Exploiting the Strained Ion-Pair Interactions of Sterically Hindered Pyridinium Salts Toward SN2 Glycosylation of Glycosyl Trichloroacetimidates

We demonstrate here that strained and sterically hindered protonated 2,4,6-tri-tert-butylpyridinium (TTBPy) tetrafluoroborate, a crystalline, bench stable salt serves as a mild and efficient organocatalyst for the S(N)2 type displacement of glycosyl trichloroacetimidates toward the stereoselective synthesis of both alpha- and beta-glycosides. The strained ion-pair interactions between the sterically hindered pyridinium cation and the tetrafluoroborate anion infuse unusual reactivity to the ions resulting in the unique anion assisted activation of alcohol. This mild activation of alcohol facilitates the S(N)2 type displacement of glycosyl alpha-trichloroacetimidates into beta-glycosides in a highly diastereoselective manner. These unique interactions were established based on extensive infrared and H-1, F-19, B-11 NMR studies and theoretical studies. [GRAPHICS] .