Synthesis of new 5-alkylidene-4-chloro-5H-1,2,3-dithiazoles and their stereochemistry

A variety of 5-alkylidene-4-chloro-5H-1,2,3-dithiazoles (9-25) have been prepared from 4-chloro-5H-1,2,3-dithiazolium chloride, active methylene compounds, and pyridine. The reactions with ethyl nitroacetate ((Z) > (E)), ethyl 3-nitrobenzoylacetate ((E) > (Z)), ethyl 2-fluorobenzoylacetate ((E) > (Z)), and tetronic acid ((Z) > (E)) gave a mixture of (E)- and (Z)-isomers, whereas those of benzoylnitromethane (Z), 5,6-dihydro-4-hydroxy-6-methyl-2H-pyran-2-one (E), 4-hydroxy-6-methyl-2-pyrone (E), 4-hydroxycoumarin (E), 6-chloro-4-hydroxycoumarin (E), and 6-bromo-4-hydroxycoumarin (E) afforded only single stereoisomers. The reactions with 4-hydroxy-1-methyl-2(1H)-quinolone, 2-hydroxy-1,4-naphthoquinone and homophthalic anhydride gave only single stereoisomers whose stereochemistry is uncertain. It appears that geometrically more rigid cyclic 1,3-dicarbonyl compounds give better yields of dithiazol-5-ylidenes than the corresponding acyclic compounds. (C) 1999 Elsevier Science Ltd. All rights reserved.