To a solution of 2-mercaptoimidazolo[4,5-b]pyridine (38.1 mg, 0.252 mmol) in DMF (0.5 ml) were added 18-crown-6 (6.6 mg, 0.025 mmol) and potassium carbonate (63 mg, 0.454 mmol) and the mixture was stirred. Then to the mixture was dropped a solution of crude 2-[4-(2-methanesulfonyloxyethyl)piperazin-1-yl]-N-[2,4-bis(methylthio)-6-methyl-3-pyridyl]acetamide in DMF (1 ml) and stirred at 80° C. for 2 hours. The reaction mixture was diluted with water and extracted with ethyl acetate. The organic layer was washed with water and a saturated sodium chloride solution successively, dried over anhydrous sodium sulfate and concentrated in vacuo. The resulting residue was purified by a silica gel preparative thin layer chromatography (developing solvent; chloroform:methanol=10:1) to provide 51 mg (yield 40%) of the desired compound as colorless powdery crystals. IR (KBr) cm−1: 3140, 1688, 1600, 1564, 1485. 1H-NMR (CDCl3) δ: 2.38 (3H, s), 2.46 (3H, s), 2.49 (3H, s), 2.72-3.00 (10H, m), 3.29 (2H, s), 3.37 (2H, t, J=5.8 Hz), 6.63 (1H, s), 7.12 (1H, dd. J=7.8, 4.9 Hz), 7.82 (1H, d. J=7.8 Hz), 8.25 (1H, d. J=4.9 Hz), 8.49 (1H, br s).
