SYNTHESIS OF CHOLEST-4-EN-3-ONE

• Place a mixture of 20 g (0.052 mol) of pure cholesterol (m.p. 1 47- 1 50 °C; dried to constant weight at 80-100 °C), 150 ml of dry acetone and 200 ml of sodium-dried benzene in a dry, 1 -litre, two-necked round bottomed flask fitted with a reflux condenser and dropping funnel both protected with calcium chloride tubes. 

• Add a few carborundum chips and heat to boiling in an oil bath at 75-85 °C. 

• Add from the dropping funnel, in one portion, a solution of 16 g (0.065 ml) of aluminium t-butoxide in 100 ml of anhydrous benzene. 

• The reaction mixture becomes cloudy and develops a yellow colour in about 10 to 15 minutes. 

• Continue gentle boiling at a bath temperature of 75-85 °C for 8 hours. 

• Treat the cold mixture with 40 ml of water and then with 100 ml of 10 per cent sulphuric acid. 

• Shake vigorously and transfer to a 1 -litre separating funnel. Dilute the mixture with 300 ml of water, shake for 5 minutes (filter, if necessary), then run off the yellow aqueous layer into a second separating funnel and extract the latter with 25 ml of benzene. 

• Wash the combined benzene extracts thoroughly with water, dry with magnesium sulphate and remove the solvent on a rotary evaporator under reduced pressure. 

• The yellow oily residue solidifies when it is cooled in an ice-salt bath and scratched with a glass rod; keep a small portion for seeding in the subsequent crystallisation. 

• Dissolve the solid in a warm mixture of 14 ml of acetone and 20 ml of methanol, allow the solution to cool very slowly and seed, if necessary. 

• When the bulk of the solid has crystallised, keep the mixture at °C for 24 hours, filter with suction, wash with 20 ml of ice-cold methanol and dry in a vacuum desiccator. The yield of almost colourless cholest-4-en-3-one, m.p. 79-80 °C, is 17 g (85%).