3β,24-Bis(tert-butyldimethylsilyloxy)-4,4-dimethylchol-5-ene (44 g) is dissolved in a warm mixture of hexane (2 l) and benzene (540 ml). 1,3-Dibromo-5,5-dimethylhydantoin (15.32 g) is added and the mixture is refluxed for 20 min and then cooled rapidly to room temperature, and the insoluble material is removed by filtration. The filtrate is concentrated under reduced pressure and o-xylene (2 l) and quinaldine (84 ml) is added. The mixture is refluxed for 1 hour. After aqueous work-up and trituration with methanol, 3β,24-bis(tert-butyldimethylsilyloxy)-4,4-dimethylchola-5,7-diene (39 g) is isolated. Melting point: 98-106° C. 1H-NMR (CDCl3, 400 MHz): δ=5.9 (1H, m); 5.54 (1H, m); 3.58 (2H, m); 3.35 (1H, m); 0.91 (18H, s); 0.05 (12H, m).
