[C:1]([O:5][C:6]([N:8]1[CH2:13][CH2:12][CH:11]([OH:14])[CH2:10][CH2:9]1)=[O:7])([CH3:4])([CH3:3])[CH3:2].[N+](=[CH:17][C:18]([O:20][CH2:21][CH3:22])=[O:19])=[N-]>ClC(Cl)C.C([O-])(=O)C.[Rh+2].C([O-])(=O)C>[C:1]([O:5][C:6]([N:8]1[CH2:13][CH2:12][CH:11]([O:14][CH2:17][C:18]([O:20][CH2:21][CH3:22])=[O:19])[CH2:10][CH2:9]1)=[O:7])([CH3:4])([CH3:2])[CH3:3]To a solution of 4-hydroxy-piperidine-1-carboxylic acid tert-butyl ester (5.0 g, 25 mmol) and rhodium(II)acetate (180 mg) in 500 ml dichloroethane at 80° C. was added (over 90 min) ethyl diazoacetate (4.2 ml, 50 mmol) in 220 ml dichloroethane. The mixture was stirred for 7 h and quenched with aqueous sodium bicarbonate (2×100 ml). The organic phase was isolated and washed with brine (2×100 ml), dried over magnesium sulfate and concentrated in vacuo. The crude product was chromatographed on silica (80g) in petroleum ether:ethyl acetate (4:1) to give 3.4 g of 4-(ethoxycarbonylmethoxy)piperidine-1-carboxylic acid tert-butylester. The product was taken up in 40 ml of a 1 M lithium hydroxide solution in water:methanol (1:3) and stirred for 60 min. The mixture was concentrated in vacuo and dissolved in 20 ml of water, acidified to pH 4 with 1 M aqueous hydrogen chloride and extracted with ethyl acetate (3×30 ml). The combined organic layers were washed with brine (10 ml), dried over magnesium sulfate and concentrated in vacuo to give 1.79 g of 4-(carboxymethoxy)piperidine-1-carboxylic acid tert-butyl ester.
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