Trifluoroacetates gas-liquid chromatography

Most carbohydrates are not sufficiently volatile to be used for gas-liquid chromatography, and they must therefore be converted into volatile compounds. The most usual derivatives for this purpose are trimethylsilyl (Me3Si) ethers, acetates, and trifluoroacetates. An extensive review on the trimethylsilylation of organic compounds has been written by Pierce.107... 

Conversion to acetates, trifluoroacetates (178), butyl boronates (179) trimethylsilyl derivatives, or cyclic acetals offers a means both for identifying individual compounds and for separating mixtures of polyols, chiefly by gas—liquid chromatography (glc). Thus, sorbitol in bakery products is converted to the hexaacetate, separated, and determined by glc using a flame ionization detector (180) aqueous solutions of sorbitol and mannitol are similady separated and determined (181). Sorbitol may be identified by formation of its monobenzyhdene derivative (182) and mannitol by conversion to its hexaacetate (183). 

A solution of 2.0 g of t-butyl alanine (S-form) and 3.78 g of ethyl 2-bromo-4-phenylbutanoate in 25 ml of dimethylformamide was treated with 1.8 ml of triethylamine and the solution was heated at 70°C for 18 h. The solvent was removed at reduced pressure and the residue was mixed with water and extracted with ethyl ether. The organic layer was washed with water and dried over magnesium sulfate. Concentration of the solvent at reduced pressure gave the oily t-butyl ester of the intermediate which was found to be sufficiently pure by gas liquid chromatography for further use. A solution of 143.7 g of this t-butyl ester in 630 ml of trifluoroacetic acid was stirred at room temperature for one hour. The solvent was removed at reduced pressure and the residue was dissolved in ethyl ether and again evaporated. This operation was repeated. Then the ether solution was treated dropwise with a solution of hydrogen chloride gas in ethyl ether until precipitation ceased. The solid, collected by filtration, was a mixture of diastereoisomers, melting point 153°-165°C. 

After purification of human serum bile acids on an Amberlyst A-26 anion e.xchanger, Sjovall and co-workers (31, 120, 121) hydrolyzed the concentrated bile acids in alkali and extracted the free acids with ethyl acetate. After methylation, the methyl cholanoates were purified on an aluminum oxide column. The bile acid fraction then obtained was subjected to gas-liquid chromatography on CNSi columns after conversion to trifluoroacetate... 

Abbreviations DMF, A,A-dimethylformamide DMA, N,A-dimethylacetamide GC, gas chromatography HPLC, high-performance liquid chromatography NMR, nuclear magnetic resonance RCM, ring closing metathesis TFA, trifluoroacetic acid THF, tetrahydrofuran. 

Notes LOD, limit of detection MeOH, methanol EtOH, ethanol ACN, acetonitrile EtAC, ethyl acetate SPE, solid phase extraction HLB (hydrophilic lipophilic balanced) TFA, trifluoroacetic acid GC, gas chromatography TMS, trimethylsilyl MS, mass spectrometry HPLC, high-performance liquid chromatography DAD, diode array detector NMR, nuclear magnetic resonance ESI, electrospray ionization APCI, atmospheric pressure chemical ionization CE, capillary electrophoresis ECD, electrochemical detector CD, conductivity detector TLC, thin layer chromatography PDA, photodiode array detector. 

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