Stereoselective reduction of 2,3-butadione monoxime trityl ether

4 STEREOSELECTIVE REDUCTION OF 2,3-BUTADIONE MONOXIME TRITYL ETHER 

a-Diphenylpyrrolidinemethanol (127 mg) was placed in a 25 mL three-necked flask equipped with a magnetic stirrer bar, under nitrogen. A solution of trimethyl borate (62 mg) in dry tetrahydrofuran (5mL) was added. The mixture was stirred for 1 hour at room temperature. 

10 M Borane-dimethylsulfide complex (2.0 mL) was added to the resulting solution. The mixture was cooled to 0-5 °C with an ice-bath, and then a solution of 2,3-butadione monoxime trityl ether (1.72 g) in dry tetrahydrofuran (5 mL) was added dropwise via a syringe pump over 1 hour at that temperature. 

After being stirred for 0.5 hour at 0-5 °C, the mixture was allowed to warm to room temperature and heated under reflux for 18 hours. The resulting mixture was cooled to room temperature and cautiously transferred into 2N hydrochloric acid (15mL) in a 200 mL round-bottomed flask equipped with a magnetic stirrer bar using diethyl ether (10 mL). 

After being stirred for 5 hours at room temperature, the mixture was made basic with sodium hydroxide (2.4 g). The organic solvents were removed under reduced pressure using a rotary evaporator. The aqueous residue was washed with diethyl ether (2 x 10 mL) and then benzyl chloroformate (3.41 g) was added. The mixture was stirred for 20 hours at room temperature. 

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