O-Acylglycosyl halides

Because the secondary hydroxyl groups attached to the ring skeleton of steroids react sluggishly with O-acylglycosyl halides, completion of the... 

Because of the stability of (59) and (59a) (unprecedented for 2-deoxy-O-acylglycosyl halides), it was considered that coupling could be accom-... 

Initial studies58 had indicated that 1-glycopyranosyl nucleosides prepared by the mercuri procedure are identical with those prepared by the Hilbert-Johnson method, when similar poly-O-acylglycosyl halides are employed. The applicability of the mercuri procedure to the synthesis of 1-aldopento-furanosyl nucleosides was demonstrated by the preparation of 5 -methyl-uridine (LIV). Condensation of dithyminylmercury (LI) with tri-O-benzoyl-... 

The most widely used method for the formation and isolation of the unstable forms of the poly-O-acylglycosyl halides is that of Schlubach," in which the stable form of a poly-O-acylglyeosyl bromide is caused to react with freshly prepared, activated silver chloride a metathetical reaction occurs, accompanied by a Walden inversion to give the unstable form of the chloride. When a method which gives only the stable form of the halide is employed, the yields from either of the anomeric acetates are usually the same. However, Stacey and his coworkers have found that crystalline tetra-O-acetyl-a-L-arabinopyranose, on treatment with hydrogen bromide in glacial acetic acid, gives tri-0-acetyl-/3-L-arabinopyranosyl bromide in 84% yield, whereas the sirupy tetra-0-acetyl-/3-L-arabinopyranose under identical conditions affords only a 12% yield. 

The course of the reaction of the poly-O-acylglycosyl halides with nucleophilic reagents has recently received attention. Thus, Lemieux has related the theories of the mechanism of replacement reactions to some reactions in carbohydrate chemistry, and has summarized our knowledge concerning the mechanism of reaction of the poly-O-acylglycosyl halides. Hence, only those aspects of the subject which have become clarified by recent work need be discussed here. 

In the preceding Section, the poly-O-acylglycosyl halides have been discussed with particular reference to the effect of their structure on the reactivity of the halogen atom and on the configuration of the product resulting from the unimolecular, nucleophilic substitution. In the present Section is given a general account of the reactions of the halides which refers more to the type of product which may be obtained from these compounds. 

The oligosaccharides represent a most important group of glycosides. Synthesis in this field has been extensive and has been reviewed by Evans, Reynolds and TalleyThe formation of the 3-biose linkage presents no difficulty when the 2-0-acetyl group of the poly-O-acylglycosyl halide is... 

A number of important reactions of the poly-O-acylglycosyl halides have been reviewed already in Advances in Carbohydrate Chemistry and there is no need to discuss further the formation of glycals and of thio and seleno-sugars or the Grignard processes. ... 

Koenigs-Knorr synthesis. The reaction of a poly-O-acylglycosyl halide with a hydroxylic component to form a glucopyranoside" is usually effected by use of silver oxide or silver carbonate. The much less expensive yellow mercuric oxide, together with a small amount of mercuric bromide, serves equally well and is not sensitive to light. The acid acceptor, by reaction with the hydrogen bromide formed, gives more catalyst. 

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