A New Synthesis of Pyrocin and Related Compounds

Abstract

A new synthesis of pyrocin (1) and related compounds is described. The reaction of 2-benzoyl-4,4-dimethyl-2-buten-4-olide (5b) with 2-methyl-1-propenylmagnesium bromide (6) in the presence of CuCl gave α-benzoylpyrocin (7b) in 90% yield. The product of 7b with a specific rotation of +2.43° was obtained when the reaction was carried out in the presence of (−)-sparteine. α-Acetylpyrocin (7a) was prepared in the similar manner in 63% yield. The treatment of 7a and that of 7b with ethanolic sodium ethoxide gave 1 in 65% and 81% yields, respectively. The reaction of 1 with large excess of SOCl₂ in absolute ethanol gave ethyl 3-(1-chloro-1-methylethyl)-5-methyl-4-hexenoate (8) in 82% yield. Similar treatment of 7 with SOCl₂ gave ethyl 2-substituted-4-(2-methyl-1-propenyl)-5,5-dimethyl-4,5-dihydrofuran-3-carboxylate (12). Compound 8 was transformed to ethyl trans-(±)-chrysanthemate (2) in 75% yield.