[C:1]1([CH2:7][C:8]2[CH:9]=[N:10][CH:11]=[CH:12][C:13]=2[C:14]([OH:16])=[S:15])[CH:6]=[CH:5][CH:4]=[CH:3][CH:2]=1.S(=O)(=O)(O)O.[CH3:22]O>COC(OC)(C)C>[C:1]1([CH2:7][C:8]2[CH:9]=[N:10][CH:11]=[CH:12][C:13]=2[C:14]([O:16][CH3:22])=[S:15])[CH:2]=[CH:3][CH:4]=[CH:5][CH:6]=1A solution of 5.1 g (20 mmol) of 3-phenylmethylthiopyridine-4-carboxylic acid in 150 mL of methanol, 1.6 mL of 2,2-dimethoxypropane, and 5 mL of sulfuric acid was heated at reflux for 18 hours. The solution was cooled, concentrated to 25 mL, then partitioned between ether and sodium bicarbonate solution. The ether layer was washed with sodium bicarbonate solution, then brine, then dried and concentrated to afford 5.0 g (96%) of white crystals, m.p. 79°-80° C.
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