Dye-sensitized photooxygenation of sugar-furans as synthetic strategy for novel C-nucleosides and functionalized exo-glycals

The MB-sensitized photooxygenation of β-ribofuranosyl furan 1e followed by in situ Et2S treatment afforded the conformationally stable β-ribofuranoside 4e almost quantitatively. The latter was converted to pyridazine C-nucleoside 6e by cyclization with NH2NH2 and to pyrazoline 7e through a 1,3-dipolar cycloaddition with diazomethane. Attempts to epoxidize the double bond failed both by dimethyldioxirane (DMDO), which left 4e unchanged, and by NEt3/t-BuOOH or NaOO-t-Bu which respectively afforded the new and unexpected exo-glycals E,Z-8e and the novel furan derivative 9.