摘要：

IntroductionReductive desulfurization is an important step in organic synthesis for the removal of sulfur so as to manipulate the structure of the substrate. A variety of reagents have been used for the desulfurization of different classes of organic compounds. Desulfurization of thioureas with reagents such as MeSO2Cl–Et3N/DMAP,2NBS,3thionyl chloride,4di(2-pyridyl) sulfite5and di-2-pyridylthionocarbonate–DMAP6is reported to give mono- and diaryl carbodimides. Raney nickel has also been reported as a dethiating agent for thioureas, but the results are not consistent,e.g., unsubstituted thioureas gave formamidine hydrochloride in the presence of ammonium chloride7or a mixture of methane, ammonia and methylamine;8N-(o-tolyl)thiourea gaveo-toluidine;9N,N′-di- andN,N,N′-trisubstituted thioureas have been reported to yield corresponding formamidines, whileN-monosubstituted thioureas gaveN,N′-disubstituted formamidines.10The synthesis of hexahydropyrimidine-4,6-diones (some of which are of pharmaceutical importance11) by reductive desulfurization of 1,3-dihydro-1,3-diaryl-2-thioxopyrimidine-4,6(2H,5H)-diones (2-thiobarbiturates) has not received wide attention. Though sodium amalgam,12Zn-HCOOH12and Raney nickel12have been reported as dethiating agents for 2-thiobarbiturates, the yields range from low to moderate. Other factors associated with Raney nickele.g., storage, high reaction temperatures, dependence on solvent and type of Raney nickel (W-1 to W-8) also restrict its applications.12The alternative approach for the synthesis of hexahydropyrimidine-4,6-diones by the condensation of malonic acid derivatives with amidines, esters, acid chloridesetc. also suffers from certain disadvantages.13Therefore in our opinion a better synthesis of hexahydropyrimidine-4,6-diones is required. In recent years nickel boride14has been reported as a convenient and an efficient reagent for the desulfurization of mercaptans, sulfides, sulfoxides, sulfones and thioketals.15It has also been reported as a chemoselective reagent for deoxygenation of sulfoxides and selenoxides.16In view of its versatility and ease of handling, we decided to explore its application for the desulfurization of thioureas, benzimidazolinethiones and 2-thiobarbiturates.

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