Strukturelle Abwandlungen an partiell silylierten Kohlenhydraten mittels Triphenylphosphan/Azodicarbons�urediethylester, 4. Mitt.: Transformationen an Mannose und Galaktose
Studies of the selective silylation of methyl α- and β-d-aldohexopyranosides: stability of the partially protected derivatives in polar solvents
摘要:
Treatment of methyl .alpha.- (1) and .beta.-D-glucopyranosides, methyl .alpha.- (3) and .beta.-D-galactopyranosides, and methyl .alpha.-D-mannopyranoside (5) with 2, 3, or 4 mol. equiv. of tert-butyldimethylsilyl (TBDMS) chloride under two conditions afforded mixtures of TBDMS ethers which were identified. The following compounds were isolated in synthetically useful yields, the 2,6-di-TBDMS ether of 1 (70%), the 2,6-di- and 2,3,6-tri-TBDMS ethers of 3 (84% and 57%, respectively), and the 2,6-di- and 3,6-di-TBDMS ethers of 5 (50% and 80%, respectively). In dipolar solvents, no migration of the TBDMS groups was detected between partially silylated hydroxyl groups, but the addition of a base (triethylamine or imidazole) caused migration to vicinal cis positions.
Regioselective Silylation of Sugars through Palladium Nanoparticle-Catalyzed Silane Alcoholysis
作者:Mee-Kyung Chung、Galina Orlova、John D. Goddard、Marcel Schlaf、Robert Harris、Terrance J. Beveridge、Gisele White、F. Ross Hallett
DOI:10.1021/ja026723v
日期:2002.9.1
silylation of levoglucosan and 1,3,5-O-methylidene-myo-inositol. In an attempt to rationalize the observed regioselectivities, ab initio predictions (HF/3-21G) have been made on the relative energies of some of the silylated products. They suggest that the observed regioselectivities do not reflect a kinetic vs thermodynamic product distribution but are induced by the silylation agent employed. Models
作者:Alphert E. Christina、Daan van der Es、Jasper Dinkelaar、Hermen S. Overkleeft、Gijsbert A. van der Marel、Jeroen D. C. Codée
DOI:10.1039/c2cc17623h
日期:——
Upon condensation of 6-thio-6-deoxy-mannosyl donors 1,2-cis products are obtained with a high degree of stereoselectivity. Subsequent reductive removal of the 6-thio functionality gives 1,2-cisrhamnosides. The 1,2-cis-selectivity can be rationalized with a product forming 3H4-oxocarbenium, which is in equilibrium with a bridged sulfonium intermediate.
either the selective TBDMS protection of secondary alcohols or the fast per-O-trimethylsilylation of saccharide polyols. In the second part of the paper the scope of the silylation approach was significantly extended with the development of unprecedented "one-pot" and "solvent-free" sequences allowing the regioselective silylation/alkylation (or the reverse sequence) of saccharide polyols in short times
as a glycosyl acceptor for further elongation. The preparation of biologically important linear and branched oligomannoses incorporated into HIVgp120 demonstrates that iteration of this one-potsequence leads to very straightforwardoligosaccharideassembly. As an additional result, a rapid approach has been disclosed for accessing a 3,6-OH mannose building-block to be incorporated in branched structures
Regioselective silylation of pyranosides using a boronic acid/Lewis base co-catalyst system
作者:Doris Lee、Mark S. Taylor
DOI:10.1039/c3ob40981c
日期:——
The combination of a boronic acid and a Lewis base, both employed in substoichiometric amounts, enables the regioselective silylation of cis-diol groups in alkylpyranoside substrates. The proposed mode of activation involves the formation of a tetracoordinate adduct that displays enhanced nucleophilicity at the boron-bound alkoxide groups.