2-Thioparabanic acids

Thiets have been proposed as intermediates in the corresponding photoaddition of thiones to alkynes.369 The stable spirothiet (421) was obtained in this way along with the 1,2-dithiole (422) on irradiation of l,3-dimethyl-2-thioparabanic acid (423) with diphenylacetylene370 the proposed pathway is outlined in Scheme 28. Stable thiets have also been detected in the photoaddition of xanthenethione to bis(tert-butylthio)ethynes.371... 

A l 2-adduct (277) is formed on irradiation of l,3-dimethyl-2-thioparabanic acid (278) in the presence of dimethylacetylenedicarboxylate. With diphenyl-acetylene, however, the spirothiet 1 1-adduct (279) can be isolated and is formed along with the 1,2-dithiole (280). The probable mechanism for these transformations is briefly outlined in Scheme 23. 

Keywords Benzoyl thiocyanate, oxalyl chloride, primary amines, dichloromethane, room temperature, multicomponent reaction, 2-thioparabanic acids... 

To a stirred solution of primary amine (1 2 mmol) and benzoyl isothiocyanate (2 0.32 g, 2 mmol) in dichloromethane (10 mL) was added oxalyl chloride (3 0.25 g, 2 mmol) at room temperature, and stirring was continued for about 12 h at rt. On completion of the reaction, the solvent was removed under reduced pressure and the viscous residue was purified by flash column chromatography on Si02 (Merck 230-400 mesh) using hexane-EtOAc as eluent to give 2-thioparabanic acid derivatives 4, characterized on the basis of spectral and elemental analyses. 

Yavari, I., Souri, S., and Sirouspour, M. (2008). Efficient one-pot synthesis of unsymmetrical 2-thioparabanic acids from oxalyl chloride, benzoyl isothiocyanate, and primary amines. Synlett, 1633-1634. 

Iinidazolidine-4,5-dione-2-thiones (26), also known as thioparabanic acids, can be prepared by beating thioureas with oxalyl chloride or diethyl oxalate. Cyanogen may also be used if the reactants are heated in the presence of acid. Stoffel and Ulrich and Sayigh have studied in detail the reaction of thioureas with oxalyl chloride and were able to isolate intermediate thiazolidine-4,5-diones (25) which on heating rearranged to the imidazolidine-4,5-dione-2-thiones (26). Thus, in the mechanism proposed by Stoffel, initial attack of the oxalyl chloride is on sulfur to form 24, which can cyclize to 25. Further heating then eauses rearrangement of 25 to 26. 

Chloroformamidines are not produced in the reaction of thioureas with oxalyl chloride, because ring closure occurs and 2-imino-l,3-thiazolidin-4,5-diones (XV) are obtained in high yield ( ). The heterocyclic derivatives XV isomerize readily to form thioparabanic acids XVI... 

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