A surprising C-4 epimerization of 5-deoxy-5-sulfonylated pentofuranosides under Ramberg–Bäcklund reaction conditions

Treatment of methyl 5-deoxy-2,3-O-isopropylidene-5-(benzylsulfonyl)-β-d-ribofuranoside with CBr2F2–KOH/Al2O3 afforded the corresponding olefinic sugar. The methyl- and the isopropyl-analogues in contrast underwent epimerization at C-4 to generate the α-l-lyxo derivatives.