Koenigs-Knorr reaction intermediates

The synthesis93 of N-(2,4-dinitrophenyl)-3-0-(tetra-0-acetyl-/ -D-glu-copyranosyl)-L-threonine methyl ester (131) involved a two-step procedure. First, formation of the intermediate, L-threonine orthoester 130 was achieved by treatment of tetra-O-acetyl-a-D-glucopyranosyl bromide (128) with the methyl ester of N-(2,4-dinitrophenyl)-L-threonine94 (129) under the conditions of the Koenigs-Knorr reaction (see next paragraph), and this was then converted into the L-threonine glycoside 131. 

Because a substantial excess of the halide is used in the Koenigs-Knorr synthesis of cardenolides, it is usually difficult, if not impossible, to secure the acylated intermediate in crystalline form. Hence, Reichstein nd coworkers18 introduced a procedure whereby the whole of the products arising from the coupling reaction are saponified, employing a solution of potassium hydrogen carbonate in aqueous methanol. This modified transesterification procedure renders all extraneous carbohydrate materials water-soluble the methanol is removed by evaporation, leaving an aqueous solution from which the (deacylated) cardenolide may be obtained by... 

Glycosylammonium Salts. - Koenigs-Knorr syntheses of the A/-glucuronides of (S)-(-)-nicotine, cotinine and cis- and rra/u-3-hydroxycotinine have been reported. The confcnmational analysis of the glycosylnitrilium ions that are intermediates in glycosylation reactions in acetonitrile, is covered in Chapter 21. 

After Fischer and Helferich, as well as Koenigs and Knorr, introduced the use of the heavy-metal salts of a purine to direct the glycosylation reaction to N9, Todd and co-workers adapted Fischer s silver salt method for the coupling of 2,8-dichloroadenine with 2,3,5-tri-O-acetyl-D-ri-bofuranosyl bromide to provide an intermediate blocked nucleoside that was converted to both adenosine and guanosine4 ... 

---