Methyl chloroformate (75 μL, 0.92 mmol) was added dropwise to a solution of 2-(6-tert-butoxycarbonylamino-pyridin-3-ylmethyl)-2-methyl-malonic acid mono-tert-butyl ester (348 mg, 0.915 mmol) and Et3N (123 μL, 0.92 mmol) in THF (6 mL). The reaction mixture was stirred for 20 min., filtered and added dropwise to a suspension of NaBH4 (39 mg, 1.04 mmol) in THF (6 mL) at 0° C. The reaction was stirred for 16 h at room temperature. 0.2 M HCl was added followed by EtOAc. The organic phase was washed with brine and dried. Concentration under reduced pressure followed by chromathography (toluene/EtOAc, 3:1→1:3) gave 3-(6tert-butoxycarbonylamino-pyridin-3-yl)-2-hydroxymethyl-2-methyl-propionic acid tert-butyl ester (190 mg, 57%).
