Hydroxy acids, trimethylsilyl derivatives

In Pinaceae resins, for instance, fully trimethylsilylated derivatives of 7-hydroxy-DHA, 15-hydroxy-DHA and 15-hydroxy-7-oxo-DHA have been identified, as well as all the other abietadiene and pimaranediene acids present in these resins. The derivatization was ineffective on some labdane alcohols such as larixyl acetate, a marker compound for Venice turpentine, but in general labdane compounds have been identified in their trimethylsilylated form. Labdane acids, such as communic, agathic, agathalic, agatholic and acetoxy agatholic acids, that are among the most important constituents of sandarac... 

The procedure was proved to be general for the preparation of protected hydroxy acids from lactones (121). This apparently trivial process is often difficult to carry out, as the attempted derivatization of y or J-hydroxyacids frequently results in relactonization rather than hydroxyl protection. The method was applied to several aldonolactones to produce the corresponding intermediate hydroxyamides. Protection using [(2-trimethylsilyl)-ethoxy]methyl chloride, methoxymethyl chloride, ter/-butylchlorodimeth-ylsilane, or zm-butylchlorodiphenylsilane followed by ozonolysis gave the protected N-(y- or <5-hydroxyacyl)indole derivatives. Mild saponification gave indole and the acetal- or silyl-protected hydroxy acids. 

For the preparation of 5-alkyl-2-/err-butyl-l,3-dioxolan-4-ones from the bis(trimethylsilyl) derivatives of the /-hydroxy acid and the carbonyl compound catalyzed by trimcthylsilyl tri-flate see also ... 

Melphalan has been converted to its trimethylsilyl derivative with bis(trimethylsilyl)acetamide and has been analyzed by GC on a 1.8 m x 3 mm column packed with 2.5% (w/w) SE-54 on acid-washed, silanized Chromosorb W (80-100 mesh) at 210° (injector temperature, 250° flame ionization detector temperature, 215°) using nitrogen as the carrier at 30 ml/min. The order of elution from a partly hydrolyzed mixture was melphalan, mono-hydroxy-derivative VI and di-hydroxy-derivative VII (Scheme III). The same elution order was obtained on a SE-30 column it was reversed on a more polar liquid phase (OV-17). Identification of the peaks was done by mass spectrometry [52]. 

The sensitivity of Si chemical shifts to structural changes and the technique of silylating compounds for more favourable analysis or synthesis have been combined by several researchers to produce a powerful structure elucidation technique for monofunctional or polyfunctional compounds. (135-141) Specifically, the trimethylsilyl derivatives of imidophosphoryl compounds, (141) sugars, (138-140) steroids, (140) amines, amides, and urethanes, (135,136) and amino-, hydroxy-, and mercaptocarboxylic acids (137) have all been studied within the past three years. 

The trimethylsilyl group was the first to be developed and is widely used for the protection of serine and threonine (Table 6). Chlorotrimethylsilane, l,14 3,3,3-hexamethyldisilazane, and A(0-bis(trimethylsilyl)acetamide are commercially available reagents used for the conversion of alcohols into the corresponding trimethylsilyl derivatives.Furthermore, trimethylsilyl cyanide has been used to protect the side chains of serine, threonine, and ty-rosine.f This silyl protection allows the formation of A -carboxyanhydrides from H-Ser(TMS)-OH and H-Thr(TMS)-OH, and their application in peptide synthesis in the aqueous phase.f l The TMS group can be removed under various conditions, depending on the kind of functional group to which it is bound the TMS ethers are more stable than related amino or carboxy derivatives.These differences in stability allow the direct application of completely silylated hydroxy amino acids in peptide synthesis.b ... 

Amines. Chiral a-amino acids are obtained from cyanohydrins via a Mitsunobu reaction employing A-f-butoxycarbonyl-A-(2-trimethylsilyl)ethylsulfonamide as the nucleophile. The a-aminonitrile derivatives thus generated are hydrolyzed with acid. By means of an intramolecular displacement (3-hydroxy acids are transformed into (3-amino acids. Thus, subjecting the derived 0-benzylhydroxamides to Mitsunobu reaction conditions leads to (3-lactams which are readily processed (LiOH H, Pd/C). 

The most common derivatives used in the GG analysis of cuticular waxes containing hydroxy-compounds are trimethylsilyl derivatives (TMSi ethers and esters). They are volatile but not stable, particularly in acidic or wet conditions. Analytes are derivatized with common silylation reagents such as BSA [N,0-bis(trimethyl-silyl)acetamide], BSTFA [N,0-bis(trimethyl-silyl)trifluoroacetamide] and others (Blau Halket, 1995 Drozd, 1981). TMCS (trimethylchlorosilane) can be used as catalyst. TMSi derivatives should be prepared... 

Rontani and Aubert [47] studied the El mass spectral fragmentation of hydroxy carboxylic acids and trimethylsilyl derivatives of oxocarboxylic acids. Some fragmentations appeared to be specific to characterize and... 

Takahashi [12, 13] used gas liquid chromatography to determine BHA and BHT in breakfast cereals. He obtained quantitative recoveries of 20 to 30 ppm of the antioxidants added to cereals. To determine antioxidants and preservatives in soya sauce and other foods, Takemura [14] converted these compounds to trimethylsilyl derivatives and analysed the product by GC. The sample was shaken with the silation reagent (1 ml) for 30 seconds, then set aside for 5 minutes and 2 pi of the solution was analysed by GC on, for example 20% of SE-31 silicone on Celite 545 at 200 °C with helium as carrier gas (22 ml/min) and hexane as internal standard. The method was applied to 15 preservatives and 7 antioxidants well-separated peaks were obtained for 4-hydroxy benzoate estars, phenol, xylenol derivatives and salicylic acid. The calibration graphs for methyl, ethyl, propyl and butyl-4-hydroxybenzoates were rectilinear for the range 0.5 to 4 mg. The method was used to determine 4-hydroxybenzoates in soya sauce. 

Fig. 5.3 Chromatograms of urinary acidic metabolites extracted using DEAE-Sephadex and separated as their ethoxime and trimethylsilyl derivatives on OV-101 by temperature programming on a packed column (upper) and WCOT capillary column (lower). Selected peaks are identified for comparative purposes. These are 1, lactic 2, glycollic 3, oxalic 4, sulphate 5, phosphate 6, succinic 7, 4-deoxyerythronic 8, 4-deoxythreonic 9,3-deoxytetronic 10,2-deoxytetronic 11, erythronic 12, threonic 13, 3-hydroxy-3-methylglutaric 14, citric 15, uric 16, n-tetracosane (standard) 17, n-hexacosane (standard) (Tracey and Chalmers, unpublished).

A comprehensive collection of spectra from a number of sources of trimethylsilyl derivatives of organic acids of biological interest was compiled by Markey et al (1972) and subsequently included in a larger reference collection (Markey et al., 1974b) and in other library files (e.g. Mass Spectrometry Data Centre Collection, The University, Nottingham, England). In addition to spectra of trimethylsilyl (TMS) derivatives are spectra of free acids, methyl esters, methyl ester-trimethylsilyl ethers (hydroxy acids), and trimethylsilyl esters-trimethylsilyl ethers-methoximes (oxo acids or oxo hydroxy acids). 

Burkhard, C.A. (1957), Trimethylsilyl derivatives of hydroxy aromatic acids. J. Org. Chem., 22, 592. 

Petersson, G. (1974), Gas-chromatographic analysis of sugars and related hydroxy acids as acyclic oxime and ester trimethylsilyl derivatives. Carbohydr. Res., 33,47. 

Petersson, G. (1977), Retention data in GLC analysis Carbohydrate related hydroxy carboxylic and dicarboxylic acids as trimethylsilyl derivatives./. Chromatogr. ScL, 15, 245. 

Fig. 8.3 Chromatogram of acidic metabolites extracted from amniotic fluid from a normal subject using DEAE-Sephadex and separated as their ethoxime and trimethylsilyl derivatives on 10 per cent OV-101 on HP Chromosorb W (80-100 mesh) by temperature programming from 110 C to 285°C at 4 C min" with a 5 min initial isothermal delay. Peak identifications are 1, lactate 2, pyruvate 3,2-hydroxybutyrate 4, 3-hydroxybutyrate (+) 5, acetoacetate (ethoxime-TMS) 6, 2-oxocaproate 7, phosphate 8, succinate 9, fumarateplus some glycerate 10, malate 11, pyroglutamate 12, tetronates 13, 2-oxoglutarate plus 3-hydroxy-3-methylglutarate 14, pentonates 15, citrate 16, 4-hydroxyphenyl-lactate (+) 17, palmitate 18, unidentified 19, urate 20, -tetracosane (standard) 21, -hexacosane (standard).

Fig. 10.4 Chromatogram of organic acids extracted from the same urine of the patient illustrated in Fig. 10.3, separated as their O-n-butyloxime and trimethylsilyl derivatives under the conditions described in the legend to Fig. 10.2. Peak identifications are 1, lactate 2, glycollate 3, unidentified 4, 2-hydroxy-n-butyrate plus 3-hydroxy-propionate 5, sulphate 6, 3-hydroxybutyrate and 3-hydroxyisobutyrate 7, 2-hydroxyisovalerate 8, 3-hydroxyisovalerate 10, 2-hydroxyisocaproate 11,2-hydroxy-3-methyl-n-valerate 12, phosphate 13,2-oxoisovalerate (peak 1) 14,2-oxoisovalerate (peak 2) 15, 2-oxo-3-methyl-/i-valerate (peak 1) 16, 2-oxo-3-methyl-n-valerate (peak 2) 17, 2-oxoisocaproate 18, citrate 19, 4-hydroxyphenyl-lactate 20, n-tetracosane (standard) 21, n-hexacosane (standard). The separation of O-n-butyloxime-TMS derivatives of the keto acids from the TMS-hydroxy acids and other components is apparent (compare with Fig. 10.3). Note the apparent loss of 4-hydroxyphenylpyruvate. (The horizontal axis represents the time elapsed in minutes from sample injection.)...

Fig. 10.5 Mass spectra of the ethoxime-trimethylsilyl derivatives of (a) 2-oxoisovaleric, (b) 2-oxo-3-methyl-n-valeric (peak 1), (c) 2-oxo-3-methyl-/i-valeric (peak 2) and (d) 2-oxoisocaproic acids and the trimethylsilyl derivatives of (e) 2-hydroxyisovaleric, (f) 2-hydroxy-3-methyl-rt-valeric and (g) 2-hydroxyisocaproic acids.

Fig. 10.17 Chromatogram of organic acids extracted using ethyl acetate from the urine of a patient with 3-hydroxy-3-methylglutaric aciduria separated as their trimethylsilyl derivatives on 6 per cent Apiezon N on Chromosorb W (DMCS) (80-100 mesh) using temperature programming from 80 C to 250°C at 6°C min Peak identifications are 1, 3-hydroxyisovalerate 2, 3-methylglutarate 3 and 4, 3-methylglutaconate peaks 5, 3-hydroxy-3-methylglutarate. (Redrawn with modifications from Faull et al, 1976b)...

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