Neighboring Group Participation in Glycosylation Reactions by 2,6-Disubstituted 2-O-Benzoyl groups: A Mechanistic Investigation

Probing the Influence of a 4,6-O-Acetal on the Reactivity of Galactopyranosyl Donors: Verification of the Disarming Influence of thetrans–gaucheConformation of C5–C6 Bonds

“…76–77 Such subtle but important changes are likely to be induced by changes in reaction conditions, including the Lewis acid and its concentration, and the nature of the O3 protecting group such as established by the Huang and Williams groups in the glucose series. 78–79 We also consider it possible that the quantity and nature of the Lewis acid involved may influence the course of the reaction by complexation to the carbonyl oxygen of the participating group at position 2, for example, leading to silyl imidate formation and thereby limiting participation; such processes have been shown to play a major role in the chemistry of N -acetylglucosamine by Auzanneau and coworkers 80–81 following early observation by Pougny and Sinaÿ. 82 …”

Mechanisms of Stereodirecting Participation and Ester Migration from Near and Far in Glycosylation and Related Reactions

“…102,109,110 Several groups have studied the influence of steric and electronic effects in the participating ester on conversion of the intermediate dioxalenium ion to the glycoside with a view to suppressing orthoester formation and other deleterious side reactions including erosion of stereoselectivity, hydrolysis, and acyl transfer. 106,[110][111][112][113][114][115][116][117][118][119] In this regard, a recent report by Sun and coworkers according to which the 2-O-[2,2dimethyl-2-o -nitrophenyl)acetyl group provides complete stereoselectivity in formation of glycosides, without competing orthoester formation, is noteworthy. 120 On the basis of the proximity of the nitro group to the carbonyl carbon in X-ray crystal structures, the experimental requirement for an o-, as opposed to a p-nitro group, and of quantum mechanical calculations, it was suggested that the nitro group intercepts and stabilizes the bridging dioxacarbenium ion (Scheme 2).…”

Statistical Analysis of Glycosylation Reactions

“…Due to the NGP, [13] the synthesis of 1,2-trans-glycoside can be achieved. However,w ithout these directing groups,t he development of glycosylation,e specially for 1,2-cis-glycoside, has been hampered by unpredictable stereoselectivity because the combination of donor and acceptor undergoes am echanism on the spectrum between the S N 1a nd S N 2p athways, and thus, leadingt oamixtureo fa-a nd b-glycosides.…”

“…[12] Glycosyld onor 1,w ith a2 -acyl group, showst he tendency to stabilizeg lycosyl triflate intermediate 2 from the a face, which is rationalized by the identificationo fa cyloxonium ions 3 as the reactive intermediates through low-temperature NMR spectroscopy. [13] Therefore, compound 3 only allows acceptor 4 to attack from the b face to produce only 1,2-trans-glycosides 5 (Scheme 1A). However,t he preparation of 1,2-cis-glycoside is still ac hallenge because stereoselective glycosylation is highly sensitivet ot he reaction substrates (glycosyl donor 6 and acceptor 7)a nd its reaction conditions (Scheme 1B).…”