Structures, biogenetic relationships, and cytotoxicity of pimarane-derived diterpenes from Petalostigma pubescens

Extraction of Petalostigma pubescens heartwood followed by chromatographic purifications and crystallizations afforded five tricyclic diterpenes: 5,9-syn-rosanes petalostigmones A and B (1 and 2), the erythroxylane petalostigmone C (3), the norditerpene lactone pubescenone (4), and the known ent-cleistanthane diterpene (-)-sonderianol (5). The structures and relative stereochemistry were elucidated by means of spectroscopic methods, chemical correlations, and, in the cases of 1 and 4, by X-ray crystallographic analyses. The new isolates 1-4 are assumed to belong to the same absolute configurational family (9alphaCH3) of ent-pimarane-derived diterpenes as the known co-occurring (-)-5 (10alphaCH3). Biogenetic schemes originating from a common ent-copalyl diphosphate intermediate are presented to rationalize the structures of these natural products. A novel ring contraction-ring expansion mechanism is suggested to account for the 7-membered B ring of pubescenone. Compounds 1-5 were evaluated for their cytotoxicity; sonderianol (5) showed the highest activity against mouse leukemia cell lines L1210, P388 and mouse liver cancer cells HEPA1c1c7.