Chemically labile

Once the above restrictions on isotope, solubility, chemical lability and paramagnetism are met, then a very wide range of samples can be investigated. Gases can be studied, especially at higher pressures. Solutions for... 

It was observed lelatjvdy early that chemically labile compounds - such as vitamii carotenes - decomix e, either on application to the TLC l r or during the TI separation that fidlows. This phenomenoii was primarily ascribed to the presence of c ygen (oxidation) and exposure to light (photochemical reaction) in the presence of t active sorbents, which were assumed to exert a catalytic effect (photocatalytic reactioi... 

It is usually desirable to define molecular weights by analyzing underivatized samples, because chemically labile functional groups will then be observed. Unfortunately, underivatized oligosaccharides often give ambiguous sequence-data. The reasons for this may be deduced from a... 

As the first isolable intermediate in the bioconversion of arachidonic acid into prostaglandins and thromboxanes (Eq. 3), PGG2 is a bicyclic peroxide of immense biological importance. It is difficult to obtain pure from natural sources and the presence of the 15-hydroperoxide group adds a further dimension of chemical lability to that associated with the 9,11-peroxide bridge. The chemical synthesis of PGG2 is thus a landmark in prostaglandin chemistry. It also represents a pinnacle of success for the silver-salt route to bicyclic peroxides. 

Thermodynamically controlled asymmetric transformations of stereo-chemically labile diastereomers. 

As a substrate for chemical modification, vincristine (2) represents much more challenge. The extracted yield of 2 from Catharanthus is approximately 10% that of 1, making this compound a much scarcer substrate (42). Furthermore, the chemical lability of the N-1 formyl bond is extreme under a variety of conditions. The preparation of compounds in this series is usually accomplished by performing the requisite functional group interconversions with compounds bearing an N-1 methyl and reserving the oxidation of this methyl for the last step. 

Charton, M. The Prediction of Chemical Lability Through Substituent Effects, in Design of Biopharmaceutical Properties Through Prodrugs and Analogs (Roche, E. B. ed.), Am. Pharm. Assoc. Acad. Pharm. Sci., Washington, D.C. 1977... 

Charton, M. 1977.The prediction of chemical lability through substituent effecBefeign of Biopharmaceut-ical Properties through Prodrugs and Analggpdited by E. B. Roche, 22280. V foshington, D.C. American Pharmaceutical Association. 

Solution-phase chemistry is simpler than solid-phase. Solid-phase synthesis has two extra variables the resin and linker. Unwanted reactions on either can cause major problems. The reactivity of the resin, normally polystyrene, is typically very low and not a major concern. Linkers, however, are designed to be broken, so they must have some level of reactivity. The cleavage conditions must be strictly avoided during all the synthetic steps. Because photochemical reactions are uncommon in mainstream synthesis, photo-chemically labile linkers are attractive in solid-phase synthesis. 

The bases of this group, which are not methylated on the basic nitrogen and which are only known here, are chemically labile, occur in minimal amounts, are difficult to isolate, and often are contaminated with unknown substances. Consequently, mucronines-F, -G, and -H, as well as abyssenine-B were studied chemically and by mass spectrometry as their acetyl derivatives (Section VII, D). 

Another example employed Mitsunobu reaction for the inversion reaction (Figure 10(b)).A single enantiomer of a (stereo)chemically labile allylic-homoallylic alcohol was obtained in 96% yield and 91% ee from the racemate through a lipase-catalyzed kinetic resolution coupled with in situ inversion under carefully controlled (Mitsunobu) conditions. Using this reaction, the algal fragrance component, (S)-dictyoprolene, was synthesized. 

Chemically-Controlled Systems. In these systems, the polymer matrix contains chemically-labile bonds. On exposure to water or enzymes the bonds hydrolyze, erode the three dimensional structure of the polymer and release the incorporated reagent into the surrounding medium. Depending on the polymer used, the erosion products may act as interferences, such as by altering the pH of the solution. Examples of these systems are polyglycolic acid (PGA) and a polyglycolic acid - polylactic acid (PGA/PLA) copolymer. PGA hydrolyzes to hydroxyacetic acid, and PGA/PLA hydrolyzes to lactic acid and hydroxyacetic acid. Other chemically-controlled systems are based on polyorthoesters, polycaprolactones, polyaminoacids, and polyanhydrides. 

Recently, Hernandez et al. reported the first catenane system in which unidirectional rotation can be achieved in either direction [60]. Catenane 21 works by biasing Brownian motion with the aid of chemically labile kinetic barriers. The system is a simple [2] catenane in which the route that the smaller macrocycle can take between two stations on the larger can be se-... 

FAB is most often compared to the soft ionization method known as field desorption (FD) mass spectrometry, a technique in which the sample, deposited on an emitter wire coated with microcrystalline carbon needles, is desorbed under the influence of a high electric field gradient. As usual, bioorganic systems are best represented by both techniques (21, 33). Though FAB is the easier of the two, they are complementary, FAB being particularly suited for the case of extreme thermal lability and FD for the case of chemical lability or matrix interference. Cerny et al. (33) compare the two techniques for the study of coordination complexes and conclude FD is better for molecular-ion determination, while FAB provides better fragmentation information, which is useful in elucidating structures. 

When chemically labile Se compounds are to be characterized, there is a need for species-targeted sample preparation. As a representative example, the study of... 

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