The N-p-methoxybenzyloxycarbonyl-D-serine benzyl ester (720 mg, 2.00 mmol) is dissolved in dry dichloromethane (20 ml) and the solution cooled in dry ice/acetone. Boron trifluoride etherate (100 μl) is added followed by the portionwise addition of diazomethane (30 mmol in 20 ml CH2Cl2) After about 30 minutes the solution is filtered and washed once with water (10 ml), then dried (MgSO4) and evaporated to dryness. The resulting residue is chromatographed on silica gel using chloroform as eluant to give the pure title compound as an oil (520 mg, 70%) which crystallises on standing to give a colourless solid of m.p. 68°-69° C.; [α]D20 --1.3° (c 4.0, CHCl3).
