Competitive anion formation in pulegone [p-menth-4(8)-en-3-one] synthesis of phenylmethylenepyrans

Abstract

Treatment of (+)-pulegone [p-menth-4(8)-en-3-one] with strong base generates various anions which can be quenched with benzoyl chloride. Enolization towards C-4 followed by acylation leads to p-mentha-3,8(10)-dien-3-yl benzoate (2a). 2-Benzoyl-p-mentha-2,4(8)-dien-3-yl benzoate (3) results from acylation of an enol generated at position 2. The 9,10-dianion reacts with benzoyl chloride to give a diketone, which by cyclization affords 4-benzoylmethylene-5,6,7,8-tetrahydro-7-methyl-2-phenyl-4H-chromen (6a). Compound (6a) was identified by an independent synthesis involving treatment of ethyl 4-methyl-2-oxocyclohexanecarboxylate with benzoylmethylene(triphenyl)phosphorane. In addition to 5,6,7,8-tetrahydro-7-methylflavone (9a), 2-(2,6-di-phenyl-4H-pyran-4-ylidene)-5-methylcyclohexanone (5a) and its isomer (6a) are also formed in equal yields in this reaction.