Applications of the ene reaction. Synthesis and properties of substituted α-methylene-γ-lactones

Abstract

Substituted dimethyl 1,4-pentadiene-1,2-dicarboxylates (1 or 3) may be prepared by AlCl3-promoted or thermal ene reaction of olefins with dimethyl acetylenedicarboxylate. Lactonization of these adducts is catalyzed by acid. In the presence of 80% H2SO4 the predominant product (ca. 80%) is a (Z)-dihydro-3-carbomethoxymethylene-2(3H)-furanone (2 or 4). Cyclization with anhydrous HCl affords a mixture of products, the composition of which depends on the starting material; major products include the corresponding (E)- and δ-lactone isomers (6 and 7). Detailed 1H and 13C nmr data are reported for all compounds. The structures of the (E)-4,5,5-trimethyl-, (Z)-4,5,5-trimethyl-, and (Z)-4,4,5,5-tetramethyl furanone derivatives (6a, 2a, and 2b) have been established by X-ray crystallography. The properties of some of these lactones are discussed, as are possible methods of removal of the carbomethoxyl from 2.