Glycosyl heterocycles

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-... 

Analogously, Bognar and coworkers45-46 prepared the same thiourea derivative 33, and synthesized some other N-glycosyl heterocyclic derivatives, such as thiazole 36, 1,3,4-thiadiazole 38, and 1,2,3,4-thiatriazole 39, using as the starting material isothiocyanate 2, or its ureido (34) or semicar-bazide (37) analogs. It is noteworthy that treatment of semicarbazide 37 with a nitroso acid at 0° afforded the 1,2,3,4-thiatriazole derivative 39 by addition and cyclization. The same 1,2,3,4-thiatriazole derivative 39 was obtained by treatment of isothiocyanate 2 with hydrazoic acid. 

Glycosyl Imines and Amines for the Synthesis of Glycosyl Heterocycles. . 41... 

Keywords Glycosjd azides - Glycosjdamines Glycosyl enamines - Glycosyl heterocycles Triazoles Click chemistry... 

The reactions of acetylated C-(D-glycosyl) heterocycles and of D-glycosyl cyanides with NBS in hot carbon tetrachloride or with bromine in the presence of u.v, light gave axial bromination at the anoraeric centre. Both methods gave the same results, with a wide range of heterocycles. Scheme 3 shows some examples of reactions... 

Various N-glycosylated heterocycles are covered In Sections 5 and 6, and in Chapter 19. 

N-Glycosyl derivatives are conveniently named as glycosylamines. In the case of complex heterocyclic amines, such as nucleosides, the same approach is used. 

Betalains are N-heterocyclic water-soluble pigments deposited in vacuoles. Their common precursor is betalamic acid consisting of a 1,7-diazaheptamethin system, an extended Jt-electron system exhibiting a canary yellow color. Betalamic acid may condense with cyc/o-dopa to yield betanidin, the common precursor aglycon of the red betacyanins. Betanidin in turn may be glycosylated and/or acylated, yielding 29... 

Ferndndez-Bolanos JG, Lopez 6 (2007) Synthesis of Heterocycles from Glycosylamines and Glycosyl Azides. 7 31-66... 

The activity of PK and NRPSs is often precluded and/or followed by actions upon the natural products by modifying enzymes. There exists a first level of diversity in which the monomers for respective synthases must be created. For instance, in the case of many NRPs, noncanonical amino acids must be biosynthesized by a series of enzymes found within the biosynthetic gene cluster in order for the peptides to be available for elongation by the NRPS. A second level of molecular diversity comes into play via post-synthase modification. Examples of these activities include macrocyclization, heterocyclization, aromatization, methylation, oxidation, reduction, halogenation, and glycosylation. Finally, a third level of diversity can occur in which molecules from disparate secondary metabolic pathways may interact, such as the modification of a natural product by an isoprenoid oligomer. Here, we will cover only a small subsection of... 

The /3-d configuration was assigned40 to 82 on the basis of the trans rule,83 as exemplified by the reaction of glycosyl halides with heavy-metal salts of heterocyclic bases.833 Unambiguous chemical proof was secured from the transformation of the nitrile into the corresponding acid (21). 

Carbohydrate derivatives, in which one or more of the oxygen atoms bonded directly to the carbon skeleton have been replaced by sulfur, are termed thiosugars. The placement of the sulfur atom at the anomeric position constitutes a special case, because thioglycosides, alkyl, aryl and heterocyclic, occupy a very important place as versatile glycosyl donors in glycosidation methodology. Anomeric thiocarbonyl compounds, on the contrary, have been less explored, although their potential and scope is likely to be similar. 

Additions to acetylenes have been carried out with many other types of azide, including glycosyl azides, various heterocyclic azides ... 

More complex examples exist where the 2-substituent on the furan is a polyhydroxyalkyl or glycosyl moiety <89CAR(l9l)209>. In these cases, the rearrangement products provide useful intermediates for the preparation of heterocycle-substituted sugar derivatives. Sometimes Baeyer-Vil-liger-type products are observed, such as 3-keto enoic esters instead of 3-keto enol esters <81TL1443>. 

On the other hand, the presence of a glycosyl group attached to the nucleoside pyrophosphate has been found to influence the reactivity of the heterocyclic base. Thus, the catalytic hydrogenation of uridine and 6-azauridine 5 -(a-D-glucopyranosyl pyrophosphates) to the 5,6-dihydro derivatives proceeds more slowly than that of the respective nucleoside 5 -phosphates or 5 -pyrophosphates.338,339 Such differences have not been observed in comparisons of analogous derivatives of 2 -deoxyuridine, N3-methyluridine, and cytidine. 

It has been found that l,3-di(ethoxycarbonyl)allene reacts readily with such heterocyclic dienes as furan, pyrrole, and pyrone derivatives. The furan adduct 383, obtained in 87% yield, was hydroxylated, and after acetonation, was cleaved251 with ozone, to afford intermediate 384. An approach to the conversion of 384 into C-glycosyl compounds by hydride-promoted scission of the C-5-C-6 bond was disclosed.255... 

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