Nucleosides anomerization

A. Kittaka, T. Asakura, T. Kuze, H. Tanaka, N. Yamada, K. T. Nakamura, and T. Miyasaka, Cyclization reactions of nucleoside anomeric radical with olefin tethered on base Factors that induce anomeric stereochemistry,/. Org. Chem., 64 (1999) 7081-7093. 

The most important derivatives of pyrimidines and purines are nucleosides Nucleosides are N glycosides m which a pyrimidine or purine nitrogen is bonded to the anomeric carbon of a carbohydrate The nucleosides listed m Table 28 2 are the mam building blocks of nucleic acids In RNA the carbohydrate component is d ribofuranose m DNA It IS 2 deoxy d ribofuranose... 

The anomeric carbon is attached to N 1 in pyrimidine nucleosides and to N 9 in purines... 

Hydantocidin. Hydantocidin (182), C2H2QN2O3, is elaborated by S. hygroscopicus (278). It is unique in that the anomeric carbon of the ribosyl moiety forms the spHo bond of hydantoin (279). The ribofuranose moiety which has been reported to be in a Q -endo conformation (279) has been synthesized (280,281). Hydantocidin is a herbicidal nucleoside with activity against monocotyledenous and dicotyledenous plants. 

Synthesis of spiro sugars and C-nucleosides using anomeric free radicals 98SL700. 

Just as proteins are biopolymers made of amino acids, nucleic acids are biopolv-mers made of nucleotides joined together to form a long chain. Each nucleotide is composed of a nucleoside bonded to a phosphate group, and each nucleoside is composed of an aldopentose sugar linked through its anomeric carbon to the nitrogen atom of a heterocyclic purine or pyrimidine base. 

The isomerism existing between the pairs of nucleotides was attributed to the different locations of the phosphoryl residues in the carbohydrate part of the parent nucleoside,49 63 since, for instance, the isomeric adenylic acids are both hydrolyzed by acids to adenine, and by alkalis or kidney phosphatase to adenosine. Neither is identical with adenosine 5-phosphate since they are not deaminated by adenylic-acid deaminase,68 60 and are both more labile to acids than is muscle adenylic acid. An alternative explanation of the isomerism was put forward by Doherty.61 He was able, by a process of transglycosidation, to convert adenylic acids a" and 6 to benzyl D-riboside phosphates which were then hydrogenated to optically inactive ribitol phosphates. He concluded from this that both isomers are 3-phosphates and that the isomerism is due to different configurations at the anomeric position. This evidence is, however, open to the same criticism detailed above in connection with the work of Levene and coworkers. Further work has amply justified the original conclusion regarding the nature of the isomerism, since it has been found that, in all four cases, a and 6 isomers give rise to the same nucleoside on enzymic hydrolysis.62 62 63 It was therefore evident that the isomeric nucleotides are 2- and 3-phosphates, since they are demonstrably different from the known 5-phosphates. The decision as to which of the pair is the 2- and which the 3-phosphate proved to be a difficult one. The problem is complicated by the fact that the a and b" nucleotides are readily interconvertible.64,64... 

The C-nucleosides are a group of C-glycosylated heterocycles in which the anomeric carbon atom is attached to the heterocycle by a C-C bond. For a number of years after its discovery, pseudouridine1 (1) was the only representative of this class of compound it is found as a minor component in various transfer ribonucleic acids.2 Since 1959, a number of other C-nucleosides have been isolated in rapid succession, mainly from fermentation sources, and have been found to exhibit a variety of interesting biological properties.3 Thus, pyraz-... 

The Wittig reaction, or its phosphonate modification, can be a very useful reaction in the synthesis of anomerically functionalized precursors to C-nucleosides, particularly when the ring-closure reaction leads to five-membered, anhydro derivatives, and occurs with a high degree of stereocontrol. Zhdanov and coworkers,130 showed that the following five-membered, Wittig products (179-182) were formed from various free sugars and (p-methoxybenzoyl)- and (acetyl-methylene)-triphenylphosphoranes. 

David and Lubineau191 reported the synthesis of pseudocytidine [5-/3-D-ribofuranosylcytosine (270)] and its a anomer by a procedure analogous to that used in preparing pseudouridine.155-157 Thus, 2,4 3,5-di-O-benzylidene-a/de/iydo-D-ribose (223) was condensed with the dilithio derivative of 2-0,4-N-(trimethylsilyl)cytosine, and the resulting, epimeric, acyclic derivatives were subjected to acid-catalyzed cyclization. The anomeric configuration of the free C-nucleosides was ascertained by spectroscopic methods and by their transformation into a- and /3-pseudouridine in the presence of nitrous acid. The anomeric 5-(/3-D-ribofuranosyl)isocytosines have also been prepared by Fox and coworkers.1913... 

Sulfoxides have also been used in the synthesis of nucleoside analogs (Scheme 3.2). Chanteloup and Beau reported the synthesis of ribofuranosyl sulfoxide 13 and its use in the glycosylation of a series of silylated pyrimidine and purine bases.7 Although 16 is not an anomeric sulfoxide, its reaction with cytosine derivative 17 is conceptually related.8... 

Unsaturated monosaccharides [e.g. (91)] react with the Ph3P-DAD combination in the presence of phthalimide, with inversion of configuration of C-4, to form the phthalimido-derivatives (92).82 Treatment of carbohydrates having a free anomeric OH group with 6-chloropurine, DAD, and methyldiphenylphosphine gives the purine nucleosides (93).83... 

Nature has exploited ribose derivatives for a number of cmcially significant biochemicals. Many of these contain a heterocyclic base attached to the P-anomeric position of o-ribofuranose, and are termed nucleosides. Adenosine, guanosine, cytidine, and uridine are fundamental components of ribonucleic acids (RNA see Section 14.1),... 

Purine nucleosides of type 1.4 (1177) were prepared by the reductive cleavage at the anomeric position of the ribofuranosyl moiety of 1176 with diisobutylaluminium hydride (DIBALH). The reductive ring opening was explained by the initial formation of a Lewis complex (93TL4835). 

S. Z. Dong and L. A. Paquette, Stereoselective synthesis of conformationally constrained 2 -deoxy-4 -thia beta-anomeric spirocyclic nucleosides featuring either hydroxyl configuration at C5, J. Org. Chem., 70 (2005) 1580-1596. 

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