Acetonated Sorbofuranoses and Their Favored Conformations as Determined by Proton Magnetic Resonance Spectroscopy

A series of l-sorbofuranose derivatives have been studied by PMR spectroscopy in order to determine their favored conformations from the coupling constants obtained by applying the spin-decoupling techniques and the solvent effects of benzene at 100 MHz. The coupling constants J3,4, have disclosed that the furanose ring of methyl 1,3,4,6-tetra-O-acetyl-α-l-sorbofuranoside (IIa) in solutions has a conformation close to a C3-endo-C4-exohalf-chair, while that of its β-anomer (IVa) has a C3-exo-C4-endohalf-chair conformation. Their conformations are unchanged over the temperature range of 32–74°C, but they are affected by the introduction of a 4,6-O-or 2,3-O-isopropylidene group, either of which locks the furanose ring to a C3-exo-C4-endohalf-chair conformation. This effect is thought to play an important role in the exclusive formation of 2,3:4,6-di-O-isopropylidene-α-l-sorbofuranose (XII) during the acetonation of l-sorbose.