Synthesis of C-glycosides

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. 

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. 

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. 

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-... 

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. 

N d lec J-Y,P richon J,Troupel M (1997) Organic Electroreductive Coupling Reactions Using Transition Metal Complexes as Catalysts. 185 141-174 Nicotra F (1997) Synthesis of C-Glycosides of Biological Interest. 187 55-83 Nishimura J,see Inokuma S (1994) 172 87-118 Nolte RJM,see Sijbesma RP (1995) 175 25-56 Nordahl A,see Carlson R (1993) 166 1-64... 

Tanaka, T. et al.. Tannins and related compounds. CXXIV. Five new ellagitannins, platycaryanins A, B, C, and D, and platycariin, and a new complex tannin, strobilanin, from the fruits and bark of Platycarya strobilacea Sieb. et Zucc., and biomimetic synthesis of C-glycosidic ellagitannins from glucopyranose-based ellagitannins, Chem. Pharm. Bull, 41, 1708, 1993. 

C. Jaramillo and S. Knapp, Synthesis of C-aryl glycosides, Synthesis p. 1 (1994). M. H. D. Postema, Recent developments in the synthesis of C-glycosides, Tetrahedron 48 8545 (1992). 

Glycoside synthesis from 2-pyridylthio glycosides is also possible with 0-benzyl-protected donors in the presence of methyl iodide [20], silver triflate [21 ], or mercuric salts [10,13,22], The versatility of this leaving group has also been demonstrated in the synthesis of C-glycosides [23 J. 

J. Dupuis, B. Giese, J. Hartung, M. Leising, H.-G. Korth, and R. Sustmann, Electron transfer from trialkyltin radicals to nitrosugars The synthesis of C-glycosides with tertiary anomeric carbon atoms, J. Am Chem. Soc. 107 4332 (1985). 

F. Baumberger and A. Vasella, Deoxynitrosugars—stereoelectronic control in the reductive denitration of tertiary nitro ethers. A synthesis of C-glycosides, Helv. Chim. Acta <56 221C (1983). 

G. Stork, H. S. Suh, and G. Kim, The temporary silicon connection method in the control ol regio- and stereochemistry. Application to radical-mediated reactions. The stereospecific synthesis of C-glycosides, J. Am Chem. Soc. 7/3 7054 (1991). 

Grignard reactions. This anomeric activation should be useful for synthesis of C-glycosides. 

Levy, D. Tang, C. The Synthesis of C-Glycosides, Pergamon, Oxford, 1995. 

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