A new synthesis of ethyl 2-methyl-4-oxocyclohex-2-enecarboxylate (Hagemann's ester) and its methyl and t-butyl analogues

Abstract

Regioselective cyclisations of esters of 2-acetyl-5-oxohexanoic acid led to (i) alkyl 2-methyl-4-oxocyclohex-2-enecarboxylates (alkyl = Bu^t,Et or Me)(catalysed by pyrrolidinium acetate), (ii) esters of 4-pyrrolidino-2-methyl-cyclohexa-1,3-dienecarboxylic acid (in the presence of pyrrolidine), and (iii) alkyl 4-methyl-2-oxocyclohex-3-enecarboxylates (catalysed by hydrogen chloride).