To a stirred solution of 2-(trimethylacetylamino)-6-formylpyridine (3.00 g, 14.6 mmol) in 1:1 ethyl acetatetetrahydrofuran (24 mL) was added 4-amino-N-Boc-piperidine (reference example 2, 3.38 g, 18.2 mmol) and acetic acid (1.16 mL, 20.3 mmol) at 15° C. Then, sodium triacetoxyborohydride (4.50 g, 20.1 mmol) was added in 4 portions to the above homogenous solution over a period of 1.5 h at room temperature. Then, additional of tetrahydrofuran (6 mL) was added. After stirred for 1 h at room temperature, water (3 mL) was added maintaining the temperature below 15° C. with an ice-water bath. The reaction mixture was stirred for 10 min (after evolution of hydrogen) and 1N NaOH (22 mL) was added at this temperature. The mixture was extracted with ethyl acetate (3×40 mL) and the ethyl acetate extract washed with brine (40 mL). Then, the resulting solution was concentrated under reduced pressure and re-dissolved in ethyl acetate (40 mL) and treated with formic acid (1 mL). Then, the solvent was switched with methyl t-butyl ether (30 mL) and hexane (50 mL) was added dropwise. After stirring for 1.5 h at room temperature, the solvent was decanted and the residual formate salt of reductive amination product was dried to give 4.9 g (90%) as a sticky material.
