[CH:1]1([NH:5][CH2:6]/[CH:7]=[CH:8]/[C:9]([O:11][CH3:12])=[O:10])[CH2:4][CH2:3][CH2:2]1.C=O.[BH-](OC(C)=O)(OC(C)=O)O[C:17](C)=O.[Na+]>C1COCC1>[CH:1]1([N:5]([CH3:17])[CH2:6]/[CH:7]=[CH:8]/[C:9]([O:11][CH3:12])=[O:10])[CH2:2][CH2:3][CH2:4]1To a solution of (E)-methyl 4-(cyclobutylamino)but-2-enoate (19) (1.7 g, 10 mmol) in THF (30 mL) was added HCHO solution (3 g, 40 mmol, 40% solution in water). The resulting mixture was stirred for 30 min before introducing NaBH(OAc)3 (3.18 g, 15 mmol). Stirring was continued at RT overnight. After quenching with sat. aq. NaHCO3 solution, the reaction mixture was extracted with EA (3×). The layers were separated and the organic layer was dried over Na2SO4, filtered and concentrated under vacuum. The crude product was purified by flash chromatography (0-5% methanol in DCM) to provide (E)-methyl 4-(cyclobutyl(methyl)amino)but-2-enoate (20) (1.47 g, 80%) as an oil. LC-MS (ESI): m/z (M+1) 184.3.
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