C-nucleosides. Part 18. Stereoselective annulation of 6-acetoxy-6-(2,3,5-tri-O-benzoyl-β-D-ribofuranosyl)pyran-3(2H,6H)-one with ethylene glycol to pyrano-[2,3-b]-1,4-dioxine glycosides

Abstract

The synthesis of (4aS,8aS)- and (4aR,8aR)-4a-(2,3,5-tri-O-benzoyl-β-D-ribofuranosyl)-2,3,8,8a-tetrahydropyrano[2,3-b]-1,4-dioxin-7(4aH,6H)-one 9a and 9b is described. Treatment of the 6-acetoxypyranulose glycoside 2 with ethylene glycol in the presence of toluene-p-sulfonic acid in acetone for 1 h afforded the (6S)- and (6R)-6-(2-hydroxyethoxy)pyranulose glycosides 3a and 3b in 14 and 40% yield, respectively. The stereochemistry of compounds 3a and 3b was assigned as 6S and 6R by comparison of their CD spectra. When compounds 3a and 3b were treated with toluene-p-sulfonic acid at room temperature for 5 h, they reacted to give bicyclic compounds 9a and 9b in 65% yield, respectively. The stereochemistry of the bicyclic compounds 9a and 9b was established by nuclear Overhauser enhancement experiments.