Trimethylsilyl TMS Ethers

Fig. 5.4.4a Methyl ester-trimethylsilyl (TMS) ethers of BAs from a plasma sample, b n-Butyl ester-TMS ethers of BAs from a plasma sample (adapted from [15]). 1 Nor-cholic acid, 2 litho-cholic acid, 3 deoxycholic acid, 4 chenodeoxycholic acid, 5 cholic acid, 6 ursodeoxycholic acid, a cholesterol, b sitosterol)...

The volatility of GAs is increased prior to GC by forming the methyl esters with diazomethane. Hydroxylated GAs are often converted to trimethylsilyl (TMS) ethers after methylation. The mass spectra of GA methyl esters TMS ethers frequently contain intense molecular ions and characteristic fragmentation patterns, which are easier to interpret than those of the free hydroxy compounds. When recovery of GAs is required after GC, GA TMS ether esters are a convenient derivative since the free GA can easily be recovered after hydrolysis in water. 

Table 15.2 Retention times of phenol trimethylsilyl (TMS) ethers on 7% Carbowax 20M at (a) 130°C, (b) 180°C...

Reactions of polyfluoroalkylchromones with (perfluoroalkyl)trimethylsilanes proceed as a 1,4-nucleophilic per-fluoroalkylation to give 2,2-bis(polyfluoroalkyl)chroman-4-ones with high regioselectivity and good yield after acid hydrolysis of trimethylsilyl (TMS) ethers (e.g., see Scheme 52) <2003JOC7747>. 

COMPARISON OF RETENTION TIMES OF TRIMETHYLSILYL (TMS) ETHERS AND CHLOROMETHYLDIMETHYLSILYL (CDMS) ETHERS OF C19 AND C21 STEROIDS [95]... 

TrimethylsiIylethanesulfonamides, to protect amines, 382-383 Trimethylsilyl (TMS) ethers to protect alcohols, 29, 68-71, 76, 77,... 

A typical separation which can be achieved is shown in Figure 4.1, while the mass spectra of the trimethylsilyl (TMS) ethers of the derivatized cannabinoids are shown in Figures 4.2-4.S. 

One of the most common methods of alcohol protection is reaction with chlorotrimethylsilane to yield a trimethylsilyl (TMS) ether. The reaction is carried out in the presence of a base (usually triethylamine) to help form the alkoxide anion from the alcohol and to remove the HCl by-product from the reaction. 

Capillary gas chromatography-mass spectrometry is being used to separate and measure the trimethylsilyl (TMS) ethers of organic acids. A capfilary GC column is used that contains an immobilized nonpolar stationary phase. Organic acids are detected by electron impact (El) mass spectrometry performed in the scan mode m/z 50 to m/z 550 to obtain mass spectra. Identification is by comparison to a library of spectra generated by analysis of pure standard compounds integrated by published spectra, when applicable. 

Hydrolysis of trimethylsilyl (TMS) ether [129,130] and alcoholysis of a tetrahy-dropyranyl (THP) [131,132] group have been also employed in acid-catalyzed conversion to PHOST (Fig. 24) (or novolac). Another acetal-protected PHOST, poly[4-(l-phenoxyethoxy)styrene], was prepared by radical polymerization of the corresponding monomer and also by chemical modification of PHOST [133]. This acetal polymer produces a phenolic polymer and phenol upon aci-dolysis (Fig. 24). 

The ease with which alcohols and phenols can be converted to silyl ethers as compared with alkyl ethers has made silylation, generally, the derivatization method of choice for hydroxy compounds. Furthermore, intermolecular attractive forces involving trialkylsilyl (R3Si-) groups are relatively small the conversion of a typical alcohol to its trimethylsilyl (TMS) ether results in an increase of 72 mass units per molecule with only slight decrease in volatility. 

We have recently explored the application of glass capillary gc to trimethylsilyl (tms) ether derivatives of pyrrolizidine alkaloids. The high resolution capabilities of capillary columns offered promise in separating the complex and closely related alkaloid mixtures often encountered in plant extracts. The gc trace shown in Figure 5 illustrates the application of a capillary system to a mixture of tms-ethers of the 12 pure macrocyclic di-ester alkaloids shown in Figures 2 and 3. 

To determine sterols, a portion of the total lipid extract is saponified using methanolic potassium hydroxide, and sterols subsequently recovered in 2 1 hex-ane/chloroform. The sterols are converted to the corresponding trimethylsilyl (TMS) ethers using bis-N,0-(trimethylsilyl)trifluoroacetamide, BSTFA, and analyzed by capillary GC and GC with mass spectrometry. Reviews of relative retention times and mass spectra for sterol TMS ethers have been published [e.g. 73]. In some cases, sterol acetates, rather than TMS ethers, are the derivatives prepared for GC. SiHca column chromatography of the total lipid extract may also be used instead of saponification to isolate the sterol fraction [74], or even sterol subclasses such as 4,4-dimethyl, 4-monomethyl and 4-desmethyl sterols [75], prior to derivatization. However, this approach only includes free sterols in the analysis, whereas by saponifying the total extract, sterols present as steryl esters are also detected. 

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