Synthesis of Benzothiophens by Ring-closure Reactions

Synthesis of Benzothiophens by Ring-closure Reactions.—A review of the chemistry of benzothiophens, covering the period 1952—1968, has appeared.  

The mechanism of the oxidative coupling of /3-arylmercaptoacrylic acids, obtained by alkaline hydrolysis of 5-arylidenerhodanines (314), to yield 

A new simple benzo[6]thiophen synthesis consists of the reaction of cinnamic acid derivatives with thionyl chloride and pyridine, which leads to 3-chlorobenzo[Z ]thiophen-2-carbonyl chlorides. The disadvantage of the method is that activating substituents in the cinnamic acid cause chlorination in the benzene ring.  

A synthesis of 2-alkyl- or 2-aryl-substituted benzo[Z ]thiophens consists of the cyclization of an aryl thioether with an acetonyl or phenacyl group in the or/Ao-position (315) with 48% hydrobromic acid. In order to obtain (315 R = Ph), 2-methylthiobenzyl cyanide is treated with phenyl-magnesium bromide. The reaction of or//to-methylthiostyrene derivatives, especially cinnamic acids (316), with sulphuryl chloride followed by pyridine, probably gives the benzo[6]thiophens (318) via the sulphenyl chloride (317).  

Upon oxidation of 1-thiocoumarins (319) with selenium dioxide in iVTV-dimethylformamide, ring-contraction occurs, yielding benzo[ ]thio-phen-2-aldehyde in 80% yield. Possible reaction paths are discussed. A related ring-contraction to 2-bromomethylbenzo[A]thiophen occurs when 

Heating (208) with hydrobromic acid in acetic acid gave (2-p-nitrophenyl)-benzo[Z)]thiophen. The oxidation of 1-thiochromene with selenium dioxide in boiling ethanol led to 2-(2 -benzo[6]thienyl)-l-thiochromene. Substituted 2-benzoylbenzo[6]thiophen-3(2JT)-ones have been synthesized by cyclization of 

A new route to benzo[c]thiophen via the sulphilimine (220) has been found. /rfl/M-l,4-Dimethoxycarbonyl-2,3-benzodithian, obtained by condensing dimethyl aa -dibromobenzene-l,2-diacetate with potassium disulphide at low temperature. 

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