The Clemmensen Reduction of α, β-Unsaturated Ketones

Eleven structurally different α, β-unsaturated ketones were subjected to the Clemmensen reduction under anhydrous conditions using amalgamated zinc, hydrogen chloride in a solution of ethyl ether, and acetic anhydride. In all cases but one the formation of cyclopropyl acetates was observed. 4-Methyl-3-penten-2-one, methyl vinyl ketone, 2-isopropylidene-1-cyclopentanone, and 2-cyclohepten-1-one led to substituted cyclopropyl acetates. Stereospecific reactions were found with 2-ethylidene-1-cyclopentanone, 2-benzylidene-1-cyclohexanone, and methyl 1-cyclohexenyl ketone, whereas 3-penten-2-one, 3-methyl-3-buten-2-one, and 2-methyl-2-cyclohexen-1-one afforded mixtures of the isomeric cyclopropyl acetates. These results are interpreted in terms of the intital formation of an allylic anion which undergoes electrocyclic closure. A stereospecific course is followed when geometric constraints permitted. Exceptions are discussed.