Orthoesters acyl migration

The net result of all these competitive reactions is a possible mixture of normal and /3-glycosides and a possible mixture of two diastereo-isomeric orthoesters. In the course of the Konigs-Knorr procedure, water is usually formed from the reaction between hydrogen halide and silver oxide. The water formed, or deliberately employed, may take the part of the solvent alcohol, thus giving rise to products such as acidic orthoesters and normal tetra- and heptaacetates (for a disaccharide) of the sugars, and these compounds are subject to further changes, such as acyl migration. 

Despite of the presence of NaH, neither acyl migration nor orthoester formation occurred during the anomeric O-alkylation of alkaline labile acetyl-protected sugars (hemiacetal). Scheme 7 shows the results. Klotz and Schmidt reported preferential... 

Orthoesters. The value of cycHc orthoesters as intermediates for selective acylation of carbohydrates has been demonstrated (73). Treatment of sucrose with trimethylorthoacetate and DMF in the presence of toluene-/)-sulfonic acid followed by acid hydrolysis gave the 6-0-acetylsucrose as the major and the 4-0-acetylsucrose [63648-80-6] as the minor component. The latter compound underwent acetyl migration from C-4 to C-6 when treated with an organic base, such as / fZ-butylamine, in DMF to give sucrose 6-acetate in >90% yield (74). When the kinetic reagent 2,2-dimethoxyethene was used,... 

The migration of acyl groups was another field of interest. Emil Fischer had suggested that a compound having the dioxolane ring ( orthoester ), as in (1), was the intermediate in the migration, and Hibbert placed the theory on a firm footing. 

Pozsgay, V, A simple method for avoiding alkylthio group migration during the synthesis of thioglycoside 2,3-orthoesters. An improved synthesis of partially acylated 1-thio-a-L-rhamnopyrano-sides, Carbohydr. Res., 235, 295-302, 1992. 

Phosphate esters readily undergo intramolecular migration from one hydroxyl group to another under the influence of acid 171), (See Fig. 1, 172,) This migration probably proceeds through a cyclic compound similar to the orthoester structure proposed in acyl group migration (p. 147 and 433) ... 

---