A solution consisting of 2.86 g (8 mmol) N-tert-butoxycarbonyl-O-benzyl-L-tyrosinal (Example 4, Step A) and 0.846 mL (8 mmol) tert-butyl amine (Aldrich, Milwaukee, Wis.) in 36 mL methanol containing 1% acetic acid was stirred at 25° C. for 20 minutes, at which time a solution of 1.06 g (16 mmol) sodium cyanotrihydridoborate (Aldrich, Milwaukee, Wis.) in 12 mL methanol was added and the resulting mixture stirred for 48 hours. The reaction was quenched by the addition of 3 M HCl in 3 mL increments until the vigorous bubbling subsided. The mixture was neutralized with saturated NaHCO3 solution then extracted with three 100 mL portions of ethyl acetate. The combined extracts were washed with 100 mL brine, dried over anhydrous sodium sulphate and evaporated at reduced pressure to give a pale amber oil which was chromatographed on a silica gel column using 1% to 2% methanol in ethyl acetate as eluant to give 604 mg (18%) of the title compound as waxy white crystals; mp 77-79° C.
