Reaction of Copper Enolates of Esters with Propargylic Systems. Facile Preparation of 3, 4-Dienoic Esters, Stereoselective Rearrangement to (2E,4Z)- and (2E,4E)-Dienoic Esters, and Stereoselective Synthesis of a Fragrance from the Bartlett Pear

A copper enolate species derived by treatment of the lithium enolate of ethyl acetate with cuprous iodide has been found to react with propargyl bromide cleanly in an Sn2′ manner to give ethyl 3, 4-pentadienoate, a β-allenic ester. Under the same conditions, the lithium enolate gives only ethyl 4-pentynoate, the corresponding β-acetylenic ester, resulting from direct Sn2 displacement. The reaction of propargylic methanesulfonates having an alkyl group at the propargylic position shows the same remarkable effect of enolate counterion; with the copper enolate, higher homologues of the β-allenic esters can be synthesized efficiently. Copper can act in a catalytic fashion, as the reaction proceeds satisfactorily with only 0.2 equiv. Yields of β-allenic esters from reactions using the copper enolate derived from methyl propionate are lower. Extension of the reaction to a γalkylation of a vinylogous copper enolate to produce a δ-allenic a, β-unsaturated ester succeeded in the case of 3, 3-dimethylacrylic acid, but not with crotonic acid. When exposed to sodium ethoxide in ethanol, the β-allenic esters undergo rapid reconjugation to the 2, 4-dienoates in excellent yield. The 2E, 4Z isomer is the principal product upon reconjugation, and the reconjugation mixture can be equilibrated to predominantly the 2E,4E isomer by heating with thiophenol and 2, 2′-azobis(isobutyronitrile). These reactions were used for the stereoselective preparation of ethyl (2E, 4Z)-decadienoate, a component of the odoriferous principle of Bartlett pears.