A-C-glycopyranoside

C. Leteux and A. Veyrieres, Synthesis of a-C-glycopyranosides of D-galactosamine and D-glucosamine via iodocyclization of corresponding glycals and silver tetrafluoroboranuide-promoted alkynylation at the anomeric center, J. Chem. Soc., Perkin Trans. 1, (1994) 2647-2655. 

Dawe, R D, Fraser-Reid, B, a-C-glycopyranosides from Lewis acid catalysed condensations of acetylated glycals and enol silanes, J. Chem. Soc., Chem. Commun., 1180-1181, 1981. 

Some authors exemplified different regioselectivity on glycosylations of various O- and thio-p-D-glycopyranosides (phenyl, benzyl, phenylethyl, phenylthio, benzylthio, etc.), including a C-glycosyl compound.29... 

J. M. Williams and A. C. Richardson, Selective acylation of pyranosides I. Benzoylation of methyl a-D-glycopyranosides of mannose, glucose and galactose, Tetrahedron, 23 (1967) 1369-1378. 

T. Granier and A. Vasella, Synthesis and evaluation as glycosidase inhibitors of I //-iinidazol-2-yl C-glycopyranosides, Helv. Chim. Acta, 78 (1995) 1738—1746. 

C. Bumouf, J. C. Lopez, F. G. Calvo-Flores, M. D. Laborde, A. Olesker, and G. Lukacs, ir-Facial selectivity in Diels-Alder reactions of 2-C-vinylglycals. Stereocontrolled route to annulted C-glycopyranosides, J. Chem. Soc. Chem. Commun. p. 823 (1990). 

The axial a-D-tribuyltin-glycopyranoside can be obtained by reaction of 3 with lithium naphthalenide and then with Bu3SnCl. Transmetallation (BuLi) and reaction with an electrophile results in a-C-glycosides. 

Alkylation of dihydropyranylacetates. The Pd(0)-catalyzed alkylation of allylic acetates by stabilized carbanions originally reported by Trost and Verhoeven (8,475 9, 451 -457)4 has been extended to alkylation of dihydropyranyl acetates as a route to natural C-glycopyranosides.5 Again the displacement is regioselective and results in net retention of configuration in reactions with most carbanions. However, alkylation with phenylzinc chloride or vinylzinc chloride occurs with inversion. 

C-Glycosides. Axial C-glycopyranosides arc obtained exclusively by photolysis of a mixture of an ot-D-glycopyranosyl bromide, acrylonitrile, and tributyltin hydride. The diastereoselectivity may involve an anomcric effect or rapid trapping of the initial radical before inversion can occur. 

C-Allylated glycopyranosides.4 a-Glycopyranosides or a-glycopyranosyl chlorides are converted stereoselectively into a-C-allylated glycopyranosides on reaction with al-lylsilanes catalyzed by trimethylsilyl triflate. Iodotrimethylsilane is less effective as the catalyst. 

Boschetti, A, Nicotra, F, Panza, L, Russo, G, Vinylation-electrophihc cyclization of aldopentoses easy and stereoselective access to C-glycopyranosides of rare sugars, J. Org. Chem., 53,4181-4185, 1988. 

Kozikowski, A P, Sorgi, K L, A mild method for the synthesis of anomerically allylated C-glycopyranosides and C-glycofuranosides, Tetrahedron Lett., 23, 2281-2284, 1982. 

For cyclic systems even more than for acyclic systems, chirality is readily introduced from carbohydrate precursors thus providing a simple method for the stereoselective construction of unsaturated C-glycopyranosides. An early example is the Claisen rearrangement of 19409. 

Scheme 7. Highly stereoselective synthesis of a-linked C-glycopyranoside using V-70 [6a]...

C-Glycopyranosides may be obtained from glycopyranosyl halides via intermolecular addition of glycopyranosyl radicals [129]. In a more useful example, the a-aminoacrylate 192 was used as the radical acceptor for preparation of C-glycosyl amino acids 193 and 194 [130] (Scheme 66). In a concise synthesis of showdomycin (197), Barton utilized the trigger reaction of the 7V-hydroxy-2-thiopyridone derivative and the exceptional radicophilicity of tellurides in concocting the conditions for the conversion from the anisyl telluride 195 to the intermediate 196 after oxidative elimination [131] (Scheme 67). In Keck s synthesis of (-t-)-pseudomonic acid C (201), the intermediate 200 was prepared via stereocontrolled intermolecular addition of the radical generated from the iodide 198 to the allylic sulfone 199 [132] (Scheme 68). 

This analysis was supported by the fact that conformationally labile c-glycopyranosides were found to react with ozone. Similar results were also observed with a and B-glycofuranosides as these compounds are not maintained in a rigid conformation. 

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