Carbohydrates glycals

D. P. Curran and Y. Suh, Selective mono-Claisen rearrangement of carbohydrate glycals. A chemical consequence of the vinylogous anomeric effect, Carbohydr. Res. 777 161 (1987). 

As the project progressed, a major study was made of the electrophilic addition of various fluorinated species to the double bonds of peracylated carbohydrate glycals,... 

Keywords Carbohydrates, Glycals, 0-Glycosides, C-Glycosides, N-Glycosides, S-Glycosides, Nucleosides, AUylic rearrangements... 

Scheme 8.41 Utilization of glycals and a-glycal epoxides in carbohydrate chemistry.

A conceptually new direct oxidative glycosylation with glycal donors, employing a reagent combination of triflic anhydride and diphenyl sulfoxide, has recently been reported by Gin [83], This new 3-glycosylation method works very well with hindered hydroxy nucleophiles, including sterically shielded carbohydrate hydroxy systems, and can be run on large scales. 

E. Wieczorek and J. Thiem, Preparation of modified glycosyl glycerol derivatives by glycal rearrangement, Carbohydr. Res., 307 (1998) 263-270. 

P. Tiwari, G. Agnihotriand, and A. K. Misra, Synthesis of 2, 3-unsaturated C-glycosides by HC104-Si02 catalyzed Ferrier rearrangement of glycals, Carbohydr. Res., 340 (2005) 749-752. 

Tochtermann reported the addition of dichlorocarbene to the racemic glycal 52, whose cyclopropyl-allyl rearrangement leads to the 2//-pyran. The synthesis of the optically pure (+)-(2S,3R,7S) and (-)-(2f ,3S,7f )-glycal precursors has also been achieved. As pointed out, optically pure glycals are versatile precursors for carbohydrate synthesis <00EJO1741>. 

The term glycal is used to define sugar derivatives having a double bond between C-1 and C-2. Accordingly, C-1 glycals are A1,2 unsaturated carbohydrate derivatives with a carbon substituent at the anomeric position. These compounds are versatile synthetic intermediates, owing to the variety of transformations associated with their enol ether functionality, and have found ample use in the preparation of C-glycosyl compounds,1 carbohydrate mimics,2 and natural products.3... 

The preparation of C-1 glycals has been largely addressed by synthetic modifications on cyclic carbohydrate derivatives, although strategies that rely on ring forming reactions from acyclic derivatives have recently emerged (Fig. 1). 

Among the approaches to C-1 glycals from cyclic carbohydrate precursors, those based on the formation of the C-1-C-1 bond on a glycal have been by far the more exploited. Other methods based on the formation of the anomeric C-C bond followed by the installation of the A1,2 unsaturation have also been described. 

Synthesis from carbohydrate derived 2-deoxy lactones. Routes to C-l glycals from 2-deoxy sugar pyranolactones have been described. The general strategy is outlined in Scheme 9. 

The convergent synthesis of glycopeptides in which the carbohydrate moiety is attached to the solid support proved an efficient alternative methodology.64 The carbohydrate structure itself was synthesized on solid support via glycal assembly and anomeric amine synthesis (Scheme 13.20). 

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