C-Glycosyl compounds, synthesis

Moffett and coworkers203 reported the synthesis of several 4-/3-D-ribofuranosylpyrazoles, such as 284(a-c), by 1,3-dipolar cycloaddition of diazoalkanes to the alkenic C-glycosyl compound 283, followed by dehydrogenation of the resulting pyrazolines. In view of the known biological activities of several nucleosides containing the... 

Scheme 29 One-pot synthesis of a-C-glycosyl compounds from glycals using organo-aluminium and organoboron reagents.

Radical reduction of 1-nitro-C-glycosyl compounds. In 1983, Vasella and co-workers125 reported a stereoselective method for the synthesis of a-C-mannopyranosyl compounds by reduction of 1-nitro-C-mannopyranosyl derivatives with Bu3SnH in the presence of a,a -azoisobutyronitrile (AIBN) radical initiator. These reactions involve the formation of anomeric centered radicals. Thus, in the case of d-manno configuration as in 140, the 1,2-cts-C-pyranosyl compound 145 was obtained in 84% yield. The intermediate pyranosyl radicals 143 prefer a-attack by the tin hydride. Thus for D-glucopyranosyl derivatives, the corresponding l,2-tra x-C-pyranosyl compound 144 is obtained preferentially (Scheme 47). 

This strategy has been extensively used by Parker s group in the synthesis of C-glycosyl aromatic antibiotics (Scheme 2a,c,d,e) by transformation of the easily accessible quinol C-l glycals into aryl C-l glycals, and thence to C-glycosyl compounds.19... 

Carbene-mediated methylenation of aldonolactones provides a direct route to 1-methylene sugars, which may be used as intermediates for the preparation of furanoid or pyranoid C-glycosyl compounds, or chiral precursors for the synthesis of natural products. 

Methylene sugars are versatile starting compounds for the synthesis of aromatic and hydroxymethyl C-glycosyl compounds (71) and of double-ended C-glycosyl compounds (70). The double bond undergoes 1,3-dipolar cycloadditions to give isoxazoline derivatives (71), such as that used as a model for the synthesis of tunicamycin. Also, reaction with iodonium sym-... 

Anhydroalditol compounds of a rather unusual type are the C-glycosyl compounds (erroneously called C-glycosides ). As pointed out by Treibs,33 these are not glycosides, but they are anhydro-alditols.3 A variety of approaches has been employed for synthesis of... 

Considerable effort has heen directed towards the synthesis of monosaccharide derivatives, and, in particular, C-glycosyl compounds, from Diels-Alder adducts offuran. Just and his group investigated the reaction of furan with a number of dienophiles, and described many interesting transformations of the adducts obtained from... 

In connection with the synthesis of C-glycosyl compounds, Moffatt and coworkers182 developed the selective debenzoylation of 2,3,5-tri-0-benzoyl-/3-D-ribofuranosyl cyanide, which had been reported earlier163 reaction in methanolic ammonia-chloroform at 0° gave the crystalline 5 -0-benzoyl derivative in 83% yield, without recourse to chromatography. 

Viewed (and drawn) in a terpene-like perspective, C-glycosyl compounds are in fact functionalized tetrahydrofurans and tetrahydropyrans. As such, they can be considered as versatile chirons for the synthesis of a variety of natural products containing cyclic ether motifs, such as in the ionophores. 

The next two chapters give the reader an excellent perspective and laboratory-proven methods for the synthesis of C-glycosyl compounds in general, and glycosylarenes in particular, utilizing free-radical reactions and Lewis acid-catalyzed oxonium ion chemistry, respectively. 

The synthesis of C-glycosyl compounds, commonly known as C-glycosides, in ionic reactions relies on the electrophilicity of the anomeric center and, therefore, involves the attack of an appropriate C-nucleophile. An umpolung method has been developed, and is described in the previous chapter. But instead of going from a carbocation to a carbanion, one can also consider homolytic or radical reactions to reverse the philicity (Scheme 1). 

The theme of C-glycosyl compounds is covered by two chapters on free radical- and cwis acid-mediated transformations that address the stereocontrolled synthesis of carbon abstitnents at the anomeric position. 

C.-D. Chang and T. L. Hullar, C-Glycosyl compounds. Stereospecific synthesis of... 

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