Nitrobenzene → 5,7-Dihydroxy-8-propionyl-4-propyl-2H-chromen-2-one

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A three-neck flask (500 mL) equipped with an efficient methanical stirrer, thermometer and addition funnel was charged with 5,7-dihydroxy-4-propylcoumarin, 2, (25.0 g, 0.113 mol), aluminum chloride (62.1 g; 0.466 mol), and nitrobenzene (150 mL) and the mixture was stirred until a solution was obtained, which was cooled to 0° C. in an ice bath. A solution of propionyl chloride (15.2 g; 0.165 mol) in carbon disulfide (50 mL) was added dropwise at such a rate that the reaction temperature was maintained at 8°-10° C. Addition was completed over a period of 1 hour with vigorous stirring. The reaction was monitored by TLC using a mobile phase of 50% ethyl acetate/hexane. After three hours, an additional portion of propionyl chloride (2.10 g; 0.0227 mol) in carbon disulfide (10 mL) was added. Immediately after the TLC analysis indicated the total consumption of starting material, the reaction mixture was poured onto ice, and allowed to stand overnight. The nitrobenzene was removed by steam distillation, and the remaining solution was extracted several times with ethyl acetate. The extracts were combined and dried over Na2SO4. The crude product obtained by evaporation in vacuo was purified by chromatography on a silica gel column eluting with 50% ether/hexane to provide the desired propionylated coumarin 3, mp (corr) 244°-246° C. 1H-NMR (DMSO-d6) δ 0.96 (3H, t, J=7.3 Hz, CH3); 1.10 (3H, t, J=7.2 Hz, CH3); 1.60 (2H, m, CH2); 2.88 (2H, t, J=7.7 Hz, CH2); 3.04 (2H, q, J=7.2 Hz, CH2); 5.95 (1H, s, H3); 6.31 (1H, s, H6); 11.07 (1H, s, OH); 11.50 (1H, s, OH); MS (EI): 277 (6.6, M+1); 276 (9.0, M+); 247 (100, M--C2H5); IR (KBr): 3239 (s and broad, OH); 1693 (s, C=O), 1625 and 1593 (s) cm-1 ; Anal. calcd. for C15H16O5 : C, 65.21; H, 5.84; Found: c, 64.92; H, 5.83. The isomer assignment was made by analogy to precedent.15

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