Koenigs-Knorr type reaction

The major products 265 and 266 arise from an attack of oxidant anti to the pyranosic oxygen atom and substituent in position 6. Activation of the anomeric carbon was achieved through NBS mediated C-2-C-3 dibromination. Dibromides 267 have been shown to serve as glycosyl donors in Koenigs-Knorr type reaction, since blocking the 3-axial position prevent the competitive elimination, often observed during glycosylation reactions of 2-ulosonic ester halides [149], The racemic 4-e/w-DAH acetylated esters 268 were thus obtained. 

A number of 5- and N-ketosyl derivatives of NANA were prepared by Privalova and Khorlin (1969). The use of p-nitroaniline in a Koenigs-Knorr type reaction with the acetylglycosyl chloride methyl ester of NANA gave (after subsequent deacylation) the 2-deoxy-2-p-nitropheny-lamino A-glycoside of NANA, assigned structure XXXI (Figure 10). 

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... 

Koenigs-Knorr type synthesis or fusion reaction. 

The synthesis of C-glycosylflavones has been achieved mainly by Friedel-Crafts-type reactions or Koenigs-Knorr glycosylations. However, yields were poor due to steric hindrance of the substrates. Several other procedures have been proposed in the last decade. 

Helferich, Bohm and Winkler1 have reported that the use of iodine catalyzed the Koenigs-Knorr reaction, which was exceedingly slow when calcium chloride was used as an internal desiccant. This catalyst has since been used by a number of other workers.79 111-114 The work of Talley, Reynolds and Evans79 on the synthesis of the orthoester type of oligosaccharides, indicated that the presence of iodine favored the formation of a normal biosidic linkage, whereas the absence of iodine favored the formation of an orthoester linkage. 

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. 

The glycosidic hydroxyl undergoes preferential alkylation when the sugar is alkylated with one equivalent of dimethyl sulfate and alkali 49,4 0 ). The procedure is particularly valuable for obtaining methyl glycosides of the D-mannose type for which the Koenigs-Knorr reaction fails because of orthoester formation. Alkylation of tetra-0-acetyl-/3-D-fructopyranose with silver oxide and methyl iodide leads to the methyl /8-D-fructopyranoside tetraacetate 50). 

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