SYNTHESIS OF ᴅⳑ-TRYPTOPHAN [2-Amino-3-(3-indolyl)propanoic acid]

Phenylhydrazone of 4-acetamido-4,4-diethoxycarbonylbutanal

CAUTION: Carry out all operations in an efficient fume cupboard. 

• Place 43.5g (0.2 mol) of diethyl acetamidomalonate and 70 ml of benzene (CAUTION) in a 250-ml three-necked flask fitted with a stirrer and dropping funnel and surrounded by a bath of water at room temperature. 

• Stir mechanically, add about 0.5 ml of a concentrated solution of sodium ethoxide in ethanol and then add slowly from the dropping funnel a solution of 12g (14ml, 0.215 mol) of acrylaldehyde [acrolein - CAUTION: highly toxic and irritant vapour (1)] in 14 ml of benzene; adjust the rate of addition so that the temperature of the reaction mixture does not exceed 35 °C. 

• When the addition is complete, stir for 2 hours more and filter off any traces of insoluble material. 

• Add 5 ml of glacial acetic acid and 24g (22 ml, 0.22 mol) of redistilled phenylhydrazine [CAUTION: (2)], warm to 50 °C and leave the resulting orange solution at room temperature for 2 days. 

• Collect the crystalline phenylhydrazone by filtration and wash it thoroughly by trituration with two 40 ml portions of benzene. 

• The yield of off-white crystals, m.p. 141 °C, is 50 g (69%). If the yield is low, warm the filtrate to 50 °C and set it aside for a further 2 days, when a further crop of the product may be obtained.

Diethyl (3-indolylmethyi)acetamidomalonate:

• Add 47g (0.13 mol) of the phenylhydrazone to 300 ml of water containing 14 ml of concentrated sulphuric acid in a 500-ml two-necked flask fitted with a sealed stirrer unit and a reflux condenser. 

• Boil the mixture under reflux with vigorous stirring for 4.5 hours; the suspended solid liquefies and then solidifies during this time. 

• Cool, filter off the resulting product (in the form of hard nodules) and wash it thoroughly by grinding it with water and re-filtering. 

• Recrystallise the product from 1:1 aqueous ethanol to obtain the purified malonate derivative; yield 32g (71%). The product melts at 143 °C, re-solidifies and then melts at 159 °C.

DL-Tryptophan:

• Boil 31g (0.09 mol) of the above product under reflux for 4 hours with a solution of 18g (0.45 mol) of sodium hydroxide in 180 ml of water. 

• Add a little decolourising charcoal, filter and cool the filtrate in an ice-salt bath. 

• Acidify the filtrate by adding about 55 ml of concentrated hydrochloric acid slowly and with shaking, keeping the temperature below 20 °C. 

• Cool the resulting suspension at 0°C for 4 hours and then collect the crude (indolylmethyl)malonic acid, a pale buff solid, by filtration. 

• Boil the crude product under reflux with 1 30 ml of water for 3 hours; decarboxylation ensues and some N-acetyltryptophan separates. 

• Add a solution of 16g (0.4 mol) of sodium hydroxide in 30 ml of water, continue to boil under reflux for 20 hours and then add about 1g of decolourising charcoal and filter. 

• Cool, acidify the filtrate by adding 24g (23 ml, 0.4 mol) of glacial acetic acid and cool the mixture at °C for 5 hours. 

• Collect the crude tryptophan by filtration, dissolve it in a solution of 5g of sodium hydroxide in 200 ml of water and warm to 70 °C. 

• Dilute the solution with 100 ml of ethanol at 70 °C and decant it from a little gummy precipitate which separates. 

• Acidify the hot solution with 7.5 ml of glacial acetic acid and allow to cool slowly. 

• When crystallisation is complete, filter off the purified DL-tryptophan and wash it successively with ice-cold water (2 x 40 ml), ethanol (2 x 40 ml) and ether (2 x 40 ml). The yield of colourless plates is 15g (82%), m.p. 283-284 °C (decomp.).

Notes to keep in mind:

1. Acrolein is usually supplied in sealed amber-coloured ampoules stabilised by the addition of a little hydroquinone. The ampoule should be cooled thoroughly before being opened with great care. It has been recorded that opened samples of acrolein stored in screw-capped bottles may explode violently, presumably as the result of rapid exothermic polymerisation.

2. Phenylhydrazine is highly poisonous and produces unpleasant burns in contact with the skin. Always wear disposable gloves. If any liquid does come in contact with the skin wash off immediately with 2 per cent acetic acid, then with soap and water.