19-nor-pimaranes from Icacina trichantha

Four new unusual 19-nor-pimarane-type diterpenes were isolated from the tuber of Icacina trichantha (Icacinaceae, Oliv.). The structures were elucidated based on spectroscopic and HRMS analysis. The absolute configurations were determined by electronic circular dichroism. All four compounds are structural analogues of icacinol and humirianthol, but do not demonstrate the same cytotoxic activity. A plausible biogenetic pathway is proposed.