Synthesis of 3-Ethenyl-4-methylcyclohex-2-en-1-one from 4-methylanisole

Abstract

Epoxidation of 8-methyl-1,4-dioxaspiro[4,5]dec-7-ene (13) with peroxybenzoic acid afforded 6'-methylspiro[1,3-dioxolan-2,3'-[7]oxabicyclo[4,1,0]heptane] (12), isomerization of which with a catalytic amount of lithium perchlorate in refluxing benzene yielded 8-methyl-1,4-dioxaspiro-[4,5]decan-7-one (15). Reaction of this with vinylmagnesium bromide gave one diastereoisomer of 7-ethenyl-8ξ-methy1-1,4-dioxaspiro[4,5]decan-7-ol (18a) or (18b). Attempts to dehydrate this alcohol resulted in either no reaction, or in formation of ketonic products including 3-ethenyl-4-methyl-cyclohex-2-en-1-one (4). Mild treatment of the hydroxy vinyl acetal (18a) or (18b) with dilute sulfuric acid in dioxan afforded crystalline 3-ethenyl-3-hydroxy-4-methylcyclohexanone (19) which when allowed to react with dilute sulfuric acid in ethanol at room temperature gave 3-ethenyl- 4-methylcyclohex-2-en-1-one (4).

The crystalline dithioacetal (20), obtained from 3-ethenyl-3-hydroxy-4-methylcyclohexanone (19), gave a complex mixture of products upon attempted dehydration with thionyl chloride in pyridine.