Capillary gas chromatographic behaviour of tert.-hydroxylated steroids by trialkylsilylation

Capillary gas chromatographic behaviour was studied for a variety of structurally different bile acids and sterols having one to two tert.-hydroxy groups, together with several sec.-hydroxy groups, at positions C-3, -5, -7, -12, -14, -17, -20, -24, and/or -25. The tert.-hydroxylated steroids were subjected to trimethylsilylation with hexamethyldisilazane/trimethylchlorosilane /pyridine and N,O-bis(trimethylsilyl)acetamide/N-trimethylsilylimidazole/trimethylchlorosilane, and dimethylethylsilylation with N,N,-dimethylethylsilylimidazole. The methylene unit values of the resulting trialkylsilylation products were used for determining their structures of partially and/or fully derivatised ethers. The reactivity of the trialkylsilylation of tert.-hydroxy groups was found to be significantly dependent not only on the derivatisation reagents and conditions used, but also on the position and steric factor of the tert.-hydroxy groups. The following general order of the decreasing reactivity of tert.-hydroxy groups in steroids by trialkylsilyl etherification was observed: 25>20, 24>5beta>17alpha>>14alpha.