Synthesis of Polycyclic Oxanorbornanes via a Sequential Epoxyhexopyranoside Ring Contraction-Intramolecular Diels-Alder Reaction.

Ring contraction of methyl 2,3-anhydro-6-deoxy-6-(furfurylthio)-alpha-D-allopyranoside with LiBr/TMU in boiling toluene gave the corresponding alpha,beta-unsaturated furanosidic aldehyde 1 together with a small amount of the corresponding oxanorbornene Diels-Alder adduct 2. Pumping a mixture of heptane and Et(3)N slowly through a SiO(2) column containing 1 and 2 shifted the ratio strongly toward 2. The products were eluted from the column by EtOAc/EtOH and chromatographed to give pure 2 and 1 in 79% and 15% yield, respectively. Compound 2 was submitted to a number of oxidations and reductions, which gave the oxanorbornanes 4-15. The structures were determined by 2D (1)H and (13)C NMR techniques and by a single-crystal X-ray investigation of compound 5.