Thermodynamic stabilities and conformational analyses of 4,6-O-acetalized 1,5-anhydro-5-thio-dl-threo-2-enitols

4,6-O-Methylidene and 4,6-O-neopentylidene derivatives of 1,5-anhydro-2,3-dideoxy-5-thio-dl-thero-hex-2-enitol having the C-inside form were found to be thermodynamically more stable than the corresponding O-inside conformers. Thermodynamic stabilities, as well as the conformation of sulfoxide and sulfone derivatives of the 4,6-O-neopentylidene compound were examined by experiment and ab initio MO and DFT calculations. These thermodynamic stabilities, and the most stable conformations determined by NMR data, were corroborated by calculations.