Ketones, polyhalo Favorskii rearrangement

The reaction of a polyhalo ketone with base (equation 3) was described several years before Favorskii described a similar rearrangement (equation 4). Simple a,a- and a,a -dihalo ketones yield the same product (Scheme 14), so dibromination can be followed by Favorskii rearrangement without purification. These rearrangements are usually stereospecific, though subsequent cis to trans isomerization, under the influence of base, often obscures this feature. Various explanations have been provided for this stereospecificity, but the one due to Rappe (illustrated in Scheme 15) seems most plausible. ... 

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