[N:1]([C:4]1[CH:23]=[CH:22][C:7]([CH2:8][C:9]2[C:10]([CH2:20][CH3:21])=[N:11][N:12]3[C:17]([CH3:18])=[CH:16][C:15]([CH3:19])=[N:14][C:13]=23)=[CH:6][CH:5]=1)=[N+:2]=[N-:3].[CH2:24]([O:26][CH:27]([O:30][CH2:31][CH3:32])[CH2:28][CH3:29])[CH3:25]>C(#N)C.CCOC(C)=O.[Cu]I>[CH2:24]([O:26][CH:27]([O:30][CH2:31][CH3:32])[C:28]1[N:3]=[N:2][N:1]([C:4]2[CH:23]=[CH:22][C:7]([CH2:8][C:9]3[C:10]([CH2:20][CH3:21])=[N:11][N:12]4[C:17]([CH3:18])=[CH:16][C:15]([CH3:19])=[N:14][C:13]=34)=[CH:6][CH:5]=2)[CH:29]=1)[CH3:25]A solution of 3-(4-azido-benzyl)-2-ethyl-5,7-dimethyl-pyrazolo[1,5-a]pyrimidine (20) (2 g, 6.53 mmol), propionaldehyde diethylacetal (1.294 g, 9.79 mmol) and copper(I) iodide (1.492 g, 7.83 mmol) in 33 ml of acetonitrile was stirred at r.t. for 24 h. The reaction was diluted in EtOAc and washed with sat aq Na2 CO3, water and brine. The organic layer was dried over anhydrous Na2 SO4, filtered and concentrated under HV. The crude product was purified by flash chromatography (silica gel, EtOAc/DCM 0% to 50%) to give a pale yellow oil.
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