A propos of glycosyl cations and the mechanism of chemical glycosylation

Modulating glycosylation with exogenous nucleophiles: an overview

“…(Scheme 1). 10,11 These ion-pairs or covalent adducts react with an acceptor in different pathways ranging from S N 1-like to S N 2-like mechanisms depending on a combination of factors. For example, a change of stereochemistry of a mannosyl donor can alter the mechanism of glycosylation.…”

A propos of glycosyl cations and the mechanism of chemical glycosylation; the current state of the art

“…1 We concluded that ‘glycosyl cations are likely to have a real existence in dichloromethane solution at low temperature and that their observation only awaits the development of a suitable “flash” technique for their generation in the probe of an NMR spectrometer’. In the update that follows we survey critically evidence presented in the intervening four years for the transient existence of glycosyl oxocarbenium ions in some glycosylation reactions, while stressing the important role of the counter ion in others.…”

Influence of the solvent in low temperature glycosylations with O-(2,3,5,6-tetra-O-benzyl-β-d-galactofuranosyl) trichloroacetimidate for 1,2-cis α-d-galactofuranosylation

“…However, as the activator is TMSOTf, a covalent triflate intermediate could be involved. [35][36][37][38] In this sense, low-temperature NMR studies of thioarabinofuranosyl constrained donors activation allowed the identification of a triflate arabinofuranosyl intermediate in 1,2-trans relationship. 38 Questions also arise on the selectivity obtained because benzyl protected acceptor 2 should be more nucleophilic than the benzoyl counterpart 3.…”

“…17 Acetonitrile is a participating solvent which is frequently used for 1,2-trans b-linkage construction in galacto-and glucopyranosyl donors lacking a participating group at the vicinal 2-position. [35][36][37][38]43 It is accepted that nitrile effect is due to the formation of a a-D-glycopyranosyl nitrilium ion, which is further displaced by the acceptor. Reactions in this solvent at À40°C (method C) favored the b-D-isomer in almost all cases, with the exception of lactone acceptor 6.…”