Ring Cleavage of N-Alkylisothiazolilm Salts by Complex Metal Hydrides. Synthesis of β-Enaminothioketones

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Abstract
N-Alkylisothiazolium salts undergo ring cleavage when treated with complex metal hydrides to give β-enami-nothioketones in high yields. The isothiazole nucleus is remarkably stable to chemical attack by reducing agents.1 In fact, little work has been published on reactions involving reductive cleavage of the isothiazole ring. Thus, other azole systems as isoxazole are readily cleaved by catalytic hydrogenolysis whereas the isothiazole nucleus, in the same conditions, remains unchanged.2 However there are instances in which ring cleavage occurs. For example, cleavage of isothiazole during lithiation at the 5-position gives a small proportion of a β-mercaptoenone resulting from the cleavage of the N-S bond.3,4 Isothiazoles are reductively desulfu-rized by Raney nickel and this method has been used by Woodward in a remarkable synthesis of colchicine.5