Hydroxy compounds, chromatography

This extension of the Smith analysis, involving methylation of the polyalcohol, has been studied in detail by Bose, both on model systems and on the Ti fructan.84 When a ffuctan is used, this method may give l-hydroxy-3-methoxy-2-propanone and l,3-dimethoxy-2-propa-none, together with methylated glycerols. These compounds may react further under methanolysis conditions to yield 2,5-dimethoxy-2,5-bis(methoxymethyl)-l,4-dioxane and l,3-dimethoxy-2-propanone dimethyl acetal, respectively. Several of these compounds may be obtained from sucrose by a model reaction. In view of the wide variety of hydroxy compounds obtained in this study,84 their separation and identification as benzeneboronates was investigated, and certain of the latter derivatives were purified by gas-liquid chromatography. The characterization of 1,2- and 1,3-diols by the mass spectra of their cyclic benzeneboronates has also been described.634... 

D. R. Luffer and M. V. Novotny, Capillary supercritical fluid chromatography and microwave-induced plasma detection of cyclic boron-ate esters of hydroxy compounds, J. Microcolumn Sep., 3 39-46 (1991). 

Even the simplest bifunctional compounds of these classes being analyzed on nonpolar phases indicate broad nonsymmetrical peaks on chromatograms. This leads to poor detection limits and reproducibility of retention indices (the position of peaks maxima depends on the quantity of analytes) compared with nonpolar compounds. The general way to avoid these problems is based on the conversion of hydroxy compounds to thermally stable volatile derivatives. This task is a most important purpose of derivatization (see the entry Derivatization of Analytes in Chromatography, General Aspects). This chemical treatment may be used not only for nonvolatile compounds but also for volatile substances. The less polar products typically yield narrower chromatographic peaks that provide the better signal-to-noise ratio and, hence, lower detection limits. Nonpolar derivatives have much better interlaboratory reproducibility of retention indices compared with this parameter for initially polar compounds. 

The stereoselectivity of the reaction was determined by converting both p-lactams 204 and 205 into the corresponding hydroxy compounds 206 and 207, followed by removal of the chiral auxiliary under Birch reduction conditions and further benzoylation. The resulting P-lactams 208 and 209 were subjected to HPLC analysis indicating that the ratio of cis and trans isomers was 95 5 for both compounds. After purification by column chromatography the optical purity of the major isomer was found to be 70% ee. 

In the examination of plant waxes various analytical methods have been used, including ultraviolet and infrared spectroscopy, and thin-layer, column, and gas-liquid chromatographies (Eglinton and Hamilton, 1967). The infrared spectrum has been used to indicate hydroxy compounds, ketones, ethers, and lactones, as well as the more common esters and hydrocarbons. In the use of thin-layer or column chromatography, infrared spectra can be used to make sure that there is no carryover of one class of components into another. 

The v/c-diols are cleaved by periodic acid (to aldehydes) and by permanganate (to acids). After suitable derivatization the hydroxy compounds can be separated by gas chromatography and identified by mass spectrometry (Section 10.4.1). 

Hydroperoxides have been distinguished by gas chromatography after reduction to hydroxy compounds. These are usually examined as trimethylsilyl ethers but advantages are claimed for the methyl... 

Examples of mass spectra of derivatives of 3,6-, 3,7- and 3,12-dihydroxy-cholanoates are shown in Figs. 4 and 5. At the temperatures used in gas chromatography-mass spectrometry, molecular ions are seen only with the free hydroxy compounds and the trimethylsilyl ether derivatives (intensity usually less than one percent of base peak). Molecular ions with a relative intensity of 5-10% are obtained from the methyl esters or acids when a direct probe is used (14, 15). Loss of 2 and 4 mass units from the molecular ions of compounds with free hydroxyl groups is most probably thermal and is not seen at low temperatures. 

Petrol ether-diethyl ether (95 + 5) is especially suitable for chromatography of vegetable and animal waxes on siHca gel G. The solvent petrol ether-diethyl ether-acetic acid (90 + 10 + 1) has been that mostly used in the TLC of lipids. It is particularly suitable for fractionating animal fats (Figs. 131 and 135). Vegetable oils which contain epoxy-and hydroxy-compounds are chromatographed with mixtures of petrol ether-diethyl... 

The m-hydroxy compounds (673) and (674), which are analogous to (670) and (671), were obtained as a mixture of E and Z isomers from the Cj5 aldehyde (4) and the phosphonium salt (672) (Dawson et aL, 1980b). The mixture of (673) and (674) was acetylated with acetic anhydride to give the isomeric acetates (675) and (676), which could be separated from one another by preparative high-performance liquid chromatography. 

To the hydroxy-compound (up to 10 mg) is added pyridine (0.5 mL), hexamethyidisilazane (0.15 mL) and trimethylchlorosilane (0.05 mL). The mixture is shaken for 30 seconds and then is allowed to stand for 5 min. An aliquot can then be injected directly into a gas chromatography column alternatively, the reaction mixture can be taken to dryness on a rotary evaporator, the products extracted with hexane (5 mL), the hexane layer washed with water (1 mL) and dried over anhydrous sodium sulfate, before the solvent is removed. The derivatives are stored in fresh hexane and are stable at -20°C for long periods". 

A large number of variously 2-, 4-, and 5-substituted thiazoles with alkyl, aryl, hydroxy, methylthio, mercapto, halo, and nitro groups have been analyzed by thin-layer chromatography on silica and alumina by the Stahl s technique (167, 170, 172). Among the many systems recommended for the elution of these compounds are the following ... 

Ebumamonine was assembled utilizing a Pictet-Spengler cyclization of hydroxy-lactam 52 in the presence of trifluoroacetic acid at low temperature to give a mixture of diastereomers 53 in 95% yield. These compounds were readily separated by chromatography and the a-epimer was further elaborated to give the natural product. 

Figure 10.1 Analysis of racemic 2,5-dimethyl-4-hydroxy-3[2H]-furanone (1) obtained from a strawbeny tea, flavoured with the synthetic racemate of 1 (natural component), using an MDGC procedure (a) dichloromethane extract of the flavoured strawbeny tea, analysed on a Carbowax 20M pre-column (60 m, 0.32 mm i.d., 0.25 p.m film thickness earner gas H2, 1.95 bar 170 °C isothermal) (b) chirospecific analysis of (1) from the sti awbeny tea exti act, ti ansfened foi stereoanalysis by using a pemiethylated /3-cyclodextrin column (47 m X 0.23 mm i.d. canier gas H2, 1.70 bar 110 °C isothemial). Reprinted from Journal of High Resolution Chromatography, 13, A. Mosandl et al., Stereoisomeric flavor compounds. XLIV enantioselective analysis of some important flavor molecules , pp. 660-662, 1990, with permission from Wiley-VCH.

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