A new method for enol lactone synthesis by a Michael addition/cyclization sequence

Reactions of cyclic 1,3-dicarbonyl compounds with 1-(2-alkenoyl)-4-bromo-3,5-dimethylpyrazoles under the double catalytic activation conditions using both catalytic amounts of Lewis acid and amine catalysts provide a new direct synthetic route to enol lactones. Thus, 1,3-cyclohexanedione is allowed to react with 4-bromo-1-crotonoyl-3,5-dimethylpyrazole, in tetrahydrofuran at room temperature in the presence of both catalytic amounts (10 mol% each) of nickel(II) perchlorate hexahydrate and 2,2,6,6-tetramethylpiperidine, to give 4,7,7-trimethyl-3,4,5,6,7,8-hexahydrobenzopyran-2(H),5-dione in a good yield. This reaction does not proceed or is too slow under the reaction conditions other than the double catalytic activation conditions.