Stereoselective synthesis of cyclohexa-2,4-dien-1-ones and cyclohex-2-en-1-ones from phenols

A convenient synthetic method for the synthesis of substituted cyclohex-2-en-1-ones by the direct alkylation of phenols has been developed. Furthermore, enantiomerically enriched 2,6-dimethyl-6-(3-methylbut-2-enyl)-cyclohexa-2,4-dienone was prepared by the deprotonation of 2,6-dimethylphenol with a sparteine–lithium complex followed by alkylation with 1-chloro-3-methylbut-2-ene. 2,6-Dimethyl-6-(3-methyl-but-2-enyl)-cyclohex-2-enone was prepared from the corresponding cyclohexa-2,4-dien-1-one by selective hydrogenation of the 4,5-double bond. The method was extended to 2-methyl-naphthalen-1-ol and 1-methyl-naphthalen-2-ol resulting in 2-(R)-methyl-2-(3-methylbut-2-enyl)-2H-naphthalen-1-one and 1-(S)-methyl-1-(3-methylbut-2-enyl)-1H-naphthalen-2-one, respectively.

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2-(R)-Methyl-2-(3-methyl-but-2-enyl)-2H-naphthalen-1-oneC16H18O[α]D25=+118.8 (c 1.0, EtOH)Source of chirality: asymmetric synthesisAbsolute configuration: (2R)

1-(S)-Methyl-1-(3-methyl-but-2-enyl)-1H-naphthalen-2-oneC16H18O[α]D25=+14.9 (c 1.0, EtOH)Source of chirality: asymmetric synthesisAbsolute configuration: (1S)