Ethyl chloroformate → carbonic acid 9-benzhydryloxy-7-(2,4-dimethoxy-benzyl)-6,8-dioxo-7,8-dihydro-6H-pyrrolo [3,4-g]quinolin-5-yl ester ethyl ester

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7-(2,4-Dimethoxy-benzyl)-5,9-dihydroxy-pyrrolo[3,4-g]quinoline-6,8-dione 300 (1.1 g, 2.9 mmol) was dissolved in dioxane (14.5 ml) and H2O (9.7 ml) and cooled to 0° C. To this reaction mixture was added 1.0 M NaOH (5.8 ml, 5.8 mmol), followed by ethylchloroformate (347.3 mg, 3.2 mmol). After stirring at 0° C. for 30 minutes, dioxane (10 ml) and ethylchloroformate (51 mg, 0.5 mmol) were added and the reaction stirred for another 30 minutes at 0° C. The reaction mixture was quenched with the addition of acetic acid (0.6 ml) and concentrated in vacuo. The crude mixture was diluted with ethyl acetate and washed once with 5% citric Acid (aqueous), twice with water, once with brine, and dried over magnesium sulfate. The resulting residue was dissolved in 1,2-dichloroethane (30 ml) and diphenyl-methanediazonium 38 (diphenyldiazomethane, 1.1 g, 5.6 mmol) was added. The reaction mixture was then stirred overnight at room temperature. Following dilution with dichloromethane, the reaction mixture was washed with once with water, once with brine, dried over magnesium sulfate, and concentrated in vacuo. The residue was purified by silica gel chromatography (1/1-Hexanes/Ethyl Acetate) to afford carbonic acid 9-benzhydryloxy-7-(2,4-dimethoxy-benzyl)-6,8-dioxo-7,8-dihydro-6H-pyrrolo [3,4-g]quinolin-5-yl ester ethyl ester 301 (1.2 g, 1.9 mmol, 66%). 1H NMR (CDCl3) δ 9.10 (dd, 1H), 8.40 (dd, 1H), 7.95 (s, 1H), 7.68 (m, 1H), 7.60 (d, 4H), 7.15 (m, 6H), 7.0 (d, 1H), 6.40 (d, 1H), 6.36 (d, 1H), 4.80 (s, 2H), 4.35 (q, 2H), 3.75 (s, 3H), 3.73 (s, 3H), 1.31 (t, 3H). MS: 641.2 (M+23).

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