Potassium Carbonate → 2-(3,5-Dichlorophenyl)-1,3-dioxo-hexahydro-imidazo[1,5-a]pyrazine-7-carboxylic Acid Tert-Butyl Ester

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3,5-Dichlorophenyl isocyanate (2.85 g, 15.2 mmol) was added portionwise to a mixture of piperazine-1,3-dicarboxylic acid 1-tert-butyl ester 3-ethyl ester (2.61 g, 10.1 mmol, prepared as described in Bioorg. Med. Chem. Lett. 1999, 9, 1121-6) and potassium carbonate (1.68 g, 12.1 mmol) in DCM (30 mL) at 5° C. After 30 min at 5° C., the reaction mixture was stirred at 20° C. for 48 h. The white precipitate was discarded and the filtrate concentrated in vacuo to give an oil (5.5 g) which was chromatographed over silica gel (CH2Cl2), to yield the above compound (2.77 g) as a white solid. Mp=144° C.

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