Stereoselectivity C-glycoside synthesis

Kunz, H, Weismuller, J, Muller, B, Stereoselective C-glycoside synthesis through titanium(IV)-catalyzed addition of silyl enol ethers to 2-acyloxy-3-ketoglycal, Tetrahedron Lett., 25, 3571-3574, 1984. 

Du Y, Linhardt RJ, Vlahov IR (1998) Recent advances in stereoselective C-glycoside synthesis. Tetrahedron 54 9913-9959... 

The enolate reactivity associated with this approach to C-glycoside synthesis was, however, first developed with octulosonic acid derivatives, such as 283 [114], 284 [115], and 285 [116], and a series of examples involving aldehyde and halide-based electrophiles are shown in Scheme 74. Related studies involving stereoselective protonation of this class of exocyclic enolate have also been described [13] and Scheme 75 illustrates this with an example of reductive samariation (using ethane-1,2-diol, but no HMPA) of an anomeric acetate 286 [117]. 

Craig, D, Munasinghem, V R N, Stereoselective template-directed C-glycosidation. Synthesis of bicyclic keto oxetanes via intramolecular cyclization reactions of (2-pyridylthio)glycosidic silyl enol ethers, J. Chem. Soc., Chem. Commun., 901-903, 1993. 

Herscovici, J and Antonakis, K, Recent developments in C-glycoside synthesis. In Studies in Natural Products Chemistry, Vol. 10, Stereoselective Synthesis, Part F, Atta-ur-Rahman, Ed., Elsevier, Amsterdam, The Netherlands, pp. 337-403, 1992. 

Craig, D., Pennington, M. W. and Warner, P. 1993. Stereoselective template-directed C-glycosidation. Synthesis of 5-membered oxygen heterocycles via cation-mediated intramolecular cychzation reactions. Tetrahedron Lett. 34 8539-8542. 

Stereoselective inverse-demand hetero (4 + 2) cycloadditions. A Chiral Template for C-Aryl Glycoside Synthesis. Chiral allenamides2 4 had been used in highly stereoselective inverse-demand hetero (4 + 2) cycloaddition reactions with heterodienes.5 These reactions lead to stereoselective synthesis of highly functionalized pyranyl heterocycles. Further elaboration of these cycloadducts provides a unique entry to C-aryl-glycosides and pyranyl structures that are common in other natural products (Scheme 1). 

Vasella has applied the concept of anomeric anion stabilization by a nitro group to the /3-D-JV-acetyl-D-glucosamine derivative 177, available in four steps from N-acetyl-n-glucosamine [52] (Scheme 39). Reaction of the tetraethylammonium nitronate derived from 177 with aldehyde 178 provides anti-179 which then undergoes stereoselectively reduction (see Sect. 2.2.1) to provide -C-glycoside 180, intermediate in a synthesis of N-acetyl-neuraminic acid. 

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. 

---