To a solution of 2.14 mL (15.3 mmol) of N,N-diisopropylamine in 50 mL of tetrahydrofuran at −78° C. was added 7.96 mL (15.6 mmol, 2.5M in hexane) of n-butyl lithium and the mixture was stirred at −78° C. for 10 min. The reaction was warmed to 0° C. and stirred at 0° C. for 15 min. The mixture was cooled down to −78° C. again, and a solution 2.50 g (11.7 mmol) of tert-butyl 2-methyl-3-oxopiperidine-1-carboxylate (Step C) in 5 mL of tetrahydrofuran was added to the mixture slowly. The mixture was stirred at −78° C. for 1 h. Then 1.77 mL (14.9 mmol) of trans-cinnamaldehyde in 5 mL of tetrahydrofuran (pre-cooled to −78° C.) was added to the reaction mixture and the reaction was stirred at −78° C. for 5 h. The reaction was quenched with saturated aqueous sodium bicarbonate solution and extracted with three portions of ether. The combined organic layers were washed with brine, dried over magnesium sulfate, and concentrated. Purification by flash chromatography on a Biotage® system (silica gel, eluting with 10% ethyl acetate/hexane to 30% ethyl acetate/hexane) gave the tide compound. LC/MS 368 (M+23).
