditions (Stiasny precipitates) are (﹢)-catechin-polymers, which consist of two moles of (﹢)-catechin and three moles of formaldehyde. The methylene bridges are formed mostly between A-rings, and less frequently, between A- and B-rings and between B-rings of the catechin units.