To a three-necked flask equipped with a mechanical stirrer, temperature probe and nitrogen inlet was added dibenzyl cis-N-benzylazetidine-2,4-dicarboxylate (173 g, 416 mmol) followed by methanol (1.14 L) and tetrahydrofuran (560 mL). The mixture was stirred at room temperature under nitrogen protection until homogeneous. Subsequently, the solution was cooled to an internal temperature of -4 °C in a dry ice/acetone bath. Sodium borohydride (79.0 g, 2.08 mol) was added in batches over a period of 1-1.5 hours, during which a slight exotherm was observed and the maximum temperature rose to 6 °C. After addition, the mixture was stirred under nitrogen protection and gradually warmed up to room temperature overnight (note: secondary exotherm was observed during warming up to a maximum temperature of 27 °C, controlled by an ice bath). Aliquots of the reaction mixture diluted in methanol were analyzed by LCMS to confirm the consumption of raw materials and generation of products (containing benzyl alcohol). The reaction mixture was cooled to an internal temperature of 8 °C in an ice-water bath, and the reaction was quenched by dropwise addition of water (100 mL) through a charging funnel. Most of the volatiles were removed by concentration under reduced pressure. The residue was partitioned with 10% methanol/dichloromethane (500 mL) and water (200 mL). After partitioning, the organic phase (upper layer) and the aqueous phase were collected. The aqueous phase was again extracted with 10% methanol/dichloromethane (2 x 500 mL; the organic layer was the lower layer at this point). Analysis of aliquots of the aqueous phase showed no product residue. The combined organic extracts were washed sequentially with water and saturated aqueous sodium chloride. Analysis of the aqueous phase (water plus saturated aqueous sodium chloride) showed the presence of product (containing benzyl alcohol), so the aqueous phase was back-extracted with 10% methanol/dichloromethane (3×). All methanol/dichloromethane extracts were combined and dried over anhydrous sodium sulfate. After filtration and concentration under reduced pressure, 177 g of yellow oil was obtained. It was further concentrated in a water bath at 75 °C under high vacuum to remove most of the benzyl alcohol, leaving 100 g of oil at constant weight. The substance was dissolved in dichloromethane and concentrated under reduced pressure to give a semi-solid. The semi-solid was suspended twice in hexane and concentrated under reduced pressure. The remaining solid was suspended in hexane (260 mL) and stirred overnight. The suspension was filtered and the solid was washed with hexane (2×). LCMS analysis of a sample of methanol-solubilized solid showed a product purity of about 90% (containing benzyl alcohol). The solid was re-suspended in hexane (200 mL) and stirred (on a rotary evaporator) for 1 hour at 30°C. Filtration was performed and the solid was washed with hexane (1×). LCMS analysis of the methanol solubilized solid sample showed product purity >96%. The solid was air dried to constant weight to give 81.9 g (95% yield) of cis-N-benzyl-2,4-bis(hydroxymethyl)azetidine (Intermediate 5) as a white solid.1H NMR (CDCl3, 300 MHz): δ 7.28 (m, 5H), 4.24 (dd, 2H), 3.63 (s, 2H), 3.12 (dd, 4H), 3.05 (m, 2H), 2.00 (dd, 1H), 1.57 (dd, 1H); LCMS (m/z): 208 (M + 1).
