Trimethylbromosilane catalyzed trimethylsilylation of slowreacting hydroxy- and oxosteroids in gas chromatographic-mass spectrometric analysis

The trimethylsilylation of different hydroxy- and oxosteroids has been studied using hexamethyldisilazane and trimethylbromosilane instead of trimethylchlorosilane as the catalyst. Steroids with hydroxyl groups in positions 3, 11, 17, 20 and 21 (e.g. α- and β-cortol), react to form persilylated derivatives in high yields in 24 hours at room temperature. Steroid ketones easily react to form enol trimethylsilyl ethers. Significant enolization of a 17-oxo-(dehydroepiandrosterone) and a 20-oxo- (pregnenolone) group occurred after 5 minutes, and within 30 minutes of an 11-oxo compound (3α,20β-dihydroxy-5β-pregnan-11-one). After 3 hours a complete enol trimethylsilylation was obtained with pregnenolone but not with dehydroepiandrosterone. No complete persilylation was obtained with 11-oxo- or 20-oxo-17α-hydroxy compounds within 24 hours. It is recommended that the trimethylbromosilane reagent is used after conventional trimethylsilylation of polyhydroxy steroids. It is advisable that 3- and 20-oxo-steroids are converted into O-methyloxime derivatives before persilylation is made with the trimethylbromosilane reagent.