In a dry flask (3R,4R)-4-aminopiperidine-1,3-dicarboxylic acid 1-tert-butyl ester 3-ethyl ester (3.07 g, 11.3 mmol) was dissolved in dichloromethane (100 mL) and triethylamine (2.1 mL, 15.1 mmol) and benzyl chloroformate (2.0 mL, 12.6 mmol) were added. The mixture was stirred for 22 hours. Water (30 mL) was added and the layers separated. The aqueous layer was extracted with dichloromethane (30 mL). The combined organic layers were dried with magnesium sulfate, filtered, and concentrated in vacuo to give a crude oil (4.91 g). Purification by flash column chromatography (40% ethyl acetate/hexanes) provided a colorless oil (2.37 g, 51%). 1H NMR (300 MHz, CDCl3), δ: 7.33 (m, 5H), 5.08 (s, 2H), 4.71 (s, 1H), 4.12 (m, 4H), 3.90 (m, 1H), 2.98 (bs, 1H), 2.85 (t, 1H, J=13), 2.37 (m, 1H), 2.06 (d, 1H, J=7), 1.45 (s, 9H), 1.37 (m, 1H), 1.20 (t, 3H, J=7).
