2-Bromo-4-chloro-3-methylbenzo

The synthesis of nonsymmetric cyclopentenes, containing both thien-3-yl and benzo[Z7]thien-3-yl units, was aimed at more sophisticated photochromic molecules (Scheme 54) [73]. By the application of glutaric anhydride with 2-chloro-5-methylthiophene and 5-bromo-2-methylbenzo[h]thiophene in the presence of AICI3, keto acids 52 and 53 were obtained. Conversion of the keto acids into acid anhydrides then Friedel-Crafts acylations produced the desired diketone albeit in only 31% yield (over 2 steps) in the first case and in 14% yield (over two steps) in the second case (Scheme 54) [73]. 

The thioanilides of a -cyanomalonic ester (520), which exist preferentially in their enethiol form, are cyclized by bromine to benzothiazolines (521) which can be decarboxylated to the corresponding 2-methylbenzo thiazoles (522 Scheme 282) (69CB351). 2-Bromo- or 2-chloro-thioacetanilides (523) undergo an intramolecular displacement of halogen on treatment with sodium hydride in TV-methylpyrrolidone or sodium methoxide in DMF, to give a correspondingly substituted 2-methylbenzothiazole (522 Scheme 283) (76S730). 

Most recent publications on the sulfonation of benzo[h]thiophenes confirm existing knowledge.1,2 Benzo[b]thiophene and its 5-bromo and 7-chloro derivatives undergo sulfonation (H2S04-Ac20) in the 3-position.550 3-Methylbenzo[b]thiophene 1,1-dioxide is chlorosulfonated in the 2-position551 the apparent absence of substitution in the 6-position is unusual for a 1,1-dioxide. 

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