Compounds labile

Since allylation with allylic carbonates proceeds under mild neutral conditions, neutral allylation has a wide application to alkylation of labile compounds which are sensitive to acids or bases. As a typical example, successful C-allylation of the rather sensitive molecule of ascorbic acid (225) to give 226 is possible only with allyl carbonate[l 37]. Similarly, Meldrum s acid is allylated smoothly[138]. Pd-catalyzed reaction of carbon nucleophiles with isopropyl 2-methylene-3,5-dioxahexylcarbomite (227)[I39] followed by hydrolysis is a good method for acetonylation of carbon nucleophiles. 

FAB has been used with a wide variety of substances, including thermally labile compounds such as peptides, sugars, carbohydrates, and organometallics. 

Applications. The primary advantage of an Ic/ms is in combining the separation of large, potentially thermally labile compounds with the... 

A chloro group in the 5-position in a pyrido[4,3-d]p37rimidine proved to be labile, " compound 159 being readily converted into the 5-hydrazino, oxo, ethoxy, and thioxo derivatives. 

The position of quaternization in the isomeric ring system, 1,2,9-tri-azaphenanthrene, was not settled so clearly. The methiodide of 4-amino-10-phenyl-1,2,9-triazaphenanthrene (153) is converted by the action of alkali into a labile compound, which regenerates the quaternary salt on treatment with hydriodic acid. The labile compound gave 3-amino-2-phenylquinoline on heating with soda lime. 

An erratic TIC trace is also obtained if the belt is moving too slowly but in these circumstances this is due to the formation of droplets rather than the spreading of mobile phase on the belt. An additional problem encountered when droplets are formed on the belt is that more heat is required to evaporate the solvent and with this comes the increased likelihood of decomposition of any thermally labile compounds that may be present. 

Problems may be encountered in the analysis of thermally labile compounds, as heat is required for mobile-phase removal and for the transfer of analyte from the belt into the source of the mass spectrometer, and highly involatile compounds which cannot be desorbed from the belt, unless FAB is used for ionization. 

The full potential of LC-MS could not be exploited until it was possible to study involatile and thermally labile compounds for which electron and chemical ionization are not appropriate. A relatively small number of reports of the use of the moving-belt interface with fast-atom bombardment ionization for the study of these types of compound have appeared. 

Thermally labile compounds may also be studied - for example, the El specffa from the condensation products from the reaction between dimedone and substituted phenylbenzopyrans obtained via a particle-beam interface show less thermal degradation than do the mass spectra obtained using a direct-insertion probe [10]. 

Ionization occurs directly from solution and consequently allows ionic and thermally labile compounds to be studied. 

The APCl ionization regime is much more harsh that ESI and this precludes its use for the study of large biomolecules, with the mass limit for APCl being generally considered as below 2000 Da. Having said this, as will be shown later, the technique may still be used for the analysis of many thermally labile compounds without their decomposition, and small peptides have been studied. 

Thermally labile compound A compound that decomposes under the influence of heat. 

It was observed leldivdy early that chonically labile compounds - such as vitamins, carotenes - decomixise, either on application to the TLC layer or during the TLC separation that follows. Ibis phenomenon was primarily ascribed to the presence of oxygen (oxidation) and exposure to light (photochemical reaction) in the presence of the active sorbents, which were assumed to exert a catalytic effect (photocatalytic reaction). 

RCC is very unstable and rapidly reduced to a primary fluorescent chlorophyll catabolite (pFCC) by red chlorophyll catabolite reductase (RCCR). In subsequent steps pFcc is converted to different fluorescent chlorophyll catabolites (FCCs). These chemically rather labile compounds are further converted into colorless nonfluorescing chlorophyll catabolites (NCCs) by a nonenzymatic deconjugation of the four... 

A system of parallel reactions as shown in Fig. 5.3-9 was studied by Paul et at. (1992). The reactions are an acid-base neutralization and a base-catalysed hydrolysis of product (C). The labile compound (Q is in solution in an organic solvent, and aqueous NaOH is added to raise the pH from 2 to 7. Enolization occurs under basic conditions and is accompanied by irreversible decomposition (ring opening), which is not shown in the figure. The system was studied in the laboratory using the 6-Iitre reactor shown in Fig. 5.3-10. 

Fresh plant material is mixed with two volumes of methanol containing 0.3% concentrated HCl. If particularly labile compounds are known or found to be present, methanol containing 5% acetic acid may be used to advantage because this prevents hydrolysis of the acyl groups sometimes present on the sugar moieties while maintaining the low pH required to retain the pigments in the flavinium ion form. 

Supercritical fluid chromatography (SEC) was first reported in 1962, and applications of the technique rapidly increased following the introduction of commercially available instrumentation in the early 1980s due to the ability to determine thermally labile compounds using detection systems more commonly employed with GC. However, few applications of SEC have been published with regard to the determination of triazines. Recently, a chemiluminescence nitrogen detector was used with packed-column SEC and a methanol-modified CO2 mobile phase for the determination of atrazine, simazine, and propazine. Pressure and mobile phase gradients were used to demonstrate the efficacy of fhe fechnique. 

Polar or thermally labile compounds - many of the more modern pesticides fall into one or other of these categories - are not amenable to GC and therefore LC becomes the separation technique of choice. HPLC columns may be linked to a diode-array detector (DAD) or fluorescence detector if the target analyte(s) contain chromophores or fluorophores. When using a DAD, identification of the analyte(s) is based on the relative retention time and absorption wavelengths. Similarly, with fluorescence detection, retention time and emission and absorption wavelengths are used for identification purposes. Both can be subject to interference caused by co-extractives present in the sample extract(s) and therefore unequivocal confirmation of identity is seldom possible. 

The recommended technique for the determination of oxime carbamates and their metabolites by HPLC/MS and HPLC/MS/MS is positive ESI. Electrospray is a soft ionization technique and is suitable for thermally labile compounds. Ions are produced in the liquid phase at quasi-ambient temperature and atmospheric pressure, thus leaving the fragile pesticides intact. For oxime carbamates, the molecular adducts that can be monitored during HPLC/MS analysis with electrospray in positive mode are [M- -H]+, [M- -Na]+, or [M- -NH4]+, depending on the nature of mobile phase used. ... 

IonPac KC-811 column separated the labile compounds N-acetylneuraminic acid and N-glycolylneuraminic acid released by mild acid hydrolysis of bovine vitronectin.245 Sialic acid is extremely labile to conditions of handling and must be released by mild acid hydrolysis.246 Derivatization with phe-nylisothiocyanate and separation by reversed phase chromatography was found useful in analysis of hexosamines from gastric mucosa.247 A review on separation of sugars and other carbohydrates which covers many important aspects is available.248... 

Thermal desorption of solid traps by microwave energy is unsuitable for thermally labile compounds. In microwave thermal analysis [431] the (solid) sample is heated directly via interactions of the microwaves with the sample, providing more even heating and reduction of temperature gradients in comparison to heating with electrical furnaces. By passing air over a microwave-heated volatile sample evolved gases may be collected [432]. 

Applicability for ionic, highly polar or thermally labile compounds... 

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