Thiophthalide, Phthalic Thioanhydride, and Related Compounds

Compound 100 (R = H) may be prepared by treating the thiocyanate (101a) with sulfuric acid,103 109 110 by treating o-chloro-methylbenzonitrile with alcoholic potassium hydrogen sulfide (see later),103,110 by cleavage of o-benzylthiomethylbenzonitrile (101b) 

An attempted cyclization of o-(benzylmercaptomethyl)benzoic acid (102) with polyphosphoric acid gave an excellent yield of 2-thiophthalide instead of the expected product (103).115 

2-Thiophthalide is also formed when o-xylene is oxidized with a mixture of sulfur and water at high temperatures prolonged oxidation under these conditions gives phthalic acid.116,117 The formation of 2-thiophthalide is favored if the reaction mixture is cooled under the 

Stacy and A. Y. Orita, Abstr. 160th Amer. Chem. Soc. Meeting, Chicago, Sept., 1970, Paper Orgn. 85. 

Protiva, M. Rajsner, E. Alderova, V. SeidlovA, and Z. J. VejdSlek, Collection Czech. Chem. Commun. 29, 2161 (1964). 

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