[C:1]([O:5][C:6]([NH:8][C@@H:9]1[CH2:13][CH2:12][CH2:11][C@@H:10]1[C:14]([OH:16])=[O:15])=[O:7])([CH3:4])([CH3:3])[CH3:2].[CH3:17]N(C=O)C.C(=O)([O-])[O-].[Cs+].[Cs+].CI>O.C(OCC)(=O)C>[C:1]([O:5][C:6]([NH:8][C@@H:9]1[CH2:13][CH2:12][CH2:11][C@@H:10]1[C:14]([O:16][CH3:17])=[O:15])=[O:7])([CH3:4])([CH3:2])[CH3:3]To a mixture of (1S,2R)-2-[(tert-butoxycarbonyl)amino]cyclopentanecarboxylic acid (0.88 g) and DMF (8.0 ml) were added cesium carbonate (2.5 g) and methyl iodide (0.82 g) under ice-cooling, followed by stirring at room temperature for 1.5 hours. To the reaction mixture ethyl acetate and water under ice-cooling were added. The organic layer was separated, the aqueous layer was extracted with ethyl acetate (15 ml), and the organic layer was combined and washed with a 10% aqueous citric acid solution, a saturated sodium bicarbonate solution, water, and saturated brine in this order. The organic layer was dried over anhydrous magnesium sulfate and concentrated under reduced pressure to obtain methyl (1S,2R)-2-[(tert-butoxycarbonyl)amino]cyclopentanecarboxylate (0.85 g).
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