C-Glycosylation Reactions

These model reactions were of great value in the extension of the C-glycosylation reaction with malonic esters to five-membered ring-systems. Treatment of 2,3 5,6-di-O-isopropylidene-a-D-manno-furanosyl bromide122,123 (150) with diethyl sodiomalonate led to an anomeric mixture of C-glycosyl compounds that could be separated by column chromatography, with the a (151) and /B (152) anomers in... 

Extension of the C-glycosylation reaction to the D-ribo series was at first disappointing. The sole product, obtained in high yield, from the reaction of 2,3,5-tri-0-benzoyl-/3-D-ribofuranosyl chloride (154), or the corresponding bromide (81), with diethyl sodiomalonate in 1,2-dimethoxyethane was the acetal 155, resulting102 from an attack on the dioxolenium carbon atom in the intermediate benzoxonium ion (156). 

Similarly, C-glycosylation reaction of I-0-acylsugars with silylated carbon nucleophiles, such as silyl enol ethers, proceeded to give the corresponding C-glycosides in good yields in the presence of a catalytic amount of trityl perchlorate ( ). This reaction was also catalyzed by ploymer-bound trityl perchlorate (Mukaiyama, T. Kobayashi, S. Carbohydrate Research, in press.)... 

Anomeric triphenylphosphonium salts have been used as well as phenylsul-fides,but in the latter case extra stabilization is necessary (see below). Anomeric nitrosugars, which have been extensively studied in C-glycosylation reactions by Vasella, will be covered in Sect. 2.2.1 and ester enolates derived from 3-deoxy-2-ketoulosonic acids (sialic acid and KDO derivatives), which bear a structural similarity to 2-deoxy pyranosides, will be covered in Sect. 4.4. Deprotonation of anomeric phenylsulfones has been discussed in Sect. 2.1.1 and additional transformations on closely related compounds are presented in Scheme 14 [20]. Alkylation of phenylsulfone 54 with epoxide 55 provides adduct 56 which eliminates benzenesulfinic acid at room temperature to give the C(l)-alkylated glycal 57 a similar elimination is also observed with adducts derived from... 

Scheme 57 illustrates an example of this process coupling of the lithio reagent derived from vinyl stannane 237 with epoxide 242 leads to the C-disaccharide derivative 243 in good yield. A double transmetallation to give an organocopper reagent 244 can facilitate the C-glycosylation reaction with allyl halides [80] (Scheme 58). 

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... 

Next, the scope of this C-glycosylation reaction was investigated. The NeuSAc chloride was found to serve as a donor in samarium mediated C-glycosylation, unfortunately, purification of C-glycoside from excess chloride donor is often problematic, making the NeuSAc phenyl sulfone the donor of choice. A variety of acceptors were also evaluated including alkenes, epoxides, vinyl esters, aldehydes and ketones. Only the aldehydes and ketones afforded the desired C-glycoside products. 

C-Glycosylation reactions of glycosyl-halide, -ester and -ether with carbon nucleophiles are divided into two types the Sn2 type of substitution with carbon nucleophiles such as Grig-nard reagents, and addition of carbon nucleophiles to oxocarbenium cations generated from... 

A useful C-glycosylation reaction employing a cobalt catalyst is shown in Scheme 76 [107]. Glycosyl acetate was treated with diethylmethylsilane and catalytic amounts of Co2(CO)g under a carbon monoxide atmosphere, stereospeciflc hydroxymethylation occurred in 1,2-tmns manner through neighboring group participation with the acetyl group. 

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. 

This C-glycosylation reaction developed in our laboratory turned out to be applicable to several glycosyl donors, as shown in Table 3.5 [29]. 

Analyzing the reactions involving highly oxygenated five-membered ring oxocarbenium ions added another level of complexity. For example, analyzing the C-glycosylation reactions of ribose derivatives (Fig. 4.7) [30] required not... 

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... 

Abe H, Shuto S, Matsuda A (2001) Highly a- and P-selective radical C-glycosylation reactions using a controlling anomeric effect based on the conformational restricrion strategy. A study on the conformation - anomeric effect - stereoselectivity relationship in anomeric radical reactions. J Am Chem Soc 123 11870-11882... 

Eco-cJesign of new biomimetic carbohydrates fundamental interest of Lubineau s C-glycosylation reaction... 

We dedicate this ehapter to the memory of Professor Lubineau. We thank Pr. Lubineau s team for their essential eontribution to the elaboration of the C-glycosylation reaction in water. Special thanks to all L Oreal participants, being from Research or Industry for their valuable contribution. 

In addition, there are many methods for C-glycosylation reactions using protected and activated carbohydrates in multistep sequences [9]. However, these transformations are not the matter of the discussion here. 

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