S. Gaskell, C. Brooks
Feb 1, 1976
Citations
0
Influential Citations
32
Citations
Journal
Biochemical Society transactions
Abstract
Hydroxylic compounds are usually converted into derivatives for analysis by g.1.c. and gas chromatography-mass spectrometry (g.1.c.-m.s.). Trimethylsilyl ethers have found the widest application, but other alkylsilyl derivatives (cf. Harvey & Paton, 1975) and halogenated analogues are of complementary value. Particular interest attaches to t-butyldimethylsilyl ethers ; they are more stable towards hydrolysis and have been used as protecting groups in synthesis (e.g. Corey & Venkateswarlu, 1972; Ogilvie, 1973). Studies have been reported of the g.1.c.-m.s. of t-butyldimethylsilyl ethers of prostaglandins and sterols (Kelly &Taylor, 1974) and nucleosides (Quilliam et al., 1974). The predominant feature of the mass spectra is the abundance of M-57 ions due to loss of the t-butyl group. The application of these to the selective detection of sterols has been suggested (Kelly &Taylor, 1974) and such ions have been used in the analysis (by selected ion monitoring) of glycerol 1-monophosphate dimethyl ester di-t-butyldimethylsilyl ether (Heron & Caprioli, 1974). We now report a convenient method of preparing t-butyldimethylsilyl ethers of a variety of steroids, and describe features of their g.1.c.-m.s. properties. t-Butyldimethylchlorosilane was redistilled and immediately dissolved in pyridine (freshly distilled over NaOH) to give a 1 M solution, The reagent so prepared was stored in an air-tight vial at -20°C. t-Butyldimethylsilyl derivatives of steroids were prepared by treatment with approx. lomolar proportions of reagent in pyridine solution at room temperature. Conversion of non-sterically hindered hydroxyl groups into the t-butyldimethylsilyl derivatives occurred quantitatively in 3-15 h in the absence of imidazole as catalyst (cf. Corey & Venkateswarlu, 1972). Solvent was removed under a stream of N2 and the residue dissolved in chloroform/n-heptane/methanol (10: 10: 1, by vol.). Excess of reagent was removed by elution through a short (5cm) column of Sephadex LH20, swollen in the eluting solvent (C. J. W. Brooks & G. E. Joannou, unpublished work; Engel et af., 1970). G.1.c. and g.1.c.-m.s. were performed on glass columns (2mx3mm) containing 1 % OV-1 on Gas Chrom Q (100-120 mesh) operated at 250" or 265°C. G.1.c.-m.s. was carried out on an LKB9000 instrument operated at 70eV ionizing voltage. Salient gas-chromatographic and mass-spectrometric data are given in Table 1. Retention indices of mono-t-butyldimethylsilyl derivatives are generally about 250 higher than for the corresponding trimethylsilyl derivatives. The mass spectra are characterized by their marked simplicity. The molecular ions and M-15 ions are absent or of low intensity, but ions (M-57) corresponding to loss of the t-butyl group are very intense and frequently constitute the base peaks. An ion at mle 258 (5 % of base peak) in the spectrum of 26-hydroxycholesterol di-t-butyldimethylsilyl ether may be attributable to a doubly charged ion of mass 516, corresponding to loss of two t-butyl groups. Doubly charged ions of analogous origin have been observed in the spectra of dialkylsilyl derivatives of aliphatic diols (Harvey & Paton, 1975).