O[C:2]1[N:7]=[C:6]([C:8]([O:10][CH2:11][CH3:12])=[O:9])[C:5]([CH3:13])=[N:4][C:3]=1[CH2:14][CH2:15][CH2:16][OH:17].C1(P(C2C=CC=CC=2)C2C=CC=CC=2)C=CC=CC=1.N(C(OCC)=O)=NC(OCC)=O>O1CCCC1>[CH3:13][C:5]1[N:4]=[C:3]2[CH2:14][CH2:15][CH2:16][O:17][C:2]2=[N:7][C:6]=1[C:8]([O:10][CH2:11][CH3:12])=[O:9]To a solution of ethyl 6-hydroxy-5-(3-hydroxypropyl)-3-methylpyrazine-2-carboxylate (1.03 g, 4.29 mmol) and triphenylphosphine (1.69 g, 6.43 mmol) in tetrahydrofuran (86 mL) was added diethyl azodicarboxylate solution (40% wt. % in toluene, 2.92 mL, 6.43 mmol) at room temperature. After being stirred for 50 min, the reaction mixture was concentrated in vacuo. The crude was purified by silica gel column chromatography (hexane:ethyl acetate=9:1 to 7:3), triturated with diisopropyl ether, and silica gel column chromatography (chloroform to chloroform:methanol=19:1) to give ethyl 2-methyl-7,8-dihydro-6H-pyrano[2,3-b]pyrazine-3-carboxylate. MS (APCI): m/z 223 (M+H).
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