SYNTHESIS OF BUTYL GLYOXYLATE

• In view of the quantities of benzene required, the entire preparation must be carried out in the fume cupboard. 

• Place a mixture of 125 ml of pure benzene and 32.5 g (0.123 mol) of dibutyl ( + )-tartrate (1) in a 500-ml three-necked flask, equipped with a Hershberg stirrer (Fig. 2.49) and a thermometer. 

• Stir the mixture rapidly and add 58 g (0.13 mol) of lead tetra-acetate in small portions over a period of 20 minutes while maintaining the temperature below 30 °C by occasional cooling with cold water. 

• Continue the stirring for a further 60 minutes. Separate the salts by suction filtration and wash with two 25 ml portions of benzene. 

• Remove the benzene and acetic acid from the filtrate by flash distillation and distil the residue under diminished pressure, preferably in a slow stream of nitrogen. 

• Collect the butyl glyoxylate (2) at 66-69 °C/5 mmHg. The yield is 26g (81%).

Notes to keep in mind:

1.  The purified commercial dibutyl ( + )-tartrate, m.p. 22 °C, may be used. It may be prepared by using the procedure described under isopropyl lactate. Place a mixture of 75g of ( + )-tartaric acid, lOg of Zerolit 225/H®, llOg (135 ml) of redistilled butan-1-ol and 150 ml of sodium-dried benzene in a 1-litre three-necked flask equipped with a sealed stirrer, a double surface condenser and an automatic water separator (cf). Reflux the mixture w'th stirring for 10 hours: about 21 ml of water collects in the water separator. Filter ol ' the ion-exchange resin at the pump and wash it with two 30-40 ml portions of hot benzene. Wash the combined filtrate and washings with two 75 ml portions of saturated sodium hydrogen carbonate solution, followed by 100 ml of water, and dry over magnesium sulphate. Remove the benzene by distillation under reduced pressure (water pump) and finally distil the residue. Collect the dibutyl ( + )-tartrate at 150°C/1.5mmHg. The yield is 90g(69%).

2.  Store the butyl glyoxylate under nitrogen; it undergoes autoxidation in air. The product decomposes on boiling (159-161 °C) at atmospheric pressure.

Cognate preparation: (R)-2,3-0-Isopropylideneglyceraldehyde

• To a solution of l,2:5,6-di-0-isopropylidene-D-mannitol (50 g, 0.19mol) in dry benzene (500 ml) is slowly added lead tetra-acetate (90 g, 0.2 mol) with stirring. 

• After 3 hours the mixture is filtered, and the filtrate concentrated below 25 °C to a syrup from which four 30-ml portions of carbon tetrachloride are evaporated at a temperature maintained below 25 °C. 

• The syrup is distilled in vacuo. 

• The first few millilitres are discarded and the main fraction is collected, 34 g (68%), b.p. 31 °C/5mmHg (1).

Notes to keep in mind:

1.  The product is unstable on storage and changes are observed in the i.r. and p.m.r. spectra when it is kept at room temperature for several days.