Can You Really Do Chemisty Experiments About 7-Chloro-3-hydroxybenzo[b]thiophene-2-carboxylic acid

The 7-chloro and 7-nitro derivatives of benzothiophene and 1,2-benzisothiazole have been prepared from readily available precursors, which for each substituent are common to both ring systems. 7-Chlorobenzothiophene has been obtained from 3-chloro-2-mercaptobenzoic acid via 7-chlorobenzothiophen-3(2H)-one, or from 2,3-dichlorobenzaldehyde, either via beta-(2,3-dichlorophenyl)-alpha-mercaptoacrylic acid (16) or, preferably, via 7-chlorobenzothiophene-2-carboxylic acid.Hexamethylphosphoric 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 (57percent), or by heating 2,3-dichlorobenzaldehyde with sulphur and aqueous ammonia (46percent). 7-Nitrobenzothiophene 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-bytylthiobenzaldehyde behaves unexpectedly with chloramine, to give what is believed to be 7-nitro-2-t-butyl-1,2-benzothiazolium chloride (24) (73percent).