Bile acid methyl ester-trimethylsilyl ether derivatives

Sometimes the methyl esters are better separated than the free bile acids. This is especially true when neutral solvents are used. In acidic solvents the difference is small, particularly with trisubstituted bile acids. Thus, the same types of acidic solvents can be used for free and methylated bile acids. When bile acid methyl esters are acetylated or converted into trimethylsilyl ethers their mobility is considerably increased. The separation of configurational isomers is then often impaired whereas the separation of positional isomers may be improved (4). This is often the case when neutral derivatives are analyzed in neutral solvent systems (12). The influence of the nature and position of nuclear substituents on the mobility of a bile acid can be estimated... 

Improved separations of bile acid methyl esters can be observed with increasing amounts of phenyl substituents in the stationary phase (5% in SE-52, 20% in PhSi-20, and 50% in OV-17). Positional and configurational isomers are better resolved than on SE-30 or OV-1 columns. For instance the valuable separation of the trimethylsilyl ethers of 3,6,7-substituted methyl cholanoates from the corresponding cholic acid derivative on OV-17 should be noted. The pronounced effect of a 7/3-hydroxy substituent on retention times on columns of SE-52 and PhSi-20 is noteworthy. The large separation factors between the diacetate derivatives of chenodeoxy- and ursodeo.xycholic acids may be most useful in work with bile acids of biological origin. 

In our experience with several different instruments (Atlas CH-4 and LKB 9000) over a period of six years the reproducibility in analyses of bile acid trimethylsilyl ethers is very good also with respect to differences between spectra of stereoisomers. Only once, when the ion source housing was contaminated, were the spectra quite different from normal. Spectra of bile acid methyl esters and particularly of their trifluoroacetates show somewhat larger variations this is assumed to be due to the greater thermal sensitivity of these derivatives. 

Gas Chromatographic Analysis of Bile Acid Methyl Esters as Partial Tri-methylsilyl Ether Derivatives Using N,0-Bis(trimethylsilyl)trifluoro-acetamide as Silylating Reagent J. Chromatogr. 155(2) 427-431 (1978) CA 89 l42749n... 

Ethers, Esters, and Related Derivatives of Alcohols.—5a-Cholestanyl methyl ether has been cleaved inter alia) and converted into 5a-cholestanol by successive treatment with trimethylsilyl iodide and water.Pyridinium toluene-p-sulphonate has been reported as an efficient and mild catalyst for the conversion of alcohols into their tetrahydropyranyl ethers.Bile acids were efficiently performylated by treatment with 90% HCO2H-HCIO4. Selective base-catalysed... 

In the procedure of Grundy ef at. (30) the bile acids are quantitated as a group after analysis of their trimethylsilyl ether methyl esters on SE-30 columns. As can be seen from Table XII only small separation factors are obtained with this phase. Fortunately, these authors found a unit response for all bile acids analyzed in this way and were able to measure bile acids as the entire base line deflection obtained after 5a-cholestane had been eluted. With such a procedure it has to be established that compounds other than bile acid derivatives are not determined. Whether this is the case under different experimental conditions, such as drug treatment or change of diet, should always be fully explored. 

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