Rivalosides A and B, two 19-Oxo triterpenoid saponins from Galium rivale

Two new oleanene-type triterpene glycosides, rivaloside A (2alpha-acetoxy-3alpha-hydroxy-19-oxo-olean-12-en-28-o ic acid 28-O-beta-glucopyranosyl-(1-->6)-beta-D-glucopyranoside) (1) and rivaloside B (2alpha,3alpha-dihydroxy-19-oxo-olean-12-en-28-oic acid 28-O-beta-glucopyranosyl-(1-->6)-beta-D-glucopyranoside) (2), were isolated from aerial parts of Galium rivale. The structures were determined on the basis of spectroscopic data and chemical transformations. Under alkaline or acid hydrolysis, rivalosides A and B undergo allylic rearrangement with migration of the double bond. These compounds are the first representatives of 19-oxo substituted oleanenes, and they seem to be promising as taxonomic markers for G. rivale.