meta-Anisidine (1.72 g) is added at a temperature in the region of 20° C. to a solution, maintained under an argon atmosphere, of (5RS, 6SR)-5-(4-nitrophenoxycarbonyloxy)-2,6-diphenyl-5,6-dihydro-4H-1,3-oxazine (3 g) and 4-(dimethylamino)pyridine (0.3 g) in anhydrous N,N-dimethylformamide (50 cc). The solution obtained is heated to a temperature in the region of 70° C. for 6 hours and then concentrated to dryness under reduced pressure (2.7 kPa). The residue is purified by chromatography on silica (0.063-0.2 mm; 50 g) contained in a column 2 cm in diameter [eluent: cyclohexane/ethyl acetate/triethylamine (65:30:5 by volume)], collecting 25-cc fractions. Fractions 2 to 5 are combined and concentrated to dryness under reduced pressure (2.7 kPa) at 40° C. After recrystallization in diisopropyl ether, (5RS, 6SR)-5-(3-methoxyphenylcarbamoyloxy)-2,6-diphenyl-5,6-dihydro-4H-1,3-oxazine (1.8 g), m.p. 110° C. is obtained.
