Compound 14

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Synthetic procedure: See article for the definitive version of this procedure and for full experimental details.

A single neck round bottomed flask fitted with a condenser was charged with diester 13 (0.476 g, 0.936 mmol, 1 equiv.), NaOH (0.60 g, 15.0 mmol, 16 equiv.), EtOH (15 ml), and H₂O (4.5 ml). After refluxing the flask overnight, the reaction was cooled and the EtOH evaporated. Concentrated HCl was slowly added to the aqueous solution and a precipitate formed, which was isolated and washed sequentially with cold H₂O and cold MeOH to yield the diacid intermdiate as a yellow solid (0.373 g, 83%); ¹H NMR (600 MHz, DMSO-d₆, 25 °C) δ (ppm) 13.37 (s, 2H), 8.48 (s, 2H), 8.36 (s, 2H), 8.03 (d, J = 7.7 Hz, 2H), 7.92 (d, J = 7.2 Hz, 2H), 7.54 (s, 2H), 7.44-7.38 (m, 4H); ¹³C NMR (151 MHz, DMSO-d₆, 25 °C) δ (ppm) 169.03, 141.58, 139.92, 139.61, 133.16, 131.99, 130.93, 130.52, 129.30, 124.73, 124.65, 123.88, 123.25, 122.29. HRMS (ES⁺) (m/z), calculated for C₂₈H₁₇O₄S₂ (M+H)⁺ 481.0568, found 481.0569. To a suspension of the diacid (0.476 g, 0.991 mmol, 1 equiv.) in CH₂Cl₂ (20 ml) was added 3 drops of DMF followed by oxalyl chloride (0.34 ml, 3.96 mmol, 3.0 equiv.). After 1 hour, the volatiles were removed under reduced pressure. The crude acid chloride was dissolved in CH₂Cl₂ (20 ml) and solid AlCl₃ (0.661 g, 4.95 mmol, 5 equiv.) was added to the flask. The reaction was stirred overnight and then poured into an HCl-ice mixture, precipitating the dione. The solid was filtered and washed successively with H₂O, CH₂Cl₂ and MeOH to afford dione 14 as a purple solid that was too poorly soluble to obtain NMR spectra (0.41 g, 93%); HRMS (ES⁺) (m/z), calculated for C₂₈H₁₂O₂S₂ (M)⁺ 444.0279, found 444.0297.