Stereoselective C-glycosylation

In C-glycosylation reactions a new stereocenter is formed, and in general only one of the two stereoisomers is desired. Therefore, it is quite important to develop stereoselective C-glycosylation procedures, in which the stereochemical outcome can be predicted... 

C. G. Lucero and K. A. Woerpel, Stereoselective C-glycosylation reactions of pyranoses The conformational preference and reactions of mannosyl cation, J. Org. Chem., 71 (2006) 2641-2647. 

Larsen CH, Ridgway BH, Shaw JT, Smith DM, Woerpel KA (2005) Stereoselective C-glycosylation reactions of ribose derivatives electronic effects of five-membered ring oxocarbenium ions. J Am Chem Soc 127 10879-10884... 

The 3-0-diethylthiocarbamoyl libofuranosyl donor 140 was also utilized for the p-stereoselective C-glycosylation by Mukaiyama and co-workers the reaction of 140 and carbon nucleophile 144 produced the coupling product 145 (p/a = 96 4) with an excellent p-selectivity, which presumably resulted from the remote participation of the thiocarbamoyl group at 0-3 of 140 (Scheme 26) [78]. 

C27-C36 subunit via completely stereoselective C-glycosylation to the F ring- Synlett, 43—45. 

Horita, K., Sakurai, Y, Nagasawa, M., and Yonemitsu, O. (1997) Synthetic studies of halichondrin B, an antitumor polyether macrolide isolated from a marine sponge. 7. Synthesis of two C27-C36 units via construction of the F ring and completely stereoselective C-glycosylation using mixed Lewis adds. Chem. Pharm. Bull., 45,1558-1572. 

Stereoselective methods for the preparation of 1,2-cw-O-glycosides and 1,2-cw-C-glycosyl compounds are presented. 

The aim of this chapter is to describe the methods currently available for the stereoselective synthesis of compounds having 1,2-cz x-O-glycosylic or 1,2-czj-C-glycosylic linkages.11... 

Russo and co-workers98 have reported a high stereoselective method for the preparation of 1,2-czls-C-glycosyl compounds based on the iodocyclization of some hept- and hexenitols obtained by Wittig reaction of sugar... 

Starting from 3,4,6-tri-O-benzyl-D-glucal epoxide 93 (Scheme 31), a-C-glycosyl compounds 94 are obtained in good stereoselectivity by treatment with a lithium acetylide and zinc chloride.103... 

Glycosyl acetates. Reaction of l-0-acetyl-2,3,5-tri-0-benzyl-D-ribofuranose (99) with various enoxysilanes in the presence of TrC104 afforded 1,2-czj-C-glycosyl compounds with high stereoselectivity and yields higher than 90% (Scheme 34).106... 

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). 

As far as the stereochemistry concerns, when the cyclization occurs through a Michael reaction, the thermodynamic product is generally obtained, but some exceptions have been observed such as that of A-acetyl glucosamine protected as 4,6-benzylidene, the reaction of which with [(ethoxycarbonyl)methylene]triphenylphosphorane afforded the ot-C-glycosyl compound stereoselectively.28... 

Stereoelectronic effects can be invoked for the radical reaction at anomeric centre of carbohydrates. The high stereoselective preparation of a-substituted C-glycosyl phosphonates in a a p ratio of 98 2 was achieved by reductive addition of bromide 2 to a-phosphonoacrylate (Reaction 7.5) [10]. Yields (in parentheses) depend on the sugar configuration D-galacto (80%), D-manno (47 %), D-gluco (30 %) and L-fuco (62 %). 

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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