Koenigs-Knorr reaction donors

Despite its wide application in glycoside synthesis, the Koenigs-Knorr reaction suffers from several disadvantages (i) the glycosyl halides are unstable, (ii) excess toxic heavy metals are needed to activate the donor, and (iii) a desiccant (to absorb any liberated water) and an acid acceptor (to absorb the liberated hydrogen halide and can be a promoter itself) are often needed to increase the yield and suppress side reactions. 

As a first attempt to synthesise uronic acid-containing structures from this polysaccharide and from the polysaccharide of Streptococcus pneumoniae type 3 (Fig. 6) [67], the common Koenigs-Knorr type donor methyl (2,3,4-tri-O-acetyl-a-D-glucopyranosyl bromidejuronate (easily obtained from the 3,6-glucurono-lactone and recrystallised from ethanol [68]) was tried in coupling reactions with the relevant acceptors 44 and 45 (for structures see Scheme 14, p. 189) and... 

Notes and discussion. The 2-chloride derivative can be prepared via several protocols [8, 9, 10, 11]. This protocol is the simplest and safest method, and allows facile large-scale preparation. The 2-chloro derivative is often used to prepare the 2,3-dehydro derivative [8b], which is pivotal for access to C-3 appended type donors. It is also central to strategies involving Koenigs-Knorr reactions with simple alcohols. Immediate use is recommended after preparation, despite the moderate shelf life of the title compound in the freezer. 

Sugar halides are important glycosyl donors. They are usually activated by means of heavy metal salts in glycosylations referred to as the Koenigs-Knorr reaction. The first sugar anomeric dihalide reported was 2,3,4,6-tetra-O-acetyl-l-bromo-D-glucopyranosyl bromide isolated in a 17% yield by Blattner and Ferrier from products of photobromination of P-o-glucopyranose pentacetate with bromine in the absence of base. This method is synthetically useful, as a C-5 brominated... 

This is the first glycosylation in this synthesis. There are. several methods to build a new glycosydic C-0 bond. Very important methods are the Koenigs-Knorr and related reactions and the trichloroacetimidate method. It depends on the carbohydrates themselves, what kind of method is useful and which donor and acceptor you have to use. 

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