Enantioselective Construction of Natural (+)-Pallescensin A. A Sigmatropic Pathway to Furanosesquiterpenes

Article abstract of DOI:10.1021/jo00052a026

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 β-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.