SYNTHESIS OF ᴅⳑ-PHENYLALANINE (2-Amino-3-phenylpropanoic acid)

• In a 1-litre three-necked flask, fitted with a reflux condenser, a sealed stirrer unit and dropping funnel, place 25g (0.1 mol) of 4-benzylidene-2-phenyloxazol-5-one 20g (0.65 mol) of purified red phosphorus and 135g (125 ml, 1.32 mol) of acetic anhydride. 

• Add with stirring over a period of 1 hour 125 ml of hydriodic acid (d 1.56; 50%). 

• Reflux the mixture for 3 hours, cool and filter with suction; wash the unreacted phosphorus on the filter with two 5 ml portions of glacial acetic acid. 

• Evaporate the filtrate and washings on a water bath (rotary evaporator) and collect the distillate (which may be used for another reduction) in a flask cooled in ice. 

• Add 100 ml of water to the dry residue and repeat the evaporation to dryness. 

• Shake the residue in the flask with 150 ml of water and 150 ml of ether until solution is complete; separate the acqueous layer and extract it with three 75 ml portions of ether. 

• Discard the ether extracts. 

• Introduce 2-3 g of decolourising carbon and a trace of sodium sulphite into the aqueous phase, heat on a water bath until the dissolved ether has been removed, filter, heat the filtrate to boiling and neutralise to Congo red with ammonia solution (d 0.88; about 25 ml are required). 

• When cold, filter the colourless dl-phenylalanine at the pump and wash with two 30-ml portions of cold water and finally with a little cold ethanol. The yield is 11g (67%), m.p. 284-288 °C (decomp., rapid heating).