Diphenylphosphoryl azide (25.0 mL, 116.0 mmol, 1.5 eq) is added at room temperature to a stirred solution of 4-benzyloxy-6-methoxy-[1,5]naphthyridine-3-carboxylic acid (24.0 g, 77.34 mmol, 1.0 eq) in N,N-dimethylformamide (300 mL), followed by tert-butanol (8.5 mL, 89.71 mmol, 1.16 eq) and triethylamine (104.5 mL, 773.4 mmol, 10.0 eq). After 30 minutes stirring at 70° C. and 2 hours at 100° C., the reaction mixture is cooled down to room temperature, solvent is removed and the residue is extracted with ethyl acetate (3×200 mL) and a saturated sodium hydrogen carbonate aqueous solution (200 mL). The combined organic layers are dried over sodium sulfate, filtered and concentrated to afford crude (4-benzyloxy-6-methoxy-[1,5]naphthyridin-3-yl)-carbamic acid tert-butyl ester that is directly engaged in the next step.
