Favorskii rearrangement carbanion intermediate

A common intermediate is required in this Favorskii rearrangement to set the same product. Removal of an aH by OH- is followed by an SN2 displacement of Cl- to give a cyclopropanone ring. Ring opening occurs to give the more stable benzylic carbanion. 

The generally accepted mechanism for the Favorskii rearrangement involves the formation of reactive cyclopropanone intermediate C. Base abstracts the a-hydrogen from A to give the carbanion B, which undergoes intramolecular Sn2 displacement of the halide ion. The resulting cyclopropanone intermediate C is opened under the reaction conditions to give the more stable carbanion D, which takes proton from solvent to furnish the final product, an ester E (Scheme 2.25). 

Another common and useful rearrangement is the Favorskii rearrangement, which involves a carbanion intermediate. An example is shown in Eq. 11.60. It involves the base-induced conversion of an a-halo carbonyl to a carboxylate. If the starting material is a cyclic ketone, a ring contraction results, and this is one of the most useful applications of the Favorskii rearrangement. 

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