TERPENOIDS TO TERPENOIDS - ENANTIOSELECTIVE CONSTRUCTION OF 5,6-FUSED, 5,7-FUSED, AND 5,8-FUSED BICYCLIC SYSTEMS - APPLICATION TO THE TOTAL SYNTHESIS OF ISODAUCANE SESQUITERPENES AND DOLASTANE DITERPENES

The prevalence of a C12 common core (10) in C15-C30 terpenoids has been recognized. Construction of two ''chirons'' ((-)-11 and (-)-12a,b), corresponding to 10, from abundantly available (R)-(+)-limonene has been achieved through diastereoselective [3s.3s] sigmatropic processes 16 --> 11 and 19 --> 12, respectively. Chirons 11 and 12 have been successfully annulated to bicyclic hydrindanones 21, 23, and 30, hydrazulenoids 31-33, and 5,8-fused system 42 through methodologies that are short and practical. Thus, these enantiomerically pure bicyclics are available as advanced building blocks for higher terpene synthesis. One of the hydrazulenoids ((-)-31) has been elaborated to isodaucane sesquiterpenes (+)-aphanamol I (2) and (+)-2-oxoisodauc-5-en-12-al (46) through a novel restructuring protocol (31 --> 50). The stereo- and enantioselective synthesis reported here has established the absolute stereochemistry of isodaucane sesquiterpenes. The hydrazulenoid (-)-31 has also been deployed for the first enantioselective synthesis of oxygenated dolastane diterpenes (+)-isoamijiol (63) and (+)-dolasta-1(15),7,9-trien-14-ol (64). The key step in this venture was the stereoselective annulation of a six-membered ring through radical-induced alkyne-carbonyl cyclization (67 --> 68).