5-(3-Hydroxy-propyl)-3-methyl-1H-pyrrole-2,4-dicarboxylic acid 2-tert-butyl ester 4-ethyl ester 1f (9.20 g, 30 mmol) was dissolved in 150 ml of dichloromethane under stirring, and added with triethylamine (7.0 ml, 50 mmol) to the solution while maintaining the temperature at about −10° C. in an ice-salt bath under an argon atmosphere. Upon completion of the addition, the mixture was added slowly with methanesulfonyl chloride (3.5 ml, 45 mmol). After stirring to mix well, the reaction system was allowed to warm up to room temperature and stirred for 4 hours. After thin lay chromatography showed the disappearance of starting materials, the reaction mixture was quenched with ice. The reaction mixture was washed successively with dilute hydrochloric acid (0.5 mol/L, 80 ml×2) to remove triethylamine, saturated sodium carbonate solution (80 ml×2) to remove excess hydrochloric acid and saturated brine (80 ml), and concentrated under reduced pressure to obtain the title compound 5-(3-methanesulfonyloxy-propyl)-3-methyl-1H-pyrrole-2,4-dicarboxylic acid 2-tert-butyl ester 4-ethyl ester 1 g (11.4 g, yield 99%) as a brown oil.
