To a cold (−25° C.) solution of methyl (6S)-8-ethyl-10-(methoxy)-6-methyl-1,9-dioxo-1,2,6,7,8,9-hexahydropyrido-[3′,4′:4,5]pyrrolo[1,2-a]-pyrazine-4-carboxylate (0.29 g, 0.87 mmol) in anhydrous DMF (15 mL) under an atmosphere of nitrogen, a solution of lithium bis(trimethylsilyl)amide in THF (1 M, 0.96 mmol) was added. The reaction mixture was stirred at −25° C. for 25 minutes and treated with 3-chloro-4-fluorobenzyl bromide (0.27 g, 1.2 mmol; passed through activated basic alumina). The reaction mixture was stirred at 0° C. for 40 minutes and concentrated under vacuum. The residue was partitioned between ethyl acetate and dilute aqueous HCl. The organic extract was washed with brine, dried over anhydrous sodium sulfate, filtered, and concentrated under vacuum. The residue was subjected to column chromatography on silica gel eluted with ethyl acetate-hexane gradient. Collection and concentration of appropriate fractions provided the titled product.
