A solution of [trans-4-(2-oxo-ethyl)-cyclohexyl]-carbamic acid tert-butyl ester (3.1 g, 25.1 mmol, 1.0 eq), 3-chloro-6-methoxy-[1,5]naphthyridine-4-carbaldehyde (5.6 g, 25.1 mmol, 1.0 eq) and L-proline (1.16 mg, 10.1 mmol, 0.4 eq) in dimethyl sulfoxide (100 mL) and water (15 mL) is stirred at room temperature for 16 hours. The reaction mixture is then extracted with ethyl acetate (500 mL) and water (500 mL). The organic layer is washed with brine (300 mL), dried over magnesium sulfate, filtered and concentrated to give a residue that is purified by column chromatography (silica gel, eluent: ethyl acetate:petroleum ether, 1:1, v/v) to afford {trans-4-[2-(3-chloro-6-methoxy-[1,5]naphthyridin-4-yl)-1-formyl-2-hydroxy-ethyl]-cyclohexyl}-carbamic acid tert-butyl ester as a light yellow solid (4.5 g, 39% yield).
