Carbohydrate rings

Modification of the Carbohydrate Ring Substituents. Replacement of hncomycin s 2-hydroxy group by methoxy or by hydrogen (74), or inversion of the configuration of the hydroxy group at C-4 or at C-2 destroys activity (75). 

The diastereodifferentiating effect of the galactosylamine template in these Strecker reactions is rationalized in terms of a preferred conformation 5 of the Schiff bases which is stabilized by a (7i-double bond into the [Pg.795]

A very important group of secondary hydroxyl modifications represent fluoroglycosides.104 When they interact with a retaining glycosidase, the highly electronegative fluorine on the carbohydrate ring causes the destabilization of both transition states and the reaction proceeds at a... 

The number of naturally occurring, organic fluorine compounds is very small indeed. Compound 1 is1 a nucleoside antibiotic that contains fluorine attached to the carbohydrate ring, replacing H-4 in the D-ribosyl moiety. The presence of fluorine was shown by -n.m.r.-, 19F-n.m.r.-, and mass-spectral data, and the structure of this compound has been confirmed by independent synthesis.9,10... 

These methods have many stages and require preliminary protection of the OH-groups, followed by the removal of protective functions. The advantages of E. Fishers method include its universality - the ability to introduce ureido (thioureido) fragments in any position of the carbohydrate ring with the presence of amino group in this position. 

D. Sabatino and M. J. Damha, Oxepane nucleic acids Synthesis, characterization, and properties of oligonucleotides bearing a seven-membered carbohydrate ring, J. Am. Chem. Soc., 129 (2007) 8259-8270. 

For a highlight on carbohydrate ring contraction reactions see H. Redlich, Angew. Chem. Int. Ed. Engl. 1994, 33, 1345. 

The combination of a 5-membered carbohydrate ring and a purine or pyrimidine is called a nucleoside. The rings are numbered as shown in the following figure. The two rings of a nucleoside or nucleotide must be distinguished from each other, so the positions of the sugar carbons are denoted with a (prime) notation. If one or more phosphates exist on the carbohydrate, the combination is called a nucleotide. For example, ATP is a nucleotide. 

Periodic Acid Cleavage of Carbohydrates Another method used to determine the size of carbohydrate rings is cleavage by periodic acid. Recall that periodic acid cleaves vicinal diols to give two carbonyl compounds, either ketones or aldehydes, depending on the substitution of the reactant (Section 11-1 IB). 

These compounds illustrate the structural diversity of carbohydrates. Glucose is the most common simple sugar, whereas cellulose, which comprises wood, piant stems, and grass, is the most common carbohydrate in the piant world. Doxorubicin, an anticancer drug that has a carbohydrate ring as part of its structure, has been used in the treatment of ieukemia, Hodgkin s disease, and cancers of the breast, biadder, and ovaries. 2 -Deoxyadenosine 5 -monophosphate is one of the four nucieotides that form DNA. 

Remote Stereochemical Effect. The stereochemistry of the various chiral centers in a given carbohydrate ring may affect the spectra. This is mainly the result of the degree of binding that can be accomplished and the number of... 

Most of the key transformations described here involve carbon-carbon bond formation. Carbohydrate functional group manipulations are a long-standing research area in carbohydrate chemistry often reviewed in specialized books and series. We hope to provide the reader with a set of different chemical modifications of carbohydrate ring systems or acyclic carbohydrates available to devise new synthetic routes to biologically relevant targets. 

It is probably superfluous to say that radical reactions are suitable for the construction of carbocycles. What is working to achieve carbohydrate ring closure is obviously suitable to form... 

Nucleosides with conformationally restricted carbohydrate rings (locked nucleic acids—LNA) are RNA mimics containing the 2 -0,4 -C-methylene linkage. The bicyclic structure locks... 

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