[CH2:1]([O:3][C:4]([C@@H:6]1[CH2:10][C:9](=[O:11])[CH2:8][C@H:7]1[C:12](=[O:17])[NH:13][CH2:14][C:15]#[N:16])=[O:5])[CH3:2].CN(C=O)C.[BH4-].[Na+]>C1COCC1>[CH2:1]([O:3][C:4]([C@@H:6]1[CH2:10][C@H:9]([OH:11])[CH2:8][C@H:7]1[C:12](=[O:17])[NH:13][CH2:14][C:15]#[N:16])=[O:5])[CH3:2]To a suspension of (1R,2R)-2-(cyanomethyl-carbamoyl)-4-oxo-cyclopentanecarboxylic acid ethyl ester (2.7 mmol) in THF (12 ml) was added DMF (1 ml), the solution was cooled to 0° C., sodium borohydride (3.3 mmol) was added and stirring was continued for 2 h. The mixture was partitioned between 1N aqueous HCl and ethyl acetate, the organic layer was dried, evaporated and the residue chromatographed on silica using dichloromethane/MeOH (97:3) to give in the first fraction (1R,2R,4R)-2-(cyanomethyl-carbamoyl)-4-hydroxy-cyclopentanecarboxylic acid ethyl ester as a white solid. MS: 241.4 (M+H)+. The second fraction contained the desired isomer,
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