[CH3:1][O:2][C:3](=[O:17])[C@@H:4]1[CH2:8][C@@H:7]([OH:9])[CH2:6][N:5]1[C:10]([O:12][C:13]([CH3:16])([CH3:15])[CH3:14])=[O:11].[CH3:18][C:19]1[CH:24]=[CH:23][C:22]([S:25](Cl)(=[O:27])=[O:26])=[CH:21][CH:20]=1.CN(C=O)C>N1C=CC=CC=1.C(Cl)Cl>[CH3:1][O:2][C:3]([CH:4]1[CH2:8][CH:7]([O:9][S:25]([C:22]2[CH:23]=[CH:24][C:19]([CH3:18])=[CH:20][CH:21]=2)(=[O:27])=[O:26])[CH2:6][N:5]1[C:10]([O:12][C:13]([CH3:14])([CH3:16])[CH3:15])=[O:11])=[O:17]To a solution of trans-N-Boc-4-hydroxy-L-proline methyl ester (7) in pyridine and dry DCM at 0° C. is added 4-methyl-benzenesulfonyl chloride. After adding, the mixture is refluxed overnight. The solvent is evaporated and then the residue is dissolved in CH2Cl2. After removal of solvent, the crude 4-(toluene-4-sulfonyloxy)-pyrrolidine-1,2-dicarboxylic acid 1-tert-butyl ester 2-methyl ester is obtained, which is used in the following reaction without further purification. To a solution of 4-(toluene-4-sulfonyloxy)-pyrrolidine-1,2-dicarboxylic acid 1-tert-butyl ester 2-methyl ester thus obtained above in dry DMF is added NaN3 in one portion, and the reaction mixture is stirred at about 50° C. for 5 h. The resulting mixture is purified to give 4-azido-pyrrolidine-1,2-dicarboxylic acid 1-tert-butyl ester 2-methyl ester (8).
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