Favorskii rearrangement enolate intermediate

McGrath, M. J. A. Favorskii rearrangements. I. One electron transfer from a -enolate intermediates to triplet oxygen in aprotic, polar protic, and mixed media. Tetrahedron 1976, 32, 377-387. 

By 1950 five distinct mechanisms had been suggested to account for the formation of the major products of the Favorskii rearrangement. Four involved epoxide, ketene, enol, and carbene intermediates. A fifth mechanism related to the benzylic acid rearrangement was also proposed. Then, in 1951 Loftfield isolated two esters with identical isotope distributions at their a and P carbons from treatment of a radiolabeled, cyclic a-chloroketone with an alkoxide. These two products suggested a symmetrical intermediate, leading Loftfield to postulate the existence of a cyclopropanone along the reaction pathway. ... 

A Favorskii-like intermediate (655), and the rearranged product (657), arise in a novel way from fragmentation of the a-ketol enol-sulphite (654). Loss of sulphur dioxide is thought to afford the zwitterion (655), which gives the methoxy-ketone (656) in methanol, but the A-nor carboxylic ester (657) with sodium methoxide in ether. ... 

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