Synthetic approaches to guanacastepenes. Enantiospecific syntheses of BC and AB ring systems of guanacastepenes and rameswaralide.

[reaction: see text] A simple and efficient approach for the BC and AB ring systems of the novel diterpenes guanacastepenes and rameswaralide starting from the readily and abundantly available monoterpene (R)-carvone employing RCM reaction as the key step is described.