title: Theoretical study on the effects of a 4,6-O-diacetal protecting group on the stability of ion pairs from D-mannopyranosyl and D-glucopyranosyl triflates.
abstract: Ion pair formation from 2,3-di-O-methyl-4,6-O-formylidene-α-D-mannopyranosyl triflate αTMan and its D-glucopyranosyl counterpart αTGlc was investigated at the DFT(M06-2X) level of theory, for the purpose of clarifying the effects of the 4,6-tethering on the structure and stability of α- and β-contact ion pairs and solvent-separated ion pairs at -78 °C. In both mannopyranosyl and glucopyranosyl triflates, the 4,6-O-formylidene group destabilized (4)H3-type conformers of the α-contact ion pairs, rendering B2,5-types the exclusive conformers for this species. The B2,5-like α-contact ion pair from αTMan was 3.5 kcal/mol more stable than that from αTGlc, probably due to the stabilizing effect by the planar O-5-C-1-C-2-O-2 dihedral angle of the former. This difference in stability of the α-contact ion pair between the mannopyranosyl and glucopyranosyl series gives insights into the mechanisms underlying the reported experimental observation that highly β-selective mannosylation can be achieved with 4,6-O-diacetal mannopyranosyl donors.
authors.affiliation: Department of Chemistry, University of Natural Resources and Life Sciences, Muthgasse 18, A-1190 Vienna, Austria. Electronic address: takashi.hosoya@boku.ac.at.
authors.forename: Takashi
authors.initials: T
authors.lastname: Hosoya
authors.affiliation: Department of Chemistry, University of Natural Resources and Life Sciences, Muthgasse 18, A-1190 Vienna, Austria.
authors.forename: Paul
authors.initials: P
authors.lastname: Kosma
authors.affiliation: Department of Chemistry, University of Natural Resources and Life Sciences, Muthgasse 18, A-1190 Vienna, Austria.
authors.forename: Thomas
authors.initials: T
authors.lastname: Rosenau
mesh_terms: D005960:Glucosides / Q000737:chemistry; D006027:Glycosides / Q000737:chemistry*; D008358:Mannose / Q000737:chemistry; D008698:Mesylates / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D012997:Solvents; D013816:Thermodynamics
chemical_list: D005960:Glucosides; D006027:Glycosides; D008698:Mesylates; D012997:Solvents; D008358:Mannose
keywords: 4,6-O-Diacetal; Glycoside; Ion pairs; Mechanism; Oxacarbenium ions; Synthesis