Bisrubescensins A-C: three new dimeric ent-kauranoids isolated from isodon rubescens

[reaction: see text] A phytochemical study of the secondary metabolites produced by the species of Isodon rubescens has led to the isolation of three new dimeric ent-kauranoids and two known ones. The most important of these compounds are bisrubescensin A (1), which contains an unprecedented C(23) ent-kaurane unit, and bisrubescensin C (3), which is the precursor of bisrubescensin B (2) from the viewpoint of biosynthesis. Their structures were determined on the basis of extensive spectroscopic analysis and chemical evidence.