Scaparvin A, A Novel Caged cis-Clerodane with an Unprecedented C-6/C-11 Bond, and Related Diterpenoids from the Liverwort Scapania parva

A novel caged cis-clerodane diterpenoid, scaparvin A, possessing an unprecedented C-6/C-11 bond and a ketal ring, as well as four new cis-clerodane derivatives, scaparvins B-E, were isolated from the Chinese liverwort Scapania parva. Their absolute structures were elucidated by analysis of NMR and CD data coupled with electronic circular dichroism (ECD) calculations. It was proposed that an enzymatic intramolecular aldol reaction was the key step in the biogenetic pathway of scaparvin A.