Trimethylsilyl ether-methyl esters

A linear detector response was obtained in the concentration range 0.1 - 0.5 pg when increasing amounts of trimethylsilyl ether/methyl ester derivatives of the standard compounds (HVA, dehydroxyphenyl acetic acid, VMA) were assayed. 

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. 

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. 

Attempts were made to overcome the problems of partial ether formation with hydroxy and phenolic acids by the use of the trimethylsilyl ether methyl ester derivatives. Dalgliesh et al. (1966) esterified the carboxylic acid groups in organic acids using diazomethane and a short reaction time (<1 min) with immediate evaporation of the reagent and solvent, followed by... 

ABA = abscisic acid riboside. 2ME = methyl ester. 3TMS = trimethylsilyl METMS = methyl ester GA = gibberellin IAA -ethers. trimethylsilyl ester. indole-3-acetic acid Z - zeatin ZR = zeatin... 

A feature of many aliphatic TMS-efher pectra is the low intensity of the molecular ion. However, most have an ion at M -15, due principally to a loss of a methyl group from the trimethylsilyl function, which helps to establish the identity of the molecular ion. It occurs at an odd mass value in the absence of an odd number of nitrogen atoms. In contrast, TMS-ether-methyl esters of hydroxy aromatic acids have a significant ion. 

Alkyl esters are efficiently dealkylated to trimethylsilyl esters with high concentrations of iodotrimethylsilane either in chloroform or sulfolane solutions at 25-80° or without solvent at 100-110°.Hydrolysis of the trimethylsilyl esters serves to release the carboxylic acid. Amines may be recovered from O-methyl, O-ethyl, and O-benzyl carbamates after reaction with iodotrimethylsilane in chloroform or sulfolane at 50—60° and subsequent methanolysis. The conversion of dimethyl, diethyl, and ethylene acetals and ketals to the parent aldehydes and ketones under aprotic conditions has been accomplished with this reagent. The reactions of alcohols (or the corresponding trimethylsilyl ethers) and aldehydes with iodotrimethylsilane give alkyl iodides and a-iodosilyl ethers,respectively. lodomethyl methyl ether is obtained from cleavage of dimethoxymethane with iodotrimethylsilane. 

A guinea pig ileum bioassay was used to detect and aid in the isolation of a smooth-muscle-contracting substance from the Japanese gorgonian coral Euplexaura erecta [108]. This process led to the isolation of prostaglandin F2a (17) from the MeOH extract of the coral. Its identification was based on comparison with authentic PGF2a and its methyl ester by TLC, and comparison with authentic methyl PGF2a trimethylsilyl ether by mass spectrometry. Because of the nature of the techniques employed, some aspects of the stereochemistry in this isolate of PGF2a remain uncertain. 

Different nucleophiles such as methanol, allylsilanes, silyl enol ethers, trimethylsilyl-cyanide, and arenes can be used in this process [62]. When the sulfide itself contains an unsaturated or aromatic fragment and the process is carried out in the absence of a nucleophile, an intramolecular anodic sub-stitution/cyclization might occur [61-63]. Methyl esters of 2-benzothiazolyl-2-alkyl or aryl-acetic acid, oxidized in MeOH/Et4 NCIO4 or H2SO4 in the presence of CUCI2, form 2,2-dimethoxy products (Eq. 7) [64]. 

Mass Units of Characteristic Fragment-ions A-G Used for the Identification of Natural, N,O-Acylated Sialic Acids, and of the C7 and C8 Analogs of Neu5Ac and Neu5Gc as their Methyl Esters, Trimethylsilyl Ethers, by Mass Spectrometry"... 

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)...

Even isolated mono- and disaccharides are capable of extensive hydrogen bonding, which results in extremely low vapor pressures. Attempts at vaporization of these materials results in decomposition. Volatile derivatives must be prepared and the approaches are similar to those described for amino acids. The most common derivatives are methyl or trimethylsilyl ethers and acetyl or trifluoroacetyl esters. A highly detailed review of applications of GLC to carbohydrates including over 1000 references concerning derivatization and types of columns used has been written by Dutton (69,70). 

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. 

PREPARATION OF THE METHYL ESTER—TRIMETHYLSILYL ETHER. The standard acid (0.5 mg) or the residue obtained from the urine extraction was dissolved in 0.5 cm3 of methanol. Ethereal diazomethane (1 cm3) was then added and allowed to stand for 1 min. Ethereal diazomethane was prepared by the reaction of p-tolylsulphonyl-methyl nitrosamide with an alcoholic solution of KOH. 

Procedures A and B illustrate the two current methods for preparation of N-9-phenylfluoren-9-yl derivatives of amino acids and amino acid esters. Free carboxylate (as in alanine in Step A) or free hydroxyl (e.g., serine7) functions can be blocked for the duration of the reaction as trimethylsilyl (TMS) esters or ethers, respectively, by treatment with chlorotrimethylsilane and triethylamine. The TMS group(s) are then removed by methanolysls from carboxylic acids (as in Step A) and mild acidic hydrolysis from hydroxyl groups, both being accomplished during product isolation. In addition to 2, the N-9-phenylfluoren-9-yl derivatives of serine,7 glutamic add y-methyl ester,8 and aspartic acid 3-methyl ester3 9 have been prepared in this manner. 

Chlorophenyl)glutarate monoethyl ester 87 was reduced to hydroxy acid and subsequently cyclized to afford lactone 88. This was further submitted to reduction with diisobutylaluminium hydride to provide lactol followed by Homer-Emmons reaction, which resulted in the formation of hydroxy ester product 89 in good yield. The alcohol was protected as silyl ether and the double bond in 89 was reduced with magnesium powder in methanol to provide methyl ester 90. The hydrolysis to the acid and condensation of the acid chloride with Evans s chiral auxiliary provided product 91, which was further converted to titanium enolate on reaction with TiCI. This was submitted to enolate-imine condensation in the presence of amine to afford 92. The silylation of the 92 with N, O-bis(trimethylsilyl) acetamide followed by treatment with tetrabutylammonium fluoride resulted in cyclization to form the azetidin-2-one ring and subsequently hydrolysis provided 93. This product was converted to bromide analog, which on treatment with LDA underwent intramolecular cyclization to afford the cholesterol absorption inhibitor spiro-(3-lactam (+)-SCH 54016 94. 

Mcthy lene-y-butyrolactones, 136 0-Methylene cyclic ethers, 270 6-Methylene-3-ketoA -steroids, 193 3-Methyleneoxolanes, 136 Methylenetriphenylphosphorane, 338-339 Methyl enol ethers, 273-274, 401 Methylenomycin B, 14, 162 14a-Methylestrone 3-methyl ester, 95 0-Methylhomoallyl alcohols, 143 Methylidenation, 182 Methyl jasmonate, 279, 301 Methylketcne methyl trimethylsilyl ketal, 340 Methyl lithiodithioacetate, 340 Methyl 5-lithiotetronates, 72, 73 Methyllithium-Tetramethylethylcnediamine,... 

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. 

The level of selectivity that can be achieved in the formation of trimethylsilyl ethers is illustrated by the selective protection of the hydroxy group at C-ll in the methyl ester of the prostaglandin 15-methyl PGF20 (Equation Si2.2). Thus the less sterically hindered secondary alcohol at C-ll is selectively protected in the presence of the more sterically hindered secondary alcohol at C-9 and the tertiary alcohol at C-15. 

For GC analysis, the salts of the lowest molecular weight acids present in ozonation products subjected to base-promoted hydrolysis have been converted to their benzyl esters by reaction with benzyl bromide (Bonnet et al. 1989). The salts of all acids produced have commonly been converted to the free acids, usually with the aid of a cation exchange resin. The acids have then been converted to methyl esters by reaction with diazomethane (Bonnet et al. 1989) or, more often, have been converted to trimethylsilyl (TMS) esters (Matsumoto et al. 1986, Taneda et al. 1989, Habu et al. 1990). Trimethylsilylation has the major advantage that alcoholic and phenolic hydroxyl groups are simultaneously converted to TMS ethers, thus greatly facilitating GC analysis. 

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