title: Stereoselectivity of Conformationally Restricted Glucosazide Donors.
abstract: Glycosylations of 4,6-tethered glucosazide donors with a panel of model acceptors revealed the effect of acceptor nucleophilicity on the stereoselectivity of these donors. The differences in reactivity among the donors were evaluated in competitive glycosylation reactions, and their relative reactivities were found to be reflected in the stereoselectivity in glycosylations with a set of fluorinated alcohols as well as carbohydrate acceptors. We found that the 2-azido-2-deoxy moiety is more β-directing than its C-2-O-benzyl counterpart, as a consequence of increased destabilization of anomeric charge development by the electron-withdrawing azide. Additional disarming groups further decreased the α-selectivity of the studied donors, whereas substitution of the 4,6-benzylidene acetal with a 4,6-di-tert-butyl silylidene led to a slight increase in α-selectivity. The C-2-dinitropyridone group was also explored as an alternative for the nonparticipating azide group, but this protecting group significantly increased β-selectivity. All studied donors exhibited the same acceptor-dependent selectivity trend, and good α-selectivity could be obtained with the weakest acceptors and most reactive donors.
authors.affiliation: Leiden Institute of Chemistry, Leiden University , Einsteinweg 55, 2333 CC Leiden, The Netherlands.
authors.forename: Stefan
authors.initials: S
authors.lastname: van der Vorm
authors.affiliation: Leiden Institute of Chemistry, Leiden University , Einsteinweg 55, 2333 CC Leiden, The Netherlands.
authors.forename: Herman S
authors.initials: HS
authors.lastname: Overkleeft
authors.affiliation: Leiden Institute of Chemistry, Leiden University , Einsteinweg 55, 2333 CC Leiden, The Netherlands.
authors.forename: Gijsbert A
authors.initials: GA
authors.lastname: van der Marel
authors.affiliation: Leiden Institute of Chemistry, Leiden University , Einsteinweg 55, 2333 CC Leiden, The Netherlands.
authors.forename: Jeroen D C
authors.identifier: 0000-0003-3531-2138
authors.initials: JDC
authors.lastname: Codée