δ-galactonolactone:: Synthesis, isolation, and comparative structure and stability analysis of an elusive sugar derivative

delta-D-Gluconolactone, delta-D-mannonolactone, and - for the first time - the thermodynamically unstable delta-D-galactonolactone have been prepared and isolated from DMF solution by oxidizing the corresponding sugars with Shvo's catalyst [(C4Ph4CO)(CO)(2)Ru](2) and a hydrogen acceptor. The preferred conformation of delta-D-galactonolactone in [D-6]DMSO solution has been determined by H-1 NMR spectroscopy experiments and DFT calculations to be H-4(3) and is compared to those of the previously established conformations of delta-D-gluconolactone (H-4(3)) and delta-D-mannonolactone (B-2,B-5). The conformations of the lactones suggest an explanation for their relative rates of isomerization to their respective 7-D-lactones by an intramolecular mechanism. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.