C(Cl)(=O)C(Cl)=O.CS(C)=O.[OH:11][CH2:12][CH2:13][CH2:14][CH2:15][N:16]1[C:21](=[O:22])[CH:20]=[N:19][C:18]2[CH:23]=[CH:24][C:25]([O:27][CH3:28])=[N:26][C:17]1=2.C(N(CC)CC)C.[Cl-].[NH4+]>ClCCl>[CH3:28][O:27][C:25]1[CH:24]=[CH:23][C:18]2[N:19]=[CH:20][C:21](=[O:22])[N:16]([CH2:15][CH2:14][CH2:13][CH:12]=[O:11])[C:17]=2[N:26]=1A solution of oxalyl chloride (0.155 ml, 1.81 mmol) in dichloromethane (1.5 ml) was cooled to −78° C., a solution of dimethyl sulfoxide (0.128 ml, 1.81 mmol) in dichloromethane (1.5 ml) was added dropwise under a stream of argon and the solution was stirred at the same temperature for 15 minutes. A solution of 4-(4-hydroxybutyl)-6-methoxypyrido[2,3-b]pyrazin-3(4H)-one (150 mg, 0.60 mmol) in dichloromethane (1.5 ml) was added dropwise to the reaction solution and the mixture was further stirred at the same temperature for 1.5 hours. Triethylamine (0.503 ml, 3.61 mmol) was added to the reaction solution, the temperature was raised to room temperature and a saturated ammonium chloride solution and dichloromethane were added. The organic layer was separated, washed with water and saturated sodium chloride solution, dried over anhydrous magnesium sulfate and then the solvent was removed under reduced pressure to yield 148 mg (100%) of the title compound in the form of a yellow solid. The product was used as such for the next step without purification.
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