Diastereoselective routes towards the austrodorane skeleton based on pinacol rearrangement: synthesis of (+)-austrodoral and (+)-austrodoric acid

Efficient routes towards the austrodorane skeleton from the labdane diterpene (−)-sclareol (22) are described. The processes, based on pinacol rearrangement, take place with complete diastereoselectivity. Utilizing these, the marine nor-sesquiterpenes (+)-austrodoral (1) and (+)-austrodoric acid (2) have been prepared from 22. Ketone 19, a key intermediate in the synthesis of rearranged cytotoxic diterpene lactones, such as norrisolide (3), has also been prepared in moderate yield.