Methyloxime-trimethylsilyl

O-Methyloxime—trimethylsilyl derivatives of steroids from urine [61]... 

Chromatography Steroids (54). The methyloxime-trimethylsilyl ether derivative has become the method of choice for derivatizing steroids. The success of this derivatization procedure is due to the ease by which both carbonyl and hydroxyl groups can be protected. 

Another silanization procedure for the derivatization of carbohydrates is the formation of trimethylsilyl oximes. The methyloxime is heated with hexarnethyl-disilazane trifluoroacetic anhydride (9 1 v/v) for 1 h at 100°C. Anthrone O-glucoside is an important ingredient in skin care cosmetics and can be fully silylated by reaction with A/,0-bistrimethylsilylacetamide acetonitrile mixture (1 1 v/v) for 1 h at 90°C, and subsequently separated by GC. 

Single-peak sugar derivatives which allow the resolution of sugar constituents in pectin include the trimethylsilylated methyloximes (52), acetylated aldononitriles (53), trimethylsilylated alditols T54), and acetylated alditols (51.). A comprehensive review article on GLC of sugars has been published (26). 

An alternative reagent, trimethylsilyldiazomethane in hexane, is commercially available and appears useful for preparation of bile acid methyl esters [62], Like carboxylic acid groups, hydroxyl groups also require protection prior to GC-MS analysis. A common and mild method to prepare trimethylsilyl (TMS) ethers is to react the sample with a mixture of dry pyridine, hexamethyldisilazane (HMDS), and trimethylchlorosilane (TMCS), 3 2 1 or 9 3 1 (by volume) with, or without heating [32], However, if 0X0 groups are present in the bile acid, this reaction can yield enol-TMS ethers and multiple products. This artifact can be avoided by converting the 0X0 group into an oxime, usually a methyloxime (MO). To do this, the sample is dissolved in 50 pL pyridine with 5 mg methoxyammonium chloride and heated for 30 min at 60°C [32]. 

Trimethylsilylation in combination with methyloxime (MO) formation is the most popular derivatization method for steroids [62, 63], and the literature contains many examples of corresponding fragmentation pathways and details of the sy and anti isomers formed with non-sterically-hindered keto groups. A typical steroid MO-TMS derivative mass spectrum is reproduced in Figure 7 for pregnenolone [64], together with an indication of the origins of the major ions. 

Acid, free bile acid, no protecting groups OH, free hydroxyl groups Me and Et, methyl and ethyl esters, respectively OAc, acetate TEA, trifluoroacetate TMS, trimethylsilyl ether Cath, 3-cathylate DMH, 3-dimethyihydrazone MO, O-methyloxime. 

Laine, R.A., Sweeley, C.C., Analysis of trimethylsilyl O-methyloximes of carbohydrates by combined gas-liquid chromatogtaphy-mass spectrometry. Anal. Biochem., 1971, 43, 533-538. 

Fig. 5.18 Mass spectra of the trimethylsilyl derivatives of the oximes of (a) 2-oxo-isovaleric acid and (b) 3-phenylpyruvic acid, (c) the methyloxime of 2-oxoglutaric acid, and the ethyloximes of (d) 2-oxoglutaric acid and (e) 3-phenylpyruvic acid.

---