Glycosylation isopropenyl glycosides

Building on the observations of Kahne and Takahashi, Chenault and Castro relied on the selective activation of an isopropenyl glycoside donor with TMSOTf, followed by subsequent activation of a second n-pentenyl glycoside with NIS/TESOTf.12 This one-pot glycosylation offers proof of the concept that Fraser-Reid s glycosides can be incorporated into one-pot syntheses. (See Scheme 11.7.)... 

A. Marra, J. Esnault, A. Veyrieres, and P. Sinay, Isopropenyl glycosides and congeners as novel classes of glycosyl donors Theme and variations, 7. Am. Chem. Soc. 774 6354 (1992). 

Chenault, H K, Castro, A, Glycosyl transfer by isopropenyl glycosides trisaccharide synthesis in one pot by selective coupling of isopropenyl and n-pentenyl glycopyranosides. Tetrahedron Lett., 35, 9145-9148, 1994. 

On the basis of the reaction of NPGs with electrophiles, Chenault et al. considered the possible activation of isopropenyl glycosides with electrophiles. The mechanism of activation was expected to involve initial capture of the electrophile ( ) by the vinyl ether double bond of 310 leading to the formation of cation 311 or 312 (O Scheme 50). Collapse of 311 or 312 to form glycosyl oxocarbenium ion 313 and acetone derivative 314 would be followed by nucleophilic attack on 313 to generate glycoside 315. An alternative reaction would involve direct nucleophilic attack on 311 or 312 to generate the addition product 316. 

Isopropenyl glycosides could be activated selectively in the presence of armed NPGs, and that allowed a one-pot synthesis of trisaccharide 322 involving the successive glycosyl coupling of a vinyl glycoside jS-307, and an NPG, 321 (O Scheme 52). 

This reversed approach was successfully applied to the glycosylation of 2-azido derivative 63, for which the isopropenyl glycoside itself is not available as the Tebbe reagent is not compatible with the azido group (Scheme 13b). 

---