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).