Of C-glycosides

Deslongschamps P (1993) In Thatcher GJ (ed), The Anomeric Effect and Associated Stereoelectronic Effects. American Chemical Society, Washington, DC, p 26 Recent reviews on C-glycoside synthesis (a) Du Y, Linhardt RJ, Vlahov IR (1998) Tetrahedron 54 9913 (b) Levy DE, Tang C (1995) The Chemistry of C-Glycosides. Elsevier Science, Oxford... 

A recently developed application of the Ramberg-Backlund reaction is the synthesis of C-glycosides. The required thioethers can be prepared easily by exchange with a thiol. The application of the Ramberg-Backlund conditions then leads to an exocyclic vinyl ether that can be reduced to the C-nucleoside.95 Entries 3 and 4 in Scheme 10.6 are examples. The vinyl ether group can also be transformed in other ways. In the synthesis of partial structures of the antibiotic altromycin, the vinyl ether product was subjected to diastereoselective hydroboration. 

Scheme 10.6 gives some examples of the Ramberg-Backlund reaction. Entry 1 was used to prepare analogs of the antimalarial compound artemisinin for biological evaluation. The reaction in Entry 2 was used to install the side chain in a synthesis of the chrysomycin type of antibiotic. Entries 3 and 4 are examples of formation of C-glycosides. 

KATRITZKY POZHARSKII Handbook of Heterocyclic Chemistry, 2nd Edition LEVY TANG The Chemistry of C-Glycosides... 

Waridel P, Wolfender J-L, Ndjoko K, Hobby KR, Major HJ and Hostettmann K. 2001. Evaluation of quadrupole time-of-flight tandem mass spectrometry and ion-trap multiple-stage mass spectrometry for the differentiation of C-glycosidic flavonoid isomers. J Chromatogr A 926(1) 29-41. 

KATRITZKY POZHARSKII Handbook of Heterocyclic Chemistry, 2nd Edition LEVY TANG The Chemistry of C-Glycosides LI GRIBBLE Palladium in Heterocyclic Chemistry McKILLOP Advanced Problems in Organic Reaction Mechanisms MATHEY Phosphorus-Carbon Heterocyclic Chemistry The Rise of a New Domain OBRECHT Solid Supported Combinatorial and Parallel Synthesis of Small-Molecular-Weight Compound Libraries... 

D. E. Levy and C. Tang, The Chemistry of C-Glycosides Tetrahedron Organic Chemistry Series, Pergamon Press, Oxford, 1995, vol. 13 J.-M. Beau... 

Hoffmann, H. M. R. Dunkel, R. Mentzel, M. Reuter, H. Stark, C. B. W., The total synthesis of C-glycosides with completely resolved seven-carbon backbone polyol stereochemistry Stereochemical correlations and access to L-configured and other rare carbohydrates. Chem. Eur. J. 2001,7, 4771-4789. 

Maddaford reported the diastereoselective synthesis of C-glycosides 29 using conjugate addition catalyzed by cationic rhodium catalysts such as [Rh(COD)2]BF4 (Eq. 1) [24]. Addition of phosphine hgands to the reaction system inhibited the conjugate addition. It is likely that the enone 28 derived from the pyranose is less reactive toward the conjugate addition. 

An equally intriguing report concerned the rhodium(I)-catalyzed hydroformylation of C-glycoside alkenes 12 (Scheme 5.5) [5]. As depicted, aldehydes 13 were formed with excellent diastereoselectivity. It was proposed that the diastereoselectivity is due to a strong conformational bias favoring the depicted alkene conformation. 

In the aim to prepare isosteric analogs of nucleotide-activated sugars, this straightforward method was applied to the preparation of C-glycosides related... 

The use of other 1,3-dicarbonyl compounds as nucleophilic species was also investigated in this reaction. In particular, unsymmetrical diketones were tested in the condensation, giving rise to mixtures of C-glycosides [111, 114]. 

The replacement of the anomeric oxygen of C-glycosides and C-disaccharides by a CF2 leads to difluoro-C-glycosides, difluoro-C-disaccharides, and difluoro-C-glyco-peptides. These hydrolytically stable compounds have been synthesized, but their use as inhibitors does not seem to have been biologically validated. ... 

Anomeric 2-pyridylsulfones, such as 48, were introduced by Skrydstrup and Beau [18] and have a LUMO energy level low enough that reductive samariation is possible in THF alone (no HMPA needed). Under Barbier conditions, good yields of C-glycosides 50-53 are available with aldehydes and ketones (Scheme 13) [ 19]. The low selectivity associated with 50-53 is again a consequence... 

While transition metals have been used extensive ) in C-gly coside construction, assigning a precise reactivity profile to C(l) can be difficult and this section will cover cases where nucleophilic character is apparent, albeit loosely defined. The use of palladium-mediated synthesis of C-glycosides has been reported and this topic, which involves an anomeric tin intermediate, has been covered in Sect. 2.2.2. The insertion of carbon monoxide into a carbon-metal (C(l)-metal) bond is, however, an important aspect of transition metal-mediated C-glycoside formation, and this process has been observed and exploited in a number of situations. 

Levy DE, Tang C (1995) The chemistry of C-glycosides, Pergamon, Oxford... 

Later on, in the 1980s, a renewed impulse in the synthesis of C-glycosides came from the observation that many macroUdic structures of biological inte-... 

C-Glycosides are usually synthesized starting from the parent sugar, properly protected and functionalized, in a process generally defined C-glycosylation. Examples of C-glycoside syntheses starting from non-carbohydrate substrates, mainly Diels-Alder adducts, are also known, but less common. 

This article will describe our efforts in the stereoselective synthesis of C-glycosides of biological interest, giving, in the meantime, an overview of the main C-glycosylation procedures and explaining their stereochemistry. 

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