[Li+].[BH4-].[N+:3]([C:6]1[CH:29]=[CH:28][C:9]([CH2:10][O:11][C:12]([N:14]2[CH2:19][CH2:18][N:17]3[N:20]=[C:21]([C:23](OCC)=[O:24])[CH:22]=[C:16]3[CH2:15]2)=[O:13])=[CH:8][CH:7]=1)([O-:5])=[O:4].Cl.C([O-])([O-])=O.[K+].[K+]>CO.C1COCC1>[N+:3]([C:6]1[CH:29]=[CH:28][C:9]([CH2:10][O:11][C:12]([N:14]2[CH2:19][CH2:18][N:17]3[N:20]=[C:21]([CH2:23][OH:24])[CH:22]=[C:16]3[CH2:15]2)=[O:13])=[CH:8][CH:7]=1)([O-:5])=[O:4]LiBH4 (640 mg) and MeOH (1.2 mL) was added to the THF (267 mL) solution of 6,7-dihydro-4H-pyrazolo[1,5-a]pyrazine-2,5-dicarboxylic acid 2-ethyl ester 5-(4-nitrobenzyl) ester (10 g) under a nitrogen atmosphere at room temperature and stirred for 3 h at 40° C. Additional LiBH4 (523 mg) and MeOH (1.0 mL) was added to the solution and stirred for 1 h at 40° C. and 1 h at 50° C. The mixture was acidified with 3 mol/L HCl to pH 2 and stirred for 1 h at room temperature, then solid K2CO3 was added to the solution to adjust pH to 8. The insoluble material was filtered off and the filtrate was extracted with AcOEt. The organic layer was dried (K2CO3), and concentrated under reduced pressure. The residue was purified with silica gel column chromatography (CHCl3/MeOH=49/1–19/1) to afford titled compound as pale yellow crystals (8.44 g, 95%).
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