A series of conformationally restricted
N-"furanosides" has been synthesized, where the carbons of thetetrahydrofuran ring are kept in one plane by a rigid norbornane skeleton, permitting only the ring oxygento move above or below the tetrahydrofuran ring plane. This causes the substituents of the anomericcarbon to occupy a pseudoaxial or a pseudoequatorial position. On protonation of these "norbornane-furanosides" with trifluoromethanesulfonic acid, all three compounds exhibited decreasing couplingconstants for the anomeric proton, indicating a shift toward the pseudoaxial conformation. The couplingconstant measurements were supported by volume integration of NOESY cross-peaks, which also showeda change toward the pseudoaxial conformation upon protonation of the nitrogen. These results provideno evidence for the so-called reverse anomeric effect; on the contrary they are in full agreement with asmall normal anomeric effect.
