Potassium Carbonate → 3,3-dimethyl-4-[2-(4-nitro-phenyl)-6-oxo-4-thioxo-6H-1-oxa-3b,5-diaza-cyclopenta[a]pentalen-5-yl]-butyric acid ethyl ester

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2-(4-nitro-phenyl)-4H-furo[3,2-b]pyrrole-5-carboxylic acid ethyl ester (212 mg, 0.71 mmol, 1 molequivalent), prepared as described hereinabove (see, preparation of Compound 1), was placed in a round bottom flask and 4-isothiocyanato-3,3-dimethyl-butyric acid ethyl ester (1.4 grams, 7.1 mmol, 10 molequivalents) was added thereto, followed by addition of grinded anhydrous potassium carbonate (K2CO3; 147 mg, 1.1 mmol, 1.5 molequivalents). The mixture was heated while stirring at 140° C. for 16 hours. The reaction mixture was thereafter dissolved in chloroform and 5% citric acid solution, the organic phase was extracted, and the aqueous layer was washed with chloroform several times. The combined organic layers were dried over Na2SO4, the solvent was removed under reduced pressure, and the crude product was triturated in ethanol to give Compound 7 (322 mg, 40% yield) as an orange solid.

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