title: Absence of reverse anomeric effect in furanosides.
abstract: A series of conformationally restricted N-"furanosides" has been synthesized, where the carbons of the tetrahydrofuran ring are kept in one plane by a rigid norbornane skeleton, permitting only the ring oxygen to move above or below the tetrahydrofuran ring plane. This causes the substituents of the anomeric carbon to occupy a pseudoaxial or a pseudoequatorial position. On protonation of these "norbornane-furanosides" with trifluoromethanesulfonic acid, all three compounds exhibited decreasing coupling constants for the anomeric proton, indicating a shift toward the pseudoaxial conformation. The coupling constant measurements were supported by volume integration of NOESY cross-peaks, which also showed a change toward the pseudoaxial conformation upon protonation of the nitrogen. These results provide no evidence for the so-called reverse anomeric effect; on the contrary they are in full agreement with a small normal anomeric effect.
authors.affiliation: Organic Chemistry, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden.
authors.forename: Hans
authors.initials: H
authors.lastname: Grundberg
authors.forename: Johan
authors.initials: J
authors.lastname: Eriksson-Bajtner
authors.forename: Karl-Erik
authors.initials: KE
authors.lastname: Bergquist
authors.forename: Anders
authors.initials: A
authors.lastname: Sundin
authors.forename: Ulf
authors.initials: U
authors.lastname: Ellervik