Abstract 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
The reactions of (±)-α-epoxy drimenol (4) and (±)-α-epoxy drimenyl cyanide (6) with acids (proton acid or Lewis acid) selectively gave the rearranged aldehyde (±)-13 and (±)-15 having the hydroindane skeleton, respectively, while the reactions of (±)-4 and (±)-6 with Dibal-H selectively afforded the allyl alcohol (±)-14 and (±)-16, respectively. The reactions of (8aR)-6 and (8aS)-6 with Dibal-H were applied for the determination of the absolute structure of natural 7β-acetoxy-ent-labda-8(17),13(14)E-dien-15-ol (18). The reaction of (±)-α-epoxy bicyclofarnesol (5) and (8aS)-5 with proton acid selectively provided the rearranged ketol (±)- and (8aS)-31 having the hydroindane skeleton, respectively. The optically active (8aS)-31 was converted to the natural (9S)-austrodoric acid (33).
A short and efficient synthesis of the rearranged nor-sesquiterpenes (+)-austrodoral (1) and (+)-austrodoric acid (2), recently isolated from the Antarctic marine mollusk Austrodoris kerguelenensis, fromditerpene (−)-sclareol (4) is reported. The key step of the sequence is the pinacol rearrangement of the drimanetriol 11.