Photochemical and acid-catalyzed dienone-phenol rearrangements. The effect of substituents on the regioselectivity of 1,4-sigmatropic rearrangements of the type A intermediate

Abstract

Birch reduction of isophthalic acid and 3-cyanobenzoic acid followed by (1) methylation of the resulting enolate with methyl iodide and (2) esterification with diazomethane provided 2-carbomethoxy- and 2-cyano-6-methyl-6-carbomethoxy-1,4-cyclohexadienes 9 and 25. Type A photorearrangements of a series of 2-carbomethoxy-, 2-cyano-, 2-methoxy-, and 2-methyl-4-carbomethoxy-4-methyl-2,5-cyclohexadien-1-ones 11, 26, 45a, and 45b gave 4-carbomethoxy-3-methyl-2-substituted-phenols 12, 28, 46, and 31. It has been demonstrated that the regioselectivity of type A photorearrangement of C(2) substituted 2,5-cyclohexadien-1-ones is governed by electronic rather than steric effects to give the intermediate C(1) rathen than C(3) substituted bicyclo[3.1.0 1,5 ]hex-3-en-2-ones. Regioselectivities of the acid-catalyzed dienone-phenol rearrangements of C(2) substituted 2,5-cyclohexadienones 11,45a, and 45b appear to be dependent upon the relative stabilities of carbocations resulting from migration of the C(4) carbomethoxy group