Production method of (1S,2S)-2-Methyl-3-oxo-1-phenylpropylcarbamate contains 3 steps. The synthetic routes are as follows:

A. tert-Butyl phenyl(phenylsulfonyl)methylcarbamate. A 500-mL, two-necked, round-bottomed flask is equipped with a magnetic stirring bar and fitted with a glass stopper and an argon inlet. The flask is flushed with argon and charged with tert-butyl carbamate (13.00 g, 110.9 mmol, 1.00 equiv) and 40 mL of tetrahydrofuran. (100 mL), sodium benzenesulfinate (18.21 g, 110.9 mmol, 1.00 equiv), and freshly distilled benzaldehyde (11.48 mL, 113.1 mmol, 1.02 equiv) are sequentially added in single portions, followed by formic acid (99%, 24.3 mL, 643 mmol). The reaction mixture is stirred for 18 h at room temperature under an argon atmosphere, during which time the desired product precipitates. The resulting white solid is filtered through a Büchner funnel (diameter 100 mm) and washed with distilled water (200 mL). The solid is transferred to a 500-mL single-necked, round-bottomed flask and is slurried in a mixture of hexane/dichloromethane (150/15 mL). The mixture is stirred for 2 h at room temperature, after which time it is collected by filtration in a Büchner funnel (diameter 100 mm) and washed with hexane/dichloromethane (91/9 mL). The solid is transferred to a 300-mL round-bottomed flask and dried under reduced pressure (0.5 mmHg) at 25 °C for 6 h to afford 30.79 g (80%) of the title compound 1 as a white solid.

B. (E)-tert-Butyl benzylidenecarbamate.3 A 1-L, two-necked, round-bottomed flask equipped with a magnetic stirring bar, a rubber septum, and a reflux condenser capped with an inlet adaptor connected to an argon-vaccum manifold, is charged with anhydrous potassium carbonate (71.48 g, 517.2 mmol, 6.0 equiv). The solid is placed under vacuum (0.5 mmHg) and flame-dried. The flask is purged with argon and anhydrous tetrahydrofuran (600 mL) is added via cannula under argon at 25 °C. Then, the septum is removed and compound tert-Butyl phenyl(phenylsulfonyl)methylcarbamate (29.95 g, 86.20 mmol, 1.0 equiv) is added into the flask. The septum is exchanged for a glass stopper, and the resulting suspension is heated to reflux at 80-85 °C (oil bath temp) with vigorous stirring under argon. After 15 h, the reaction is cooled to 25 °C and the solid is filtered off through alternating layers of Celite (1 cm thick)/Na₂SO₄ (1 cm thick)/Celite (1 cm thick) using a Büchner funnel (diameter 100 mm), and washed with anhydrous tetrahydrofuran (100 mL). The filtrate is concentrated at 30 °C (water bath temperature) by rotary evaporation (20 mmHg) and dried under vacuum (0.5 mmHg) to give 17.68 g (>99%) of the corresponding N-Boc-imine (E)-tert-Butyl benzylidenecarbamate as colorless oil.

C. (1S,2S)-2-Methyl-3-oxo-1-phenylpropylcarbamate. A 500-mL, three-necked, round-bottomed flask equipped with a magnetic stirring bar, thermometer, rubber septum and argon inlet is charged with N-Boc-imine (E)-tert-Butyl benzylidenecarbamate (5.00 g, 24.4 mmol, 1.0 equiv). The flask is flushed with dry argon and anhydrous acetonitrile (240 mL) is added via cannula under argon. Freshly distilled propionaldehyde (3.52 mL, 48.7 mmol, 2.0 equiv) is added quickly in one portion by syringe and the flask is sealed and purged with argon. The resulting colorless solution is cooled to 0 °C (internal temperature) with a cryostat. The septum is removed temporarily and (S)-proline (561 mg, 4.87 mmol, 20 mol %) is added. After the reaction mixture is stirred at 0 °C (internal temperature) for 12 h, it is treated with 80 mL of distilled water and warmed to 25 °C. The solution is stirred vigorously for 20 min at 25 °C, diluted with diethyl ether (100 mL), and transferred to a 500-mL separatory funnel. The layers are separated, and the aqueous layer is extracted with diethyl ether (100 mL). The combined organic phases are washed with brine (60 mL), dried over MgSO₄ (20 g), filtered through a Büchner funnel, and concentrated by rotary evaporator (30 °C, water bath temp, 20 mmHg). The resulting solid is triturated with cool iso-hexane (4 °C, 70 mL), filtered, washed with an additional portion of cool iso-hexane (4 °C, 15 mL), and dried under vacuum (0.5 mmHg, 25 °C) to afford 5.61 g (87%) of Mannich product (1S,2S)-2-Methyl-3-oxo-1-phenylpropylcarbamate.