[N:1]1[CH:6]=[C:5]([CH:7]=O)[CH:4]=[N:3][CH:2]=1.[C:9]([O:13][C:14]([N:16]1[CH2:21][CH2:20][NH:19][CH2:18][CH2:17]1)=[O:15])([CH3:12])([CH3:11])[CH3:10].C([BH3-])#N.[Na+]>C(O)C.C(O)(=O)C>[N:3]1[CH:4]=[C:5]([CH2:7][N:19]2[CH2:18][CH2:17][N:16]([C:14]([O:13][C:9]([CH3:12])([CH3:11])[CH3:10])=[O:15])[CH2:21][CH2:20]2)[CH:6]=[N:1][CH:2]=1
A solution of pyrimidine 5-carboxaldehyde (100 mg, 0.92 mmol) in ethanol (2 mL) and acetic acid (0.2 mL) at room temperature was treated with tert-butyl-1-piperazine carboxylate (2.5 eq, 2.31 mmol, 430 mg) and stirred for 5 minutes. Sodium cyanoborohydride (0.95 eq, 0.88 mmol, 55 mg) was added portionwise and the reaction then stirred for 16 h. Concentration in vacuo and preparative tlc purification (EtOAc) gave the product (141 mg, 55%) as a pale yellow oil; δH (500 MHz, CDCl3) 1.47 (s, 9H, C(CH3)3), 1.95 (br, 2H, piperazine NCH2), 2.42 (t, J=4.7 Hz, 2H, piperazine NCH2), 3.45 (t, J=4.9 Hz, 4H, piperazine N(CH2)2), 3.54 (s, 2H, CH2), 8.78 (s, 2H, pyrimidine H-4 & H-6), 9.15 (s, 1H, pyrimidine H-2);
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