Separation trimethylsilyl ethers

Figure 4.1 Total-ion chromatogram of separated trimethylsilyl ethers of the cannabinoids cannabidiol (16.5 min) A8-THC (18.3 min) A9-THC (18.4 min) cannabinol (20.1 min).

To a solution of 0.25 mol of the trimethylsilyl ether in 120 ml of dry diethyl ether was added in 20 min at -35°C 0.50 mol of ethyllithium in about 400 ml of diethyl ether (see Chapter II, Exp. 1). After an additional 30 min at -30°C the reaction mixture was poured into a solution of 40 g of ammonium chloride in 300 ml of water. After shaking, the upper layer was separated off and dried over magnesium sulfate and the aqueous layer was extracted twice with diethyl ether. The ethereal solution of the cumulenic ether was concentrated in a water-pump vacuum and the residue carefully distilled through a 30-cm Vigreux column at 1 mmHg. The product passed over at about 55°C, had 1.5118, and was obtained in a yield of 874. Distillation at water-pump pressure (b.p. 72°C/I5 mmHg) gave some losses due to polymerization. 

The composition of the enol ethers trimethylsilyl prepared from an enolate mixture reflects the enolate composition. If the enolate formation can be done with high regio-selection, the corresponding trimethylsilyl enol ether can be obtained in high purity. If not, the silyl enol ether mixture must be separated. Trimethylsilyl enol ethers can be prepared directly from ketones. One procedure involves reaction with trimethylsilyl... 

In dichloromethane, the acidic ESE cation radical undergoes a rapid proton transfer (k = 1.9 x 109 s ) to the CA anion radical within the contact ion pair (CIP) to generate the uncharged radical pair (siloxycyclohexenyl radical and hydrochloranil radical) in Scheme 6. Based on the quantum yields of hydro-chloranil radical (HCA ), we conclude that the oxidative elimination occurs by geminate combination of the radical pair within the cage as well as by diffusive separation and combination of the freely diffusing radicals to yield enone and hydrochloranil trimethylsilyl ether, as summarized in Scheme 6. 

Tsuj i and Robertson achieved the separation of neomycin B, neomycin C and neamine as the trimethylsilyl ethers on a 6ft. column of 0.75% OV-1 on Gas Chrom Q at a temperature of 290°C. The same conditions have also been shown to separate neobiosamine B, neosamine and deoxystreptamine from neomycin and neamine. Hence the method could be used to study the stability of neomycin or to monitor the biosynthetic production process. Use of the procedure to assess the stability of neomycin in pharmaceutical formulations has been demonstrated by Van Giessen and Tsuji237 with trilaurin as internal standard. However,these authors recommended a 2ft. column packed with 3%... 

On studying the KDO content of LPS from Xanthomonas sinensis, Volk and coworkers64 found that only one KDO residue per LPS chain is present in this organism. Moreover, the authors observed the formation of 4,7- and 4,8-anhydro derivatives (39 and 40) of KDO (see Scheme 14) under the conditions used for mild hydrolysis of the LPS with acetic acid. Free KDO was also shown to yield the anhydro derivatives under analogous conditions. The structures of these unusual side-products were identified by mass spectrometry, as follows. Reduction of 39 and 40 with sodium borohydride gave the two epimeric mixtu.es, 41 and 42. These were subjected to periodate treatment followed by borohydride reduction, the products separated by paper electrophoresis, and their trimethylsilyl ether derivatives analyzed by... 

The hydroperoxides obtained on thermal oxidation of cholesteryl acetate (191e) can be selectively separated by SPE and elution with a polar solvent. After reduction to the corresponding alcohols by NaBH4 and further derivatization to the trimethylsilyl ether, the products can be subjected to GLC with ion-trap MS detection. It can be thus demonstrated with the aid of standards that under the oxidation conditions (160 °C for 90 min) only the 7-position is attacked, leading to the la- and 7/3-hydroperoxy derivatives, while the plausible 4-position remains unscathed . Treatment of erythrocite ghosts with t-BuOOH causes a manyfold content increase of 5-hydroxyeicosatetraenoic acid (5-HETE), 5-hydroperoxyeicosatetraenoic acid (5-HPETE) and 5-oxoeicosatetraenoic acid (5-oxo-ETE) residues of phospholipids. These acids can be separated by HPLC, identified and quantitized by tandem MS . ... 

System (5) has been used for the direct analysis of steroids, including cortisone and cortisone acetates, as the trimethylsilyl ethers of their methoxyimmino derivatives [144], A glass column (180 cm x 4 mm) containing 1.5% SE-30 on 60 to 80 mesh Chromosorb W, operated at 235°C, is used for the separation. Nitrogen is used as the carrier gas (flow rate of 35 mL/min), and detection is by flame ionization. 

Although several advantages of dimethylsilyl (Me2HSi) ethers have been claimed,191 few examples of their use have been reported, except for the separation of cycloamyloses.133 Thus, dimethylsilyl derivatives (which are more volatile than trimethylsilyl ethers) require lower column-temperatures and have shorter retention times for example, for glucose, the per(trimethylsilyl) derivative has a retention time of 11.4 min, and the per(dimethylsilyl) derivative, 5.10 min. Both types of derivative may be analyzed on the same column, and, in certain cases, compounds inseparable as their O-trimethylsilyl derivatives may be separated as their dimethylsilyl ethers. The potential of di-methylsilylation for converting oligosaccharides into volatile derivatives should be examined. 

It so happens that the great majority of compounds separated as acetates are alditols or other polyhydric compounds, and this Section is therefore concerned with the problems of reduction and acetylation. There is no a priori reason why alditols and other polyols should not be separated as their trimethylsilyl ethers, and such methods are known (see Section VII,l,p. 57 Section XIII, p. 90 Table Va, p. 119 and Table Xlla, p. 151), but experience shows that the resolution of acyclic O-trimethylsilyl derivatives is less satisfactory than that of cyclic compounds. 

Trimethylsilylation has been used in studying the methanolysis of monosaccharides383,384 and the products obtained by reduction of 4-deoxyuronic acids.385 Various natural phenolic glycosides have been separated as trimethylsilyl ethers,386 as have glucosinolates.126... 

The fact that the common hexitols are not readily separated as their trimethylsilyl ethers may be turned to advantage in estimating total... 

Certain synthetic heptitols that could not be separated by paper chromatography or by electrophoresis have been separated as their O-trimethylsilyl derivatives.404 The procedure of reductive, alkaline hydrolysis, much used in glycopeptide research, yields alditols that have been estimated as their trimethylsilyl ethers,59,405 and D-glucitol in mammalian nerve has similarly been determined.406... 

The multiplicity of peaks obtained on trimethylsilylation of 3-de-oxy-D-en/fforo-hexosulose has already been mentioned (see p. 24) however, the isomeric 3-deoxyhexitols formed on reduction with borohydride were readily separated as their trimethylsilyl ethers.234 El-Dash and Hodge234 gave a large amount of tabular and graphical data on relative retention-times and showed that, when these values... 

The separation of disaccharides and higher oligomers is not essentially different from the separation of monosaccharides, except that the volatility rapidly decreases with increasing molecular weight. Oligosaccharides may be transformed into volatile derivatives, commonly the trimethylsilyl ethers, either directly or after reduction. Other derivatives, such as trifluoroacetates, have been used, but the acetates have low volatility. Oligosaccharides are not usually converted into their methyl glycosides prior to trimethylsilylation. Detailed examples are listed in Table VII (see p. 130). 

The separation and identification of disaccharides is often an important step in the elucidation of the structure of a natural polysaccharide, and Percival484 has published useful data on the O-trimethyl-silyl derivatives of a variety of disaccharides and their reduction products. In some instances, the trimethylsilyl ethers of the disaccharide alditols have lower retention times than those of the disaccharide derivatives. The per-O-trimethylsilyl derivatives of gentiobi-itol and maltitol were encountered in studies on the structure of Pneumococcus Type II capsular polysaccharide.4843... 

In model experiments,205 it was found that concentration of an aqueous solution of the aldonic acids in the presence of hydrochloric acid gives exclusively the 1,4-lactones, which, on subsequent trimethyl-silylation, give only one peak on the chromatogram. This method was successfully employed for the separation of D-galacturonic, D-glucuronic, and D-mannuronic acids. Of the 1,4-lactones examined, only the trimethylsilyl ether of D-mannono-1,4-lactone was obtained in crystalline form. However, all of the derivatives showed characteristic differences in their infrared spectra in the range of 1500 to 600 cm"1. When this method was applied to the determination of uronic acids in a variety of polysaccharides,205 it was impossible to find any hydrolytic conditions under which the uronic acids were quantitatively released and then reduced, a problem experienced by other workers.20 The method was, however, successful in affording a qualitative, microscale procedure for the identification of hexuronic acids (which otherwise are difficult to detect). 

Lee and Ballou111 investigated five liquid phases for the separation of inositol trimethylsilyl ethers and concluded that QF-1 gave the best results. This column packing permitted resolution of all but the myo-cis pair of inositols. They reported that the order of appearance, regardless of the stationary phase used, is alio, (neo, muco), racemic, (scyllo, epi), and (myo, cis). The order within the parentheses is sometimes reversed, depending on the liquid phase used. Wells and co-... 

Reference has already been made to the formation of anhydro sugars under the influence of acid (see Section XI, p. 87). In a similar manner, alditols may be converted into anhydroalditols, and these may be difficult to distinguish from other polyhydric compounds of low molecular weight. Such a problem arises in the hydrolyzates of cell walls, where ribitol and glycerol are present. It has been shown, however, by Gregory that glycerol and an anhydroribitol may be separated readily as their trimethylsilyl ethers.401... 

The kinetics of the reaction have been monitored350 polarimetri-cally, and by separation of the products, after various intervals, by g.l.c. of their per(trimethylsilyl) ethers. L-Fucose dissolved completely in boiling methanol within 30 min, even in the absence of the resin, and the initial solution contained 5% of L-fucofuranoses, 38% of a-L-fucopyranose, and 57% of /3-L-fucopyranose. Immediately upon adding the resin, glycoside formation occurred. After 1 min, all four isomers appeared, in approximately equimolar ratios and the furano-side content attained a maximum after 15 min (18% of a- and 34% of /3-furanoside, with 23% of a- and 21% of /3-pyranoside). The final equilibrium mixture (after 12 h at the b.p., with the resin) contained 6% of a- and 13% of /3-fucofuranoside, and 54% of a- and 27% of /3-fuco-pyranoside. 

Derivative formation prior to chromatographic analysis has been used successfully. An unidentified component of urine was found which had a retention time very close to that of pregnanediol and which could not be separated from it by thin layer chromatography. The trimethylsilyl ether derivatives and the tri-fluoroacetate derivatives of the two compounds would not provide resolution only the acetate derivatives could be separated. 

There are a number of gas chromatographic procedures for the determination of the catecholamines. Methods generally accepted are making the trimethylsilyl ether derivative of the alcoholic group after using diazomethane to prepare the methyl ester and the heptafluorobutyryl derivative. The ethyl esters have been used for the separation of HVA and isoHVA and mass spectrometry applied to study their characterization. 

The trimethylsilyl ether of noncarboxyl phenols gives a lower (1/10) detector response. Furthermore, the trimethylsilyl ether methyl esters are more readily separated on both SE-30 and XE-60 columns. 

---