The general procedure for the synthesis of N-Boc-3-thiomorpholinecarboxylic acid from 1,4-dioxane, di-tert-butyl dicarbonate and 3-thiomorpholinecarboxylic acid is as follows:
(a) Synthesis of 4-(tert-butoxycarbonyl)thiomorpholine-3-carboxylic acid: triethylamine (23.6 mL) was slowly added to a solution of DL-thiomorpholine-3-carboxylic acid (5 g) dissolved in 40 mL of a solvent mixture of 1,4-dioxane and water (1:1, v/v) at 0 °C. Subsequently, di-tert-butyl dicarbonate (8.16 g) was added and the reaction mixture was stirred at 0 °C for 30 min and then continued for 3 h at room temperature. After completion of the reaction, the solvent was removed by evaporation under reduced pressure and the residue was dissolved in 100 mL of ethyl acetate. The pH of the reaction solution was adjusted to 4 with saturated citric acid solution and then extracted with ethyl acetate. The organic layers were combined, dried over anhydrous sodium sulfate and concentrated under reduced pressure. The residue was recrystallized by hexane/ethyl acetate mixed solvent to give 4-(tert-butoxycarbonyl)thiomorpholine-3-carboxylic acid (6.0 g, 71% yield).
