Synthetic studies toward the kempane diterpenes. Approaches to the assembly of the ring system

The ring system of the kempane diterpenes has been assembled from the Diels -Alder adduct 7 by a highly chemo- and stereoselective attack of lithium et hoxyacetylide on its apparently more encumbered carbonyl (to give 11), remo val of the silyl protecting group (12 and 13), concomitant deoxygenation an d ethoxyethyne solvolysis (18 and 19), reconjugation and epimerization (21) , and then a series of reductive and protection steps before cyclization of the final seven-membered ring (31): An alternative approach is outlined wh ich was thwarted by an unusual cyclization of a dimethyl ether moiety (48) to a tetrahydrofuran (49).