Thiourea 1.3- benzoxathiol-2-ones

The asymmetric reduction of the benzoxathiin is very appealing because of its simplicity (Scheme 5.3). It was envisioned that intermediate 16 could be prepared from thiol-phenol 7 and bro moke tone 17. Scheme 5.8 summarized the synthesis for 16. The l,3-benzoxathiol-2-one 35 was prepared from 1,4-benzoquinone and thiourea following a literature procedure with minor modifications. Benzylation of 35 with benzyl bromide in the presence of KI gave benzyl ether 36 as a crystalline solid. It was observed that the benzylation gave better results when the reaction was run under anaerobic conditions. Hydrolysis of thiocarbonate 36 gave free thiophenol 7 which was used directly in the next reaction. 

SYNTHESIS To a well-stirred solution of 120 g thiourea in 800 mL 2N HCL, there was added a solution of 100 g benzoquinone in 500 mL acetic acid over the course of 15 min. Stirring was continued for an additional 0.5 h at room temperature, and then the reaction mixture was heated on the steam bath for 1 h. With cooling in ice water, a heavy crop of crystals separated. These were removed by filtration and air dried to provide 90.1 g of 5-hydroxy-l,3-benzoxathiol-2-one (2-mercaptohydro-quinone cyclic carbonate ester) with a melting point of 170.5-172.5 °C. 

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