The use of 2-O-(2-nitrobenzyl) and 2-O-(2-cyanobenzyl) groups controls stereoselective formation of 1,2-trans-glycosidic linkages via the arming participation effect. The observed stereoselectivity likely arises from the intramolecular formation of cyclic intermediate between the electron-rich substituent and the donor oxacarbenium ion providing the expected facial selectivity for attack of the glycoside acceptor. The stereodirecting effect of the 2-nitro- and 2-cyanobenzyl groups attached at the remote position (C-3, C-4, and C-6) of the donor molecule have also been investigated. To prove the postulated mechanism based on the participation effect of 2-substituted benzyl groups in the glycosylation stereoselectivity we used DFT theoretical calculation methodology.

The utility of intramolecular glycosylation for the synthesis of the 27-membered macrocyclic ring is highlighted in this first total synthesis of the most complex resin glycoside isolated to date -Calysolin IX. Oligosaccharide-containing macrolides core was effectively constructed by TfOH/NIS-promoted intramolecular glycosylation of thioglycosyl donor. As the glycosidic bond must be created en route to target structure, we [a] Mirosław Nawój,