7-Substituted benzo[b]thiophenes and 1,2-benzisothiazoles. Part 2. Chloro and nitro derivatives

The 7-chloro and 7-nitro derivatives of benzo[b]thiophene and 1,2-benzisothiazole have been prepared from readily available precursors, which for each substituent are common to both ring systems. 7-Chloro-benzo[b]thiophene has been obtained from 3-chloro-2-mercaptobenzoic acid via 7-chlorobenzo[b]-thiophen-3(2H)-one, or from 2,3-dichlorobenzaldehyde, either viaβ-(2,3-dichlorophenyl)-α-mercapto-acrylic acid (16) or, preferably, via 7-chlorobenzo[b]thiophene-2-carboxylic acid. Hexamethylphos-phoric triamide is a particularly useful solvent in which to effect the selective nucleophilic replacement of the 2-chloro substituent in 2,3-dichlorobenzaldehyde. 7-Chloro-1,2-benzisothiazole is available by treatment of 3-chloro-2-mercaptobenzaldehyde with chloramine (57%), or by heating 2,3-dichlorobenzaldehyde with sulphur and aqueous ammonia (46%).7-Nitrobenzo[b]thiophene has been obtained by treatment of 2-bromo-3-nitrobenzaldehyde with mercaptoacetic acid under alkaline conditions, followed by decarboxylation of the resulting 2-carboxylic acid. Cyclisation of 2-(n- or t-butylthio)-3-nitrobenzaldoxime with polyphosphoric acid gives 7-nitro-1,2-benzisothiazole in high yield. 3-Nitro-2-t-butylthiobenzaldehyde behaves unexpectedly with chloramine, to give what is believed to be 7-nitro-2-t-butyl-1,2-benzisothiazolium chloride (24)(73%).