[CH3:1][O:2][C:3]([CH:5]1[CH2:10][CH2:9][CH:8]([C:11](O)=[O:12])[CH2:7][N:6]1[S:14]([CH3:17])(=[O:16])=[O:15])=[O:4].B>O1CCCC1>[CH3:1][O:2][C:3]([CH:5]1[CH2:10][CH2:9][CH:8]([CH2:11][OH:12])[CH2:7][N:6]1[S:14]([CH3:17])(=[O:15])=[O:16])=[O:4]To a solution of (2RS,5RS)-1-methanesulfonyl-piperidine-2,5-dicarboxylic acid 2-methyl ester (10.7 g) in tetrahydrofuran was added a 1 M borane solution in tetrahydrofuran (215 ml) within 0.5 h at 0° C. The reaction mixture was stirred for 4 h at 0° C., poured onto ice/saturated sodium bicarbonate solution and concentrated. The aqueous residue was extracted with EtOAc. The organic phase was washed with brine, dried over magnesium sulphate and concentrated. The residue was chromatographed on silica gel using EtOAc/hexane 1:1 and 3:2 as eluents to obtain (2RS,5RS)-5-hydroxymethyl-1-methanesulfonyl-piperidine-2-carboxylic acid methyl ester (8.1 g) as a colourless solid, MS: 206 (100%, M+-59).
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