From Meso-ionic l,3,2-Oxathiazolium-5-olates

From Meso-ionic l,3,2 oxathiazolium-5 olates (Type B).— The photolysis (at 4045—4078 A) of the meso-ionic 4-phenyl-1,3,2-oxathiazolium-5-olate (1) yields, as the final products, benzonitrile, sulphur, and carbon dioxide. The suggested mechanism (1)- (2) (3) (4) receives experimental support from the fact that the ultimate intermediate (4) can be trapped (10%) as the substituted isothiazole (5) by means of dimethyl acetylenedicarboxylate, the alkyne undergoing 1,3-dipolar cycloaddition to the intermediate benzonitrile sulphide.  

Since the meso-ionic reactants (1) are readily accessible from (6) (and ultimately from mercaptoarylacetic acids), their conversion into isothiazoles (8) has been successfully developed as a synthetic route. They are treated with acetylene-mono- or -di-carboxylic acid esters in toluene at 60—100 °C, and afford, with loss of carbon dioxide, good yields of isothiazoles (8), probably by way of labile intermediates of type (7). 

Activated alkenes (e.g. dimethyl maleate, methyl cinnamate) react analogously the isothiazoles arise probably from the hypothetical bicyclic intermediate (9) by loss of carbon dioxide and dehydrogenation.  

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