Nitrile sulphides. Part 7. Synthesis of [1]benzopyrano[4,3-c]isothiazoles and isothiazolo[4,3-c]quinolines

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Abstract
O-Hydroxybenzonitrile sulphide (1a), generated by thermal decarboxylation of the corresponding 1,3,4-oxathiazol-2-one, reacts with dimethyl acetylenedicarboxylate to afford methyl 4-oxo-4H-[1]benzopyrano[4,3-c]isothiazole-3-carboxylate (6a), from which the parent ring system (6c) can be prepared by hydrolysis and decarboxylation. The same products are formed from the acetoxy analogue (1b)via hydrolysis of isothiazole (5b). O-Acetamidobenzonitrile sulphide (1c) reacts similarly forming isothiazole (5c) and subsequently isothiazolo[4,3-c]quinolin-4(5H)-one (7c) by hydrolysis, ring closure, and decarboxylation. Cycloadditions to ethyl cyanoformate, ethyl propiolate, and diethyl fumarate have also been examined.

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