Synthesis of methyl 4,6-O-benzylidene-2,3-dideoxy-5-thio-β-dl-threo-hex-2-enopyranoside via hetero-Diels–Alder reaction and unusual stabilities of 1,5-anhydro-4,6-O-benzylidene 2,3-dideoxy-5-thio-dl-threo-hex-2-enitol

The title compound was prepared via hetero-Diels–Alder reaction of 1-acetoxy-1,3-butadiene and thioaldehyde, followed by Pummerer rearrangement. Different from the corresponding sugar and 5a-carba sugar, C-inside isomer of 1,5-anhydro-4,6-O-benzylidene-2,3-dideoxy-5-thio-dl-thero-hex-2-enitol was found to be thermodynamically more stable than the corresponding O-inside one and these thermodynamic stabilities were corroborated by ab initio calculations.