One-pot depolymerizative extraction of proanthocyanidins from mangosteen pericarps

Reaction of dried mangosteen pericarps powders with mercaptoacetic acid in the presence of hydrochloric acid in methanol afforded 4β-(carboxymethyl)sulphanyl-(−)-epicatechin methyl ester, 1, in 4.5% yield. The structure of 1 was characterised by 1H, 13C NMR, and mass spectra. Two protons on the prochiral methylene carbon next to sulphur atom give rise to characteristic AB pattern peaks indicating its association to chiral environment as is the case in 1. Compound 1 is a potential synthon towards other epicatechin derivatives due to its labile C–S bond under weakly basic conditions. Reaction 1 with epicatechin gave B2. If 3,5-dimethoxyaniline was used, 4β-(2′-amino-4′,6′-dimethoxyphenyl)epicatechin, 2, was isolated in moderate yield. Reaction of 1 with mercaptoethanol leads to synthesis of 4β-(2-hydroxyethylthio)epicatechin, 3, in good yield. Finally mercaptoacetic acid was used as an alternative for successful determination of mean degree of polymerisation of PAs isolated from pine bark, grape seeds, cocoa bean, and mangosteen.