ABSTRACT
Dihalogenomethyl methyl ether/Lewis acid reagents are useful for generation of glycosyl halides from various anomeric derivatives of mono and oligosaccharides. When 1,2-trans-di-O-acyl derivatives of sugars are treated with dihalogenomethyl methyl ethers in the presence of BF3·Et2O, the thermodynamically less stable glycosyl halides can be prepared in high yield. The use of other Lewis acids normally yields the thermodynamically more stable glycosyl halides. The reagents are compatible with many blocking groups commonly used in carbohydrate chemistry, including some acid-labile groups. Prolonged treatment of sugars protected with acetal and/or benzyl ether groups may result in their replacement with selectively removable formyl groups, which may be advantageous in designing blocking strategies. A comparison is given of the reactivity of various derivatives from which glycosyl halides can be generated, and the side reactions observed during these conversions are referred to. This is expected to be helpful in a rational choice of reaction conditions for the use of dihalogenomethyl methyl ethers to generate complex glycosyl donors for oligosaccharide synthesis.
