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 with LiBr/TMU in boiling toluene gave the corresponding alpha,beta-u nsaturated furanosidic aldehyde 1 together with a small amount of the corresponding oxanorbornene Diels-Alder adduct 2. Pumping a mixture of heptane and Et3N slowly through a SiO2 column containing 1 and 2 shif ted the ratio strongly toward 2. The products were eluted from the col umn 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 oxida tions and reductions, which gave the oxanorbornanes 4-15. The structur es were determined by 2D H-1 and C-13 NMR techniques and by a single-c rystal X-ray investigation of compound 5.