C[O:2][C:3]([C:5]1[C:15]2[O:14][C:13]3[C:16]([CH:22]=[O:23])=[C:17]([OH:21])[CH:18]=[C:19]([CH3:20])[C:12]=3[C:11](=[O:24])[O:10][C:9]=2[C:8]([CH3:25])=[C:7]([O:26][CH3:27])[CH:6]=1)=[O:4].[I-].[Li+].Cl>CN(C)P(=O)(N(C)C)N(C)C>[CH3:20][C:19]1[C:12]2[C:11]([O:10][C:9]3[C:8]([CH3:25])=[C:7]([O:26][CH3:27])[CH:6]=[C:5]([C:3]([OH:4])=[O:2])[C:15]=3[O:14][C:13]=2[C:16]([CH:22]=[O:23])=[C:17]([OH:21])[CH:18]=1)=[O:24]4-Formyl-3-hydroxy-8-methoxy-1,9-dimethyl-11-oxo-11H-dibenzo[b,e][1,4]dioxepine-6-carboxylic acid methyl ester (42.0 mg) and lithium iodide (250 mg) were taken in hexamethylphosphoric triamide (5 ml). The mixture was stirred at 90° C. for 5 h and then lithium iodide (250 mg) was added again. The mixture was heated at 90° C. for 21 h. When the reaction was completed, the mixture was cooled and poured into hydrochloric acid cooled by an ice bath. The mixture was extracted with ethyl acetate. The organic extract was washed in turn with water, saturated aqueous sodium thiosulphate, and finally saturated brine, and was dried over anhydrous magnesium sulfate. The crude product was crystallized from dimethyl sulphoxide to afford psoromic acid, m.p. 264-265° C., 1HNMR (300 MHz, d6-DMSO): δ 2.20 (s, 3H), 2.46 (s, 3H), 3.83 (s, 3H), 6.83 (s, 1H), 7.08 (s, 1H), 10.46 (s, 1H).
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