Halo ketones with thiol acids

Thiols efficiently dehalogenate a-halo and a,a-dihalo ketones. Reduction is typically carried out in ethanol containing a weak base. Although this system will also reduce a-thio ketones, it is believed that the halo ketone is reduced, and that reaction proceeds as in equation (11), with a proton from the solvent serving as Lewis acid to assist dehalogenation by thiol (or thiolate). The reactions are of course accelerated by inclusion of a metal salt in the medium. ... 

Ring contractions of pyran derivatives are occasionally valuable. The contraction of 3-halo-2-pyrones to 2-furoic acids under the influence of alkali has been studied and the conditions defined.58112113 The method is adaptable to the preparation of 3-furoic acid via furan-2,4-dicarboxylic acid58 and of 3,4,5-triphenylfuran-2-carboxylic acid.113 Another ring contraction involving halides is the conversion of 4-chloromethylpyrylium salts into furylmethyl ketones as indicated in Scheme 21.114 Pyridine oxides may be transformed with unexpected ease into furans through treatment with a thiol (Scheme 22).115... 

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