A solution of 7-(dimethoxymethyl)-6-methoxy-1,2,3,4-tetrahydro-1,8-naphthyridine (intermediate 309, 76 mg, 0.319 mmol) and diphenylcarbonate (137 mg, 0.638 mmol) in THF (2 ml) at −15° C. was treated drop wise with LHMDS (1 M in THF, 0.35 ml, 0.35 mmol) and stirred for 25 min. The reaction mixture was then quenched with sat. aq. NH4Cl and extracted with EtOAc (2×). The combined organic layers were washed with brine, dried over Na2SO4, filtered and concentrated under reduced pressure. The crude material was purified by normal phase chromatography (12 g silica gel cartridge, heptanes/EtOAc 100:0 to 0:100) to give the title compound as a yellow oil. (UPLC-MS 1) tR 0.93 min; ESI-MS 360.1 [M+H]+.
