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

Four diterpenes, 5,9-syn-rosanes petalostigmones A and B (1 and 2), the erythorxylane petalostigmone C (3), and the norditerpene lactone pubescenone (4), together with the known ent-cleistanthane diterpene (−)-sonderianol, were isolated from Petalostigma pubescens. Biogenetic schemes are presented to rationalize the structures of these natural products.