Koenigs-Knorr glycosylation

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. 

SCHEME 3.7 Koenigs-Knorr glycosylation promoted by insoluble silver salts. 

Many variations of the metal promoters have been examined to probe the effect of the activator on the Koenigs-Knorr glycosylation yield and stereoselectivity. The... 

As a representative example, Paulsen and Von Reessen used a sialyl bromide to apply the Koenigs-Knorr method to the synthesis of an a-2,3-linked disaccharide (Scheme 8) [7b]. Glycosylation of the galactose acceptor 19 with sialyl bromide 18 led to the desired disaccharide 20, although with poor stereoselectivity (ca. 1 1 a j8). The low yields of Koenigs-Knorr glycosylations with secondary alcohols are partly due to elimination of the sialyl halide and formation of the 2,3-dehydro derivative 21. 

The Koenigs-Knorr glycosylation method using glycosyl bromides or chlorides as donors dates back to 1901 (13), and it is still in wide use. The glycosylation reaction was originally achieved with silver carbonate as a promoter, but other heavy metal salts, especially silver triflate (21), have been introduced as more effective... 

Glycosylation at the C-26 hemiacetal of 38 was achieved by using the classical Koenigs-Knorr glycosylation. Treatment of 38 with rhamnosyl chloride 36 and silver tiiflate in the presence of TMU provided trisaccharide 39 in 55% yield. Hydroboration followed by PDC oxidation afforded desired ketone 4 0. A palladium (II) hydroxide catalyzed debenzylation of 40 under hydrogen atmosphere gave 2. This completes the first total synthesis of a saponin however, aqueous solutions of 2 were not sweet at all. ... 

This reaction is related to the Koenigs-Knorr Glycosylation. 

Koenigs Knorr glycosylations of several natural products have... 

The commercially available but expensive w-pentenol may be successfully recovered after this procedure by distillation and saved for subsequent applications. Koenigs-Knorr glycosylation of suitable glycosyl bromides with, for example, silver triflate (suitable for perbenzoylated glycosyl bromides [47]) or silver carbonate (for peracetylated bromides [55]) also provides access to NPGs (e.g. 

The synthesis of 1-0-acyl-D-glucuronic acid derivatives has been achieved from the benzyl j3-pyranoside via its 2,3,4,6-tetra-0-(o -ethoxyethyl)derivative. The 1-0-methanesulphonyl ester of D-glucuronic acid was used to prepare acyl derivatives involving the attachment of biliverdin and bilirubin. In model experiments the sulphonate also afforded glucuronosides. D-Glucuronosides have also been prepared by standard Koenigs-Knorr glycosylation from various bile acids and A -tetrahydrocannabinol. ... 

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