3-(4-Fluorophenyl)-4-(phenylmethyl)-2-morpholinone → N-benzyl-3-(S)-(4-fluorophenyl)-1,4-oxazin-2-one

To a solution of 3-(4-fluorophenyl)-4-phenylmethyl-2-morpholinone (i.e. N-benzyl-4-fluorophenyl-1,4-oxazin-2-one) (10 g) in isopropyl acetate (110 ml) at room temperature was added a solution of (-)-3-bromocamphor-8-sulphonic acid ((-)-3BCS) (12 g) in acetonitrile (24 ml). Crystallisation began after 2-3 min. The slurry was stirred for 1 h at room temperature. Trifluoroacetic acid (7 ml) was added and the mixture stirred at 65° C. for 3 days. The mixture was cooled to 0-5° C., aged for 1 h. and the solid collected, washed with isopropyl acetate and dried in vacuo at 40° C., to give the N-benzyl-3-(S)-(4-fluorophenyl)-1,4-oxazin-2-one (-)-3BCS salt: yield 17.24 g, ee 98.6% (S) isomer. The chiral composition of the remaining liquors was determined as 79%(R), 21%(S). The liquors were stirred at 65° C. for 3 days, then cooled to 0-5° C. The solid was collected, washed with isopropyl acetate and dried in vacuo to give a further batch of the N-benzyl-3-(S)-(4-fluorophenyl)-1,4-oxazin-2-one (-)-3BCS salt: yield 0.84 g, ee 98.6% (S) isomer. The chiral composition of the remaining liquors was determined as 64%(R), 36%(S). The liquors were stripped in vacuo and the residue was dissolved in isopropyl acetate (20 ml) containing trifluoroacetic acid (1 ml) and stirred at 65° C. for 20 h. The mixture was cooled to 0-5° C. for 1 h and the solid collected, washed with isopropyl acetate and dried in vacuo to give a further batch of the N-benzyl-3-(S)-(4-fluorophenyl)-1,4-oxazin-2-one (-)-3BCS salt: yield 2.2 g, ee 99.2% (S) isomer. Total weight of (-)-3BCS salt: 20.28 g, 97% yield. A sample (0.5 g) of the (-)-3BCS salt was retained and the remainder converted back to free base. The salt was partitioned between isopropyl acetate (50 ml) and water (100 ml) containing 0.88 ammonia soln. (3 ml). The layers were separated and the aqueous phase extracted with isopropyl acetate (25 ml). The combined organic phases were washed with water (25 ml). The organic phase was concentrated to residue and flushed with isopropyl acetate to give the 3-(S)-(4-fluorophenyl)-4-phenylmethyl-2-morpholinone (i.e. N-benzyl-3-(S)-(4-fluorophenyl)-1,4-oxazin-2-one) as the free base: yield 8.7 g, 93% recovery, ee 98.4% (S) isomer.