[C:1]([O:5][C:6](=[O:16])[NH:7][C@@H:8]1[CH2:13][CH2:12][NH:11][CH2:10][C@H:9]1[O:14][CH3:15])([CH3:4])([CH3:3])[CH3:2].C=O.[BH-](OC(C)=O)(OC(C)=O)O[C:21](C)=O.[Na+].C([O-])(O)=O.[Na+]>CN(C=O)C.C(O)(=O)C>[C:1]([O:5][C:6](=[O:16])[NH:7][C@@H:8]1[CH2:13][CH2:12][N:11]([CH3:21])[CH2:10][C@H:9]1[O:14][CH3:15])([CH3:4])([CH3:3])[CH3:2]Tert-butyl((3R,4R)-3-methoxy-piperidin-4-yl)-carbamate (2.88 g) is dissolved with formaldehyde (1.79 mL, 37% solution in H2O) and acetic acid (100 μL) in DMF. Then Na(OAc)3BH (12.59 g) is added. After 20 h the reaction mixture is combined with aqueous NaHCO3 solution (saturated with NaCl) and extracted 5 times with EtOAc. The combined organic phase is dried on magnesium sulphate, filtered off from the desiccant and the solvent is eliminated in vacuo. The residue is used in the next reaction step without any further purification.
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