SYNTHEIS OF S-BENZYL-L-CYSTEINE [L-2-Amino-3-(benzylthio)-propanoic acid]

• Collect about 750 ml of liquid ammonia in a 1-litre three-necked flask, surrounded by a lagging bath of cork chips and fitted with a sealed stirrer unit, a soda-lime guard-tube and a stopper. 

• Weigh out 24g (0.1 mol) of L-cystine, and about 10g (0.48 mol) of sodium cut into small pieces under dry light petroleum. 

• Start the stirrer, add about 2g of the sodium followed by L-cystine in small portions until the blue colour has disappeared. 

• Repeat this addition sequence until all of the cystine has been added and a permanent blue colour remains. 

• Discharge the blue colour by gradually adding powdered ammonium chloride, and then add dropwise 25.3g (23 ml, 0.2 mol) of benzyl chloride. 

• Remove the stirrer and lagging bath and allow the ammonia to evaporate overnight. 

• Dissolve the residue in 100 ml of cold water and add concentrated hydrochloric acid until the resulting mass is acid to Congo red. 

• Heat the mixture gradually to boiling to dissolve the precipitated product and allow to cool. 

• Filter off the long needles of S-benzyl-L-cysteine which separate, wash with a little cold water and allow to dry in the air. The yield is 38g (90%), m.p. 214 °C (decomp.).