To a stirred solution of 0.57 g (1.6 mmol) O-(9-phenyl-9H-fluoren-9-yl)-L-serine methyl ester (Example 4a, Step 2), 0.150 g (5 mmol) paraformaldehyde and water (0.4 mL) in methanol (5 mL) was added dropwise a solution of 0.11 g (0.8 mmol) zinc chloride and 0.1 g (1.6 mmol) sodium cyanoborohydride in methanol (5 mL) at room temperature. The mixture was stirred at room temperature for 16 h, the solvent was evaporated, water (50 mL) was added and the mixture extracted with dichloromethane (3×20 mL). The combined dichloromethane extracts were washed with brine, dried (MgSO4), the solvent evaporated and the residue chromatographed on silica gel with 30% ethyl acetate in hexanes to give 0.5 g (yield 81.6%) N,N-dimethyl--O--(9-phenyl-9H-fluoren-9-yl)-L-serine methyl ester as an oil. 1H NMR (CDCl3, 300 MHz) δ: 7.68 (d, 2H), 7.40-7.10 (m, 11H), 3.73 (s, 3H), 3.40-3.15 (m, 3H), 2.29 (s, 6H); 13C NMR (CDCl3, 75 MHz) 6: 171.3, 146.6, 146.5, 143.2, 140.7, 140.5, 129.1, 129.0, 128.2, 128.1, 128.0, 127.0, 125.5, 125.4, 125.3, 120.0, 119.9, 88.7, 67.7, 61.9, 51.4, 42.6, 14.2.
