glycosylation · 1,2-cis-β-glycosylation · mannosylation · stereoselectivity · glycosyl trichloroacetimidate · glycosyl cation · SN2–SN1 borderline · 2,6-lactone
The advantages of glycosyl donors bearing a 2,6-lactone moiety in 1,2-cis-β-glycosylation reactions are discussed in the context of recent comprehension on the SN2–SN1 borderline. The 2,6-lactone structure increases the likelihood of the SN2-like reaction, analogous to 4,6-tethered structures or 2-O-electron-deficient substituents, which are known to mound the energetic barrier to SN1 reactions. Furthermore, the glycosyl cation generated from the 2,6-lactone donor seems to direct β-glycosides similar to the torsional and flipped cations generated from 4,6-tethered donors and mannuronate or 3,6-lactone donors, respectively. Overall, 2,6-lactones are suitable for use in 1,2-cis-β-glycosylations, and this novel class of donors is expected to help deepen our global understanding of glycosylation reactions.