[H-].C([Al+]CC(C)C)C(C)C.C1(C)C=CC=CC=1.C([O:20][C:21]([C@@H:23]1[CH2:28][CH2:27][CH2:26][N:25]([C:29]([O:31][C:32]([CH3:35])([CH3:34])[CH3:33])=[O:30])[CH2:24]1)=O)C.O>C(OCC)C>[C:32]([O:31][C:29]([N:25]1[CH2:26][CH2:27][CH2:28][C@@H:23]([CH:21]=[O:20])[CH2:24]1)=[O:30])([CH3:35])([CH3:34])[CH3:33]
A 1.2 M solution of diisobutylaluminum hydride in toluene (30.8 ml, 36.9 mmol) was added at −78° C. to a solution of (3R)-piperidine-1,3-dicarboxylic acid 1-tert-butyl ester 3-ethyl ester (4.13 g, 16.1 mmol) in diethyl ether (30 ml). The reaction mixture was stirred for 2.5 h at −78° C. Water (9.6 ml) was added dropwise. The reaction mixture was warmed to room temperature. The precipitation was removed by filtration through a plug of celite. The celite was washed with tert-butyl methyl ether (3×100 ml). The liquids were combined and dried over magnesium sulfate. The solvent was removed in vacuo, to give 1.94 g of crude (3R)-3-formylpiperidine-1-carboxylic acid tert-butyl ester, which was used for the next step without further purification.
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