Neighboring-Group Participation by C-2 Ether Functions in Glycosylations Directed by Nitrile Solvents

Ether-protecting functions at C-2 hydroxy groups have been found to play participating roles in glycosylations when the reactions are conducted in ni- trile solvent mixtures. The participation mechanism is based on intramolecular in- teraction between the lone electron pair of the oxygen atom of the C-2 ether func- tion and the nitrile molecule when they are positioned in a cis configuration. A 1,2-cis glycosyl oxazolinium intermediate is formed. This participation, in conjunc- tion with the anomeric effect of the glycosyl donor, confers high 1,2-trans selectivi- ties on glycosylations. Further application of this concept has led to efficient prep- arations of a-(1!5)-arabinan oligomers.