Dienone–phenol rearrangements of bicyclic cyclohexa-2,5-dienones; confirmation of a multistage mechanism

Abstract

Two mechanisms are well established for the dienone–phenol rearrangements of bicyclic cyclohexadienones of the 4a-alkyl-5,6,7,8-tetrahydronaphthalen-2(4aH)-one type, of which the 4a-methyl compound (1) and the steroidal 1,4-dien-3-ones are examples. They involve, for (1), either a direct alkyl shift from C-4a to C-4 and deprotonation to give a 4-alkyl-5,6,7,8-tetrahydro-2-naphthol or a shift of the 4a,5-bond from C-4a to C-8a with formation of a spiran intermediate and further ring migration from C-8a to C-1 to from a 4-alkyl-5,6,7,8-tetrahydro-1-naphthol. A further mechanism, for which there was little strong evidence, involves an alkyl shift from one angular position, C-4a, to the other, C-8a, followed by further ring migrations via a spiran intermediate to give an additional route to the 4-alkyltetrahydro-2-naphthol product. The products of rearrangement of 3,4a-and 1,4a-dimethyl-5,6,7,8-tetrahydronaphthalen-2(4aH)-ones in aqueous sulphuric acid, and in acetic anhydride with sulphuric acid catalysis, have been studied critically. The use of quantitative ¹³C n.m.r. spectroscopy confirms the identity and ratios of products deduced using ¹H n.m.r. spectroscopy and other techniques. The results show that the third, suspect, mechanism does indeed operate, and that it contributes significantly to the formation of 4-alkyltetrahydro-2-naphthol products from both dienones.