Isothiazoles, mesoionic

The triphenyl compound (150 X=S, R = Ph) has been prepared in high yield by phosphorus pentasulfide-pyridine treatment of the dibenzoyl isothiazole (149 X = S, R = Ph) (77HC(30)317, 79CB260). The isothiazole precursor has been obtained from the mesoionic l,3,2-oxathiazol-5-olate (148 X = S, R = Ph) by treatment with either dibenzoylacetylene (79CB260) or dibenzoylethylene (77HC(30)317). 

The mesoionic oxathiazoliuraolate (3) underwent a cycloaddition reaction with 2-tert-butyl-6,6-dimethylfulvene (4) in a [ 4tt + 2 t ] manner to give the isothiazole (6) by extrusion of carbon dioxide from the initially formed cycloadduct (5) and then spontaneous dehydrogenation <97X9921>. 

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