Labile compounds, preparation

Labile compounds, preparation of, under protective conditions, 6 144... 

SFE-GC-MS is particularly useful for (semi)volatile analysis of thermo-labile compounds, which degrade at the higher temperatures used for HS-GC-MS. Vreuls et al. [303] have reported in-vial liquid-liquid extraction with subsequent large-volume on-column injection into GC-MS for the determination of organics in water samples. Automated in-vial LLE-GC-MS requires no sample preparation steps such as filtration or solvent evaporation. On-line SPE-GC-MS has been reported [304], Smart et al. [305] used thermal extraction-gas chromatography-ion trap mass spectrometry (TE-GC-MS) for direct analysis of TLC spots. Scraped-off material was gradually heated, and the analytes were thermally extracted. This thermal desorption method is milder than laser desorption, and allows analysis without extensive decomposition. 

Vitamins are generally labile compounds and many of them are susceptible to oxidation and breakdown. Since the mid-1970s, the most applied method for vitamins analysis has been HPLC, because this technique does not need hard derivatization and its nondestructive nature allows the use of HPLC both as a preparative purification method as well as for quantification. 

Most ILMs are less acidic than the commonly used acidic matrices alone. This leads to the possibility to synthesize matrices with only weakly acidic or even neutral or basic pH values [48]. These matrices may be beneficial for the analysis of acid-labile compounds [40]. For example, these matrices were successfully used for the measurement of acid-labile compounds like sulphated oligosaccharides, which are a class of compounds with high biological relevance [49]. Using classical preparations, the detection of these challenging analytes was only possible after derivatization or in the form of noncovalent complexes formed with basic peptides. Upon use of the ILM,... 

Methylene difluorocyclopropanes are relatively rare and their rearrangement chemistry has been reviewed recently [14]. In addition, electron deficient alkenes such as sesquiterpenoid methylene lactones may be competent substrates. Two crystal structures of compounds prepared in this way were reported recently [15,16]. Other relatively recent methods use dibromodifluoromethane, a relatively inexpensive and liquid precursor. Dolbier and co-workers described a simple zinc-mediated protocol [17], while Balcerzak and Jonczyk described a useful reproducible phase transfer catalysed procedure (Eq. 6) using bromo-form and dibromodifluoromethane [18]. The only problem here appears to be in separating cyclopropane products from alkene starting material (the authors recommend titration with bromine which is not particularly amenable for small scale use). Schlosser and co-workers have also described a mild ylide-based approach using dibromodifluoromethane [19] which reacts particularly well with highly nucleophilic alkenes such as enol ethers [20], and remarkably, with alkynes [21] to afford labile difluorocyclopropenes (Eq. 7). 

These are relatively simple and robust procedures. Some of the factors to keep in mind when working with anthocyanins are excessive heat, light, oxidation, and sample handling, as these factors can alter or destroy these labile compounds. The preparation and HPLC separation of anthocyanins on silica C]8 columns (see Basic Protocol 1) is the easiest and most robust of the procedures. Typical care in filtering samples and solvents prior to HPLC analysis is about all that is necessary. 

All of the optically active compounds prepared by diastereoisomer separation and/or conversion to enantiomers have proved to be configurationally stable at the metal center as long as they are in the solid state. Concerning their behavior in solution, however, the optically active or-gano-transition-metal compounds are divided into two groups (a) compounds configurationally stable at the metal center, which do not racem-ize or epimerize with respect to the metal atom prior to decomposition and (b) compounds configurationally labile at the metal center which racemize or epimerize with respect to the metal atom prior to decomposition. 

PREPARATION OF LABILE COMPOUNDS UNDER PROTECTIVE CONDITIONS. CHROMIUM (II) SALTS... 

An important advantage of the //-phosphonatc method is that at no stage of the synthetic route, is a labile phosphotriester prepared.1036 1041 1 8 The reaction of salicylchlorophosphite (Scheme 3.19) with a glucosamine derivative furnished a phosphite triester which was subsequently hydrolysed to the //-phosphonate monoester. The latter compound was coupled... 

Silicon tethers based on bis-alkoxysilanes (silyl ketals) are commonly prepared from the dichlorosilanes by reaction with an alcohol in the presence of base. These conditions are not compatible with some base labile compounds. To make unsymmetrical bis-alkoxysilanes requires a method for breaking the symmetry of the dichlorosilane. Without such a method, one must accept a statistically determined mixture of mono-alkoxy and bis-alkoxy products. This may be acceptable for inexpensive readily available alcohols, but it precludes the use of bis-alkoxysilane tethers for high-value synthetic intermediates. To overcome these limitations to... 

This was introduced in 1988 principally by Karas and Hillenkamp [19-21], It has since become a widespread and powerful source for the production of intact gas-phase ions from a broad range of large, non-volatile and thermally labile compounds such as proteins, oligonucleotides, synthetic polymers and large inorganic compounds. The use of a MALDI matrix, which provides for both desorption and ionization, is the crucial factor for the success of this ionization method. The method is characterized by easy sample preparation and has a large tolerance to contaminantion by salts, buffers, detergents, and so on [22,23],... 

By choosing a pH for the reduction at which the product is stable, the preparation of this labile compound could be accomplished. 

For some drugS/ absorption is limited by a compound s solubility/ with dissolution being highly dependent on gastric pH. The antiretroviral agent didanosine/ for example/ is an acid-labile compound/ originally formulated as a buffered preparation to improve its bio availability. Other medications/ such as atazanavir and certain azole antifungals (particularly... 

A special procedure has been devised to synthesize dichlorocyclopropenone (35), since this compound cannot be prepared by direct hydrolysis of the cation. This labile compound can, however, be liberated by careful hydrolysis of the AICI3 adduct 36 (equation 32). 

The parent stibabenzene (60) and bismabenzene (61) have been prepared by dehydrohalogenation of 59a and 59b, respectively (equation 170 ). While stibabenzene (60) has been isolated, it is a labile compound and rapidly polymerizes at room temperature. Bismabenzene (61) exists mainly as a dimer, but has been detected spectroscopically and via chemical trapping by hexafluorobutyne (equation 171 ). 

Membrane emulsihcation has been recently proposed for the preparation of stable and uniform-sized microcapsules [27]. Membrane emulsihcation is a technology that allows to obtain uniform emulsions at low energy input compared to the emulsion prepared using high-pressure homogenizers and rotor/stator systems therefore, it is very useful for the preparation of emulsions containing labile compounds such as bioactive molecules sensihve to shear stress [28]. 

---