The 3,4-O-carbonate protecting group as a beta-directing group in rhamnopyranosylation in both homogeneous and heterogeneous glycosylations as compared to the chameleon-like 2,3-O-carbonates.
It is demonstrated that the beta-selectivity observed in the insoluble silver salt mediated couplings of 2,3-O-carbonate-protected rhamnosyl bromides is uniquely due to the heterogeneous nature of the reaction. In homogeneous solution these same donors are alpha-selective, a fact that is attributed to the half-chair conformation of these substances which reduces the energy barrier to oxacarbenium ion formation. It is suggested that the 2,3-O-carbonate group be dubbed torsionally arming in the manno- and rhamnopyranose series. When the carbonate group is removed to the 3,4-O-position a beta-selective system is formed, in both homogeneous and heterogeneous conditions, and it is demonstrated that this selectivity arises from the combination of the electron-withdrawing nature of the carbonate and its inability to take part in neighboring participation.
Crich
David
D
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7061, USA. dcrich@uic.edu
Vinod
A U
AU
Picione
John
J
D000475:Alkenes / Q000737:chemistry; D001966:Bromine / Q000737:chemistry; D002254:Carbonates / Q000737:chemistry*; D002777:Cholestanols / Q000138:chemical synthesis*; D006031:Glycosylation; D007536:Isomerism; D008362:Mannosides / Q000737:chemistry; D008698:Mesylates / Q000737:chemistry; D008956:Models, Chemical; D010087:Oxides / Q000737:chemistry; D012210:Rhamnose / Q000737:chemistry*; D018030:Silver Compounds / Q000737:chemistry
D000475:Alkenes; D002254:Carbonates; D002777:Cholestanols; D008362:Mannosides; D008698:Mesylates; D010087:Oxides; D018030:Silver Compounds; C040225:disilver oxide; C013658:propylene; C012077:trifluoromethanesulfonic acid; D012210:Rhamnose; D001966:Bromine