Isoeichlerianic acid from Aglaia silvestris and revision of the stereochemistry of foveolin B

A new series of 20,24-epoxy (tetrahydrofuryl) type dammarane triterpenoids were established. So far two series were described, based on the stereochemistries of the tetrahydrofurane ring linked to the D ring of the triterpenoid skeleton. Eichlerianic acid was characterised by the absolute configuration (20S,24S) and shoreic acid by (20S,24R). Foveolins A and B were correlated with these compounds and published also as (20S,24S) and (20S,24R), respectively. 13C NMR analysis of a further stereoisomer of the acids isolated from Aglaia silvestris allowed the conclusion that the new isoeichlerianic acid, its methyl ester and the known foveolin B belonged to the new stereochemical series (20R,24S).