[NH:1]1[CH2:6][CH2:5][CH2:4][CH2:3][C@@H:2]1[CH2:7][O:8][C:9]1[C:17]2[C:16]3[CH:18]=[C:19]([C:22]#[N:23])[N:20]=[CH:21][C:15]=3[N:14]([CH2:24][O:25][CH2:26][CH2:27][Si:28]([CH3:31])([CH3:30])[CH3:29])[C:13]=2[N:12]=[CH:11][CH:10]=1.[CH:32](=O)[CH3:33].C(O[BH-](OC(=O)C)OC(=O)C)(=O)C.[Na+]>C(Cl)Cl.C(=O)(O)[O-].[Na+]>[CH2:32]([N:1]1[CH2:6][CH2:5][CH2:4][CH2:3][C@@H:2]1[CH2:7][O:8][C:9]1[C:17]2[C:16]3[CH:18]=[C:19]([C:22]#[N:23])[N:20]=[CH:21][C:15]=3[N:14]([CH2:24][O:25][CH2:26][CH2:27][Si:28]([CH3:31])([CH3:30])[CH3:29])[C:13]=2[N:12]=[CH:11][CH:10]=1)[CH3:33]To a solution of (R)-4-(piperidin-2-ylmethoxy)-9-(2-trimethylsilanyl-ethoxymethyl)-9H-dipyrido[2,3-b;4′,3′-d]pyrrole-6-carbonitrile (96 mg, 0.22 mmol) in methylene chloride (1.4 mL) was added acetaldehyde (19 uL, 0.33 mmol) followed by sodium triacetoxyborohydride (70 mg, 0.33 mmol), and the reaction mixture was stirred at ambient temperature for 5 minutes. The mixture diluted with saturated aqueous sodium bicarbonate solution (50 mL) and methylene chloride (50 mL). The organic layer was separated, dried over sodium sulfate, filtered, concentrated in vacuo, and purified by flash chromatography (silica, 4 g, ISCO, 1-15% methanol in methylene chloride) to afford the title compound as a yellow solid, which was used in the next step without any further purification (64 mg, 87%).
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