To a solution of 3-pyridinyl benzaldehyde (10) (205 mg, 1.1 mmol) in methanol (20 ml) was added NT-Boc-1,4-diaminocyclohexane (6) (300 mg, 1.4 mmol), and the mixture was stirred at room temperature for 30 min. To this solution was added NaBH4 (0.5 g, 13.2 mmol) in portions at 0° C., and the reaction mixture was stirred at room temperature overnight. The reaction was worked up by addition of saturated aqueous Na2CO3 (2 ml), and the mixture was then extracted with chloroform (3×6 ml). The combined organic layers were dried over anhydrous MgSO4. The solvent was removed under vacuum and the residue was purified by a flash chromatography on silica gel (CH2Cl2/MeOH=10/1) to give 18 (398 mg) in 95% yield; 1H NMR (300 MHz, CDCl3): 8.66 (d, J=8.1 Hz, 2H), 7.39-7.61 (m, 6H), 4.30-4.50 (br, 1H), 3.88 (s, 2H), 3.30-3.50 (br, 1H), 2.40-2.60 (m, 1H), 1.90-2.10 (m, 4H), 1.70-1.90 (br, 1H), 1.44 (s, 9H), 1.06-1.40 (m, 4H) 13C NMR (75 MHz, CDCl3): 155.2, 150.0, 148.3, 141.5, 138.1, 129.1, 128.7, 126.6, 125.5, 121.6, 79.0, 55.6, 51.0, 49.4, 31.9, 28.3; HRMS (ESI) calcd for C23H31N3O2 (M+H+) 382.24890; found 382.24896.
