1.87 g of (2S,4S)-4-(4-methoxybenzylthio)-2-pyrrolidinecarboxylic acid [prepared as described in step (a) above] was suspended in 80 ml of a 1:1 by volume mixture of tetrahydrofuran and water. This suspension was then converted to a homogeneous solution by the addition of 7 ml of a 1N aqueous solution of sodium hydroxide. Whilst stirring this solution on an ice bath, 10 ml of a tetrahydrofuran solution containing 1510 mg of 4-nitrobenzyloxycarbonyl chloride and 7 ml of a 1N aqueous solution of sodium hydroxide were gradually added dropwise at the same time, and the resulting mixture was stirred under the same conditions for 10 minutes. At the end of this time, the tetrahydrofuran was removed by evaporation under reduced pressure. 1N aqueous hydrochloric acid was added to the reaction solution to adjust its pH to a value of between 2 and 3. The crystals which separated were collected by filtration, washed well with water and dried in the air. They were then further washed with a small amount of diethyl ether and dried, to obtain 2.42 g of the title compound, melting at 96°-98° C.
