[C:1]([S:5][CH2:6][C@@:7]1([CH3:14])[NH:11][C:10](=[O:12])[NH:9][C:8]1=[O:13])([CH3:4])([CH3:3])[CH3:2].CO.S(=O)(=O)(O)[OH:18].C(O)(C)C.OO.O>[Cl-].[Na+]>[C:1]([S:5]([CH2:6][C@@:7]1([CH3:14])[NH:11][C:10](=[O:12])[NH:9][C:8]1=[O:13])=[O:18])([CH3:4])([CH3:2])[CH3:3](5S)-5-[(Tert-butylthio)methyl]-5-methylimidazolidine-2,4-dione (1.00 eq, 46.2 mmol, 10.0 g) was charged to a vessel followed by methanol (2.64 mol, 107 mL, 84.5 g). The resulting clear solution was stirred and a solution of sulfuric acid, 4.40% w/w in iso-propanol (6.22 mmol, 13.9 mL, 13.9 g) was added. To this mixture was then added a solution of hydrogen peroxide, 11.68M, 35% w/w in water (107 mmol, 9.13 mL, 10.4 g) in one portion without external cooling (an exotherm from 22° C. to 41° C. over 10 min was noted). Hplc analysis of the reaction mixture approximately 1.5 h after the addition of the hydrogen peroxide revealed complete consumption of the starting material and the appearance of two new peaks (diastereoisomeric sulfoxides). The mixture was diluted with saturated aqueous sodium chloride solution (231 mL) and extracted with dichloromethane (2×250 mL). The combined organic phases were extracted with water (2×200 mL). The aqueous phase was evaporated to dryness under reduced pressure (water bath temperature 40° C.) to give (5S)-5-[(tert-butylsulfinyl)methyl]-5-methylimidazolidine-2,4-dione as white crystals (0.497 eq, 23.0 mmol, 5.34 g) in 50% yield as a 71:29 mixture of diastereoisomers (by comparison of 1H NMR integrals).
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