Koenigs–Knorr reaction

About: Koenigs–Knorr reaction is a research topic. Over the lifetime, 106 publications have been published within this topic receiving 4521 citations.

The Koenigs-Knorr Reaction

Abstract: Publisher Summary This chapter focuses on Koenigs–Knorr reaction used for the synthesis of glycosides. The Koenigs–Knorr reaction is applicable to the preparation of both aryl and alkyl glycosides, and is widely used for the synthesis of glycosides having complex groups attached to the anomeric carbon atom—particularly oligosaccharides. The procedure involves the treatment of a per- O acylated glycosyl halide with an alcohol in the presence of a heavy-metal salt or an organic base as the acid acceptor; the latter enhances the rate of reaction and also prevents side reactions. It is noted that under the usual reaction-conditions the more-stable anomer is obtained, and the less stable anomer can be prepared only by a kinetically controlled reaction. Per- O -acylated glycosyl bromides react faster than the corresponding chlorides, and are therefore preferred for most reactions. Silver carbonate or silver oxide is commonly used as the acid acceptor in the Koenigs–Knorr reaction. Mercury (II) acetate instead of a silver salt has been also used.

The glycosyl halides and their derivatives.

Abstract: Publisher Summary This chapter discusses that the glycosyl halides are compounds in which the hydroxyl group at the reducing carbon atom of a sugar is replaced by halogen. As such, the glycosyl halides are rare, only a few fluorides being well authenticated. However, the fully acylated glycosyl halides, particularly the acetylated compounds, sometimes referred to as “acetohalogeno sugars” or “acetohalogenoses”, are very well known. The most familiar of these is 2,3,4,6-tetra-O-acetyl-α-D-glucosyl bromide (“acetobromo-α-D-glucose”). The poly-O-acyl derivatives of the glycosyl halides are among the most important intermediates for synthesis in carbohydrate chemistry; moreover, their chemistry has considerable intrinsic interest. Most of the methods for the preparation of glycosyl halide derivatives depend on the replacement by halogen of an acyloxyl (nearly always acetoxyl) group a t the reducing carbon atom. Hence, the methods for the synthesis of a large number of the glycosyl halide derivatives can be exemplified by description of those used for converting fully acetylated sugars into O-acetylglycosyl halides. As might be expected, treatment for prolonged periods often results in replacement of a second acetoxyl group. Other methods available for the synthesis of glycosyl halide derivatives involve the replacement of hydroxyl and alkoxyl by halogen, the opening of anhydro rings with hydrogen halides or equivalent reagents, and the addition of halogen or hydrogen halide to certain unsaturated sugar derivatives. Glycosyl iodide derivatives are usually prepared from the corresponding bromides by reaction with sodium iodide.

Reactions of difluoroenoxysilanes with glycosyl donors: synthesis of difluoro-C-glycosides and difluoro-C-disaccharides.

Abstract: Difluoroenoxysilanes, prepared from acylsilanes and trifluoromethyltrimethylsilane under fluoride activation, were glycosylated with some glycosyl donors (acylglycosides, glycals) to yield difluoro-C-glycosides with a difluoromethylene group in the place of the anomeric oxygen. This reaction strongly depends on the substituent in the 2-position of the glycosyl donor. Application of this methodology to a xylose-derived acylsilane led to the formation of difluoro-C-disaccharides as an isosteric O-glycosyl mimetic.