Podoimbricatin A, a cytotoxic diterpenoid with an unprecedented 6/6/5/6-fused tetracyclic ring system from the twigs and leaves of Podocarpus imbricatus

Podoimbricatin A (1), a diterpenoid possessing an unprecedented 6/6/5/6-fused tetracyclic ring system, and podoimbricatin B (2), a new abietane-type diterpenoid, together with four previously reported diterpenoids and a cyclic peptide were isolated from the twigs and leaves of Podocarpus imbricatus. Their structures were established on the basis of extensive spectroscopic analyses and the relative configuration of 1 was confirmed by single-crystal X-ray diffraction analysis. A plausible biogenetic pathway involving hetero-Diels-Alder cycloaddition as a key reaction is proposed for 1. Compounds 1, 5, and 7 showed inhibitory effects against the A549 and NCI-H292 cancer cell lines.