Ethyl 2- methyl-4-oxocyclohex-2-enecarboxylate (Hagemann's ester) as a precursor to alkyl-substituted 3-methylcyclohexenones

Abstract

4-Alkyl-3-methylcyclohex-2-enones have been prepared from ethyl 2-methyl-4-oxocyclohex-2-enecarboxylate (Hagemann's ester)via the alkylation of its ethylene glycol acetal. Various routes to 6-alkyl-3-methylcyclohex-2-enones from Hagemann's ester have been explored; a particularly convenient route to these unsaturated ketones utilised the isomeric keto-ester, ethyl 4-methyl-2-oxocyclohex-3-enecarboxylate as starting material. The alkylation of ethyl 2-methyl-4-pyrrolidinocyclohexa-1,3-dienecarboxylate, the fully conjugated dienamine derived from Hagemann's ester, yielded 2-alkyl-3-methylcyclohex-2-enones after hydrolysis and de-ethoxycarbonylation. An improved preparation of Hagemann's ester is reported. The preparation of 4-alkyl- and 6-alkyl-3-methylcyclohex-2-enones complements the existing procedures to 2-alkyl- and 5-alkyl-3-methylcyclohex-2-enones.