A New Triterpene Saponin from Heteropappus biennis with an Unusual Carbohydrate Chain

A new saponin, O-α-D -arabinopyranosyl-(1→6)-O-[α-L -rhamnopyranosyl-(1→2)]-O-[β-D -xylopyranosyl-(1→3)]-β-D -glucopyranosyl arjunolate ( 1 ) was isolated from the flowers of Heteropappus biennis (LDB .) TAMAMSCH . The structure was established mainly by a combination of 1D selective and 2D NMR techniques like COSY, TOCSY, ROESY, HMQC, and HMBC. Molecular-modelling calculations showed that the oligosaccharide chain is rather rigid. Six minimum structures are discussed.