Highly modified nor-clerodane diterpenoids from Croton yanhuii

Phytochemical investigation of the leaves and twigs of Croton yanhuii led to the isolation of seven highly modified nor-clerodane diterpenoids (1-7), including three new ones, croyanoids A-C (1-3), along with four known analogues (4-7). Compound 1 incorporates a 5,12-epoxy ring, forming a unique cage-like, 6/6/6/5-fused tetracyclic ring system. Their structures were established by extensive spectroscopic analysis, and the absolute configurations of 1-4 were determined by a combination of circular dichroism (CD) analysis and single-crystal X-ray diffraction. All compounds were tested in an array of bioassays, but were inactive. Crotoeurin A (7), a nor-clerodane dimer with a high yield of 0.2‰ isolated in current study, was considered as a chemotaxonomic marker for this species.