[NH:1]1[CH2:8][CH2:7][CH2:6][C@H:2]1[C:3]([OH:5])=[O:4].ClCCCl.[O:13]1[CH2:18][CH2:17][C:16](=O)[CH2:15][CH2:14]1.[O-]S([O-])(=O)=O.[Na+].[Na+].C(O[BH-](OC(=O)C)OC(=O)C)(=O)C>>[O:13]1[CH2:18][CH2:17][CH:16]([N:1]2[CH2:8][CH2:7][CH2:6][C@H:2]2[C:3]([OH:5])=[O:4])[CH2:15][CH2:14]1To L-proline (1.00 g; 8.69 mmol) in 1,2-dichloroethane (10 mL) acetic acid (1.98 mL; 33.0 mmol)) is added tetrahydro-pyran-4-one (0.87 g; 8.69 mmol) and Na2SO4 (˜10 equivalents). After 45 minutes of agitation on an orbital shaker, Mp-triacetoxyborohydride resin (4.27 g; 10.42 mmol) is added. The mixture is agitated at RT overnight and filtered and the resin washed with DCM. The combined filtrate is washed with aq. sat. NaHCO3 solution and brine, dried (Na2SO4), filtered and concentrated in vacuo. Excess acetic acid is removed by successive azeotropic distillation with toluene on the rotary evaporator to afford (S)-1-(tetrahydro-pyran-4-yl)pyrrolidine-2-carboxylic acid. ESI-MS: 200 [M+H]+
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