Sodium borohydride → (2R,3R)-3-hydroxy-8-methyl-2-(4-methylphenyl)-2,3-dihydro-1,5-benzothiazepin-4(5H)-one

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A mixture of D-tert-leucine (721 mg), sodium borohydride (189 mg) and tetrahydrofuran (50 ml) is refluxed under nitrogen atmosphere for three hours. The mixture is cooled to -15° C., and thereto is added a solution of 8-methyl-2-(4-methylphenyl)-1,5-benzothiazepine-3,4(2H, 5H)-dione (991 mg) in tetrahydrofuran (3 ml). Each 0.38 ml of 25% acetic acid/tetrahydrofuran solution (1.67 mmole as acetic acid) is added thereto at 25 minutes, and 2 hours 10 minutes after the beginning of the reaction, and the reaction solution is stirred at the same temperature for 20 hours. The mixture is evaporated under reduced pressure to remove the solvent, and to the residue is added 1N hydrochloric acid (70 ml). The precipitated crystal is collected by filtration, washed with water, and dried. The collected crystal is suspended in isopropyl alcohol (12 ml), and the mixture is heated with stirring at 80° C. for three hours. The mixture is cooled to 5° C., and allowed to stand for five hours. The precipitated crystal is collected by filtration, washed with cold isopropyl alcohol, and dried to give (2R,3R)-3-hydroxy-8-methyl-2-(4-methylphenyl)-2,3-dihydro-1,5-benzothiazepin-4(5H)-one (804 mg).

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