N,N'-Carbonyldiimidazole (0.5 g) is added to a solution of (S)-3-tert-butoxycarbonylamino-2-phenylpropionic acid (0.79 g) in dry dichloromethane (20 cc), cooled to +5° C. The mixture is stirred for 1 hour 15 minutes at +5° C. and a solution of (3aR,7aR)-7,7-diphenyl-4-perhydroisoindolone hydrochloride (1 g) and triethylamine (0.4 cc) in dichloromethane (25 cc) is then added. The reaction mixture is stirred for 15 minutes at +5° C. and then for 16 hours at 20° C. The reaction mixture is washed with distilled water (3×100 cc), dried over magnesium sulphate and filtered and the filtrate is concentrated to dryness under reduced pressure (2.7 kPa). The residue is chromatographed on a silica gel column (0.2-0.06 mm, diameter 3 cm, height 30 cm), eluting with a mixture of cyclohexane and ethyl acetate (50/50 by volume) and collecting 50 cc fractions. Fractions 2 to 7 are combined and concentrated to dryness under reduced pressure (2.7 kPa). (3aR,7aR)-2-[(S)-2-tert-butoxycarbonylaminomethyl-2-phenylacetyl]-7,7-diphenyl-4-perhydroisoindolone (1.2 g) is obtained in the form of a white meringue.
