Ethyl bromopyruvate → (2S,4R)-2-(4-carboxythiazol-2-yl)-4-methanesulfonyloxy-1-(4-nitrobenzyloxycarbonyl) pyrrolidine

To a solution of (2S, 4R)-4-methanesulfonyloxy-1-(4-nitrobenzyloxycarbonyl) -2-thiocarbamoylpyrrolidine (2.0 g) in dichloromethane (40 ml) was added a solution of ethyl bromopyruvate (1.04 ml) in absolute ethanol (10 ml) at 0° C. After stirring at room temperature for 2 hours, the mixture was evaporated. The residue was dissolved in ethyl acetate, and washed with saturated sodium bicarbonate and brine successively. The dried organic layer was evaporated and the residue was dissolved in tetrahydrofuran (15 ml) and in sodium hydroxide (15 ml). After stirring at 35°-45° C. for 1 hour, the mixture was evaporated and poured into ethyl acetate. To this mixture was added 1N hydrochloric acid (20 ml), and then organic layer was separated, and washed with brine. The dried organic layer was evaporated to give (2S,4R)-2- (4-carboxythiazol-2-yl)-4-methanesulfonyloxy-1-(4-nitrobenzyloxycarbonyl) pyrrolidine (1.54 g).