To a solution of trans-N-Boc-4-hydroxy-L-proline methyl ester (45 g, 0.183 mol) in pyridine (140 ml) and dry DCM (140 ml) at 0° C. was added dropwise 4-methyl-benzenesulfonyl chloride (41.9 g, 0.22 mol). After adding, the mixture was refluxed overnight. The solvent was evaporated and then the residue was dissolved in CH2Cl2 (140 ml). The organic phase was washed with water (150 ml), brine (140 mL), and dried over sodium sulphate. After removal of solvent, the crude 4-(toluene-4-sulfonyloxy)-pyrrolidine-1,2-dicarboxylic acid 1-tert-butyl ester 2-methyl ester 73.2 g (92%) was obtained as yellow oil, which was used in the following reaction without further purification.
