ENANTIOSELECTIVE CONSTRUCTION OF NATURAL (+)-PALLESCENSIN-A - A SIGMATROPIC PATHWAY TO FURANOSESQUITERPENES

Addition of the cerium reagent derived from 3-lithiofuran and anhydroUS CeCl3 to optically pure bicyclic ketone 2 affords alcohol 4 selectively. Anionic oxy-Cope rearrangement of 4 in hot diglyme results in conversion to keto aldehyde 6, a consequence of beta-elimination following upon the [3,3]sigmatropic event. Advantage was then taken of chemoselective acetalization and unidirectional introduction of an enone double bond. Regio- and stereoselective epoxidation of 8 was a prelude to formation of the furan ring by BF3.etherate-promoted cyclization. Once the carbonyl group was reduced with alane, catalytic hydrogenation in the presence of diethylamine gave the title compound. An alternative scheme involving silylation of the furan ring as a protective maneuver, while entirely workable, was both less direct and less efficient.