Glycosyl halides acylated, preparation

Because of the high C - F bond energy, glycosyl fluorides are stable in comparison to the other glycosyl halides, and this character has attracted much attention. They have been prepared in many different ways. One of them, rather classical, is through addition of the elements of HF (for example, HF in benzene ), BrF, or IF to per-O-acylated glycals. ... 

Glycosyl halides, a very important group of carbohydrate derivatives, are commonly prepared14 from per-O-acylated sugars by reaction with hydrogen halides or halides of aluminum or titanium. The selection of the method depends mainly on the anomeric configuration of the substrate, the kind of its O-acyl groups, and the stability of the product to be prepared. 

The typical and widely used procedure for the activation of the anomeric centre involves the initial preparation of acylated glycosyl halides, which are then subjected to SN2-like displacement by a phosphate anion in the presence of a silver-containing promoter and a base. 

Another radical reaction for preparation of 2-deoxysugars utilizes acylated glycosyl halides as the starting material. As mentioned previously, when these glycosyl halides are treated with tributyltin hydride and AIBN the initially formed glycosyl radical is reduced to give anhydroalditols [211]. However, if tributyltin hydride and AIBN are added very slowly over... 

Scheme 16.9 Preparation of O-glycosides of 2-amino-2-deoxysugars. (A) Use of N-acyl-protected donors 27 results in 1,2-trans glycosylation due to neighboring group participation. (B) 1,2-cis glycosylation products 32 from ji-glycosyl halides or (C) by in situ anom-...

A Acyl Derivatives. - Oxazolines have been prepared in near quantitative yield by treatment of peracetylated 2-amino-2-deoxy-glycosyl halides with mercuric bromide and 2,4-collidine. C-Silyated byproducts 83 were formed in... 

Glycosyl fluorides may be prepared by displacement of per-O-acyl or suitably protected 2-0-acyl 1-halides (Cl or Br) with fluoride [AgF (Refs. 28 and 29), KHF2 (Ref 30) or AgBF4 (Ref 31)]. -o-Glucopyranosyl [ F]fluo-... 

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