Total synthesis of pseudoguaianes-I. Preparation of bicyclo[5.3.0]decane synthons for damsinic acid and helenanolides

A reaction sequence has been developed for preparing hydroazulenones suitable for total synthesis of pseudoguaiane sesquiterpenes (→1 →2 →4). The relative configuration of bicyclic ketone 2 at three chiral centers was established by its conversion to (±)-damsinic acid (3). Subsequently, the C-10 chiral center of 2 was inverted by employing temporary vinyl activation (2 →27 →4). Ketone 4 thus produced is a potential synthon for helenanolides, having the required configuration at carbons 1, 5 and 10 found in that class of sesquiterpene lactones.