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Best Glycosidic bond

Acetonitrile is another participating solvent, which in many cases leads to the formation of an equatorially linked glycoside [125-131], It has been proposed that these reactions proceed via an a-nitrilium ion intermediate. It is not well understood why the nitrilium ion adopts an axial orientation however, spectroscopic studies support the proposed anomeric configuration [130,131], It is known that nucleophilic substitution of the a-nitrilium ion by an alcohol leads to P-glycosidic bonds and the best P-selectivities are obtained when reactive alcohols at low reaction temperatures are employed. Unfortunately, mannosides give poor anomeric selec-tivities under these conditions. [Pg.211]

The best decomposition conditions for the fungal mycelia was treatment with 11 M NaOH at 45°C for 13 h and then treatment with 0.35 M acetic acid at 95°C for 5 h [36]. Moreover the Termamyl treatment is obviously highly efficient at separating the chitosan from the glucan and offers possibilities for the isolation of purified chitosan [4, 36]. Based on these observations, it has been proved that these two polysaccharides, chitosan and glucan, are linked by a-(l )-glycosidic bond [4, 8, 37]. The IR spectra of alkaline-soluble and alkaline-insoluble glucan, and chitosan are shown in Fig. 8. [Pg.198]

In the realm of biological chemistry, in 2008, Lagisetty et al. [46] reported for the first time a simple method for the arylation at C-8 of adenine nucleosides. The motivation for this investigation was the observation that substitution at the C-8 position can influence the syn-anti conformational equilibrium around the glycosidic bond or produce structural factors that can influence enzymatic recognition. Purine derivatives are also of great importance in medicinal chemistry as they display a broad spectrum of antiviral and antimycobacterial activity. In this account iodo-, bromo-, and even chloro-aromatics were coupled with vinyl nucleosides. The reaction was catalyzed by the simple combination of Pd(OAc)2, (o-tolljP, and EtjN. The best conditions and some of the best representative results are shown in Figure 1.5. [Pg.15]

FIGURE 18.15 In the Induced-fit model, a flexible active site and substrate both adjust to provide the best fit for the reaction. Sucrose binds to the active site to align the glycosidic bond for hydrolysis. The monosaccharide products are released, and the enzyme binds to another sucrose. [Pg.671]

Perhaps best known of the nucleoside analogues are the arabinose compounds Ara(C) (cytosine arabinoside, l-jS-D-arabinofuranosylcytosine) and Ara(A) (adenine arabinoside, 9-j8-D-arabinofuranosyladenine). Both compounds are triphosphorylated [converted to Ara(CTP) and Ara(ATP) in vivol upon which they may exert a dual action on DNA polymerase. They may inhibit DNA polymerase (for example, Ara(CTP) inhibits DNA polymerase II and III) and they may be incorporated into newly synthesized DNA. In fact, Ara(C) but not Ara(A) is also incorporated into RNA. Such uptake into DNA is lethal since a subtle difference will develop in the DNA geometry as a result of the cytosine base adapting a new glycosidic bond angle due to steric hindrance from the 2 -hydroxyl function. [Pg.138]

Cereals are considered one of the best sources of B-vitamins thiamin, riboflavin, niacin, pyridoxine, and folates, but a poor source of fat-soluble vitamins and vitamin Bi2. Most B-vitamins are associated with the aleurone. The niacin associated with cereals is found in free and bound forms. Bound niacin is not well utilized by the human system. Interestingly, the alkaline treatment of maize for production of tortillas and other traditional food products increases niacin bioavailability because the alkali and heat treatment breaks the glycosidic bond that binds free niacin with the other components (Chapters 9 and 17). [Pg.102]


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See also in sourсe #XX -- [ Pg.3 , Pg.184 , Pg.187 ]




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