Glycosyl samarium

P. de Poully, A. Chenede, J.-M. Mallet, and P. Sinay, SmI2 promoted chemistry at the anomeric center of carbohydrates. Reductive foimation and reaction of glycosyl samarium (III) reagents. Bull. Soc. Chim Fr. 730 256 (1993). 

In contrast, the Barbier reactions of 2-acetamido-2-deoxy sugars with ketones showed a preference for the formation of 1,2-m-C-glycoside products (Scheme 7.39). This was attributed to the complexation of the C2-acetamido group to the glycosyl samarium, thereby reducing the configurational lability of the organometallic species.78,80,81... 

Mazeas, D, Skrydstrup, T, Beau, J M, A highly stereoselective synthesis of 1,2-trans-C-glycosidss via glycosyl samarium(III) compounds, Angew. Chem. Int. Ed. Engl, 34, 909-912, 1995. 

Glycosylation. Samarium(III) triflate catalyzes the reaction of glycosyl 2-pyridyl sulfones with alcohols. This method is applicable to the preparation of di- and trisaccharides containing both furanose and pyranose residues. The difference in reactivity from... 

Stereoselective synthesis of l,2-trans-C-glycosid s via glycosyl samarium (III) compounds [40]. 

Other extensions of this C-glycosylation methodology have been reported for the preparation of two C-linked 1,6-disaccharides. Reaction of the pyridyl sulfone of N-acetylglucosamine 54 with aldehyde 55 produced mainly the a-C- saccharide 56 (Scheme 13) [35]. The stereoselectivity at the anomeric center is explained in this case by internal coordination of the fV-acetyl group with the metal ion of the intermediate glycosyl samarium species 57, retarding anomerization to the ther-... 

Thus, samarium(III)-triflate-promoted reactions provided O-linked disaccharides in moderate to excellent yields [308]. Szeja and Niemiec reported the use of 5-nitro-S-pirydyl glucoside as glycosyl donors in P-glucosidase-catalyzed glycosylation of alcohols [494]. 

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. 

It was clear that the protection of amino group and the presence of an a-COOH group in homoserine complicated samarium-based C-glycosylation. 

In this review, we have attempted to demonstrate how glycosyl radical intermediates prepared from the corresponding glycosyl pyridylsul-fones and samarium diiodide provide for a suitable method for the construction of cis- and rraiw-1,2-C-glycosides. These mild and simple reactions allow for a facile access to a variety of C-glycosides in a method which parallels <7-glycosylations in that intact glycosyl donors and acceptors are employed. 

Chang, G X, Lowary, T L, A glycosylation protocol based on activation of glycosyl 2-pyridyl sulfones with samarium triflate, Org. Lett., 2, 1505-1508, 2000. 

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