To a solution of cis-ethyl 2-amino-1-cyclopentanecarboxylate hydrochloride (250.0 mg, 1.28 mmol) in ethanol (5 mL) was added triethylamine (0.2 mL, 1.42 mmol), cyclopropylacetaldehyde (288.0 mg, 1.92 mmol), and sodium cyanoborohydride (59.2 mg, 0.895 mmol) at 25° C. After stirring for 12 h, the mixture was concentrated in vacuo and the residue was dissolved in ethyl acetate. The organic layer was washed with water, dried over anhydrous magnesium sulfate, and concentrated in vacuo. The crude material was purified by flash column chromatography (Teledyne Isco RediSep Flash Column; 0-50% ethyl acetate in hexanes) to afford cis-2-(2-cyclopropylethylamino)-cyclopentanecarboxylic acid ethyl ester as a yellowish oil (42.7 mg, 0.189, 15%) and trans-2-(2-cyclopropylethylamino)-cyclopentanecarboxylic acid ethyl ester as a yellowish oil (130.5 mg, 0.579 mmol, 45%). cis-2-(2-Cyclopropylethylamino)-cyclopentanecarboxylic acid ethyl ester: 1H NMR (400 MHz, CDCl3) δ 0.08-0.12 (2H, m), 0.45-0.49 (2H, m), 0.63-0.72 (1H, m), 1.29 (3H, t, J=7.0 Hz), 1.46-1.52 (2H, m), 1.58-1.77 (2H, m), 1.85-2.10 (4H, m), 2.73-2.87 (2H, m), 3.03 (1H, q, J=6.7 Hz), 3.36 (1H, q, J=7.3 Hz), 4.17 (2H, q, J=7.0 Hz). LC-MS (ESI) calcd for C13H23NO2 225.17, found 226.2 [M+H+]. trans-2-(2-Cyclopropylethylamino)-cyclopentanecarboxylic acid ethyl ester: 1H NMR (400 MHz, CDCl3) δ 0.12-0.16 (2H, m), 0.50-0.55 (2H, m), 0.65-0.72 (1H, m), 1.31 (3H, t, J=7.1 Hz), 1.53-1.59 (2H, m), 1.64-1.84 (2H, m), 1.89-2.13 (4H, m), 2.81-2.87 (1H, m), 2.95-3.01 (1H, m), 3.07 (1H, q, J=7.1 Hz), 3.44 (1H, q, J=7.2 Hz), 4.20 (2H, q, J=7.0 Hz). LC-MS (ESI) calcd for C13H23NO2 225.17, found 226.2 [M+H+].
