THE ACID-CATALYZED REARRANGEMENT OF METHYL 3,4-O-BENZYLIDENE-β-D-RIBOPYRANOSIDE

Methyl 3,4-O-(R)-benzylidene-β-D-ribopyranoside (2) rapidly rearranged, under anhydrous acidic conditions, to give methyl 2,3-O-(R)-benzylidene-β-D-ribopyranoside (6). Further rearrangement of 2 and 6 gave the (S)-isomers 4 and 8, an equilibrium mixture of the four pyranoside acetals 2, 4, 6, and 8 resulting. At higher acid concentrations, the rearrangement proceeded, with ring contraction, to give the diastereomeric forms of methyl 2,3-O-benzylidene-β-D-ribofuranoside (11 and 13) as the sole product. Treatment of methyl 3,4-O-(S′)-benzylidene-β-D-ribopyranoside (4) with acid resulted in the immediate formation of the equilibrium mixture of pyranoside acetals. On increasing the acid concentration this mixture again underwent ring contraction to give the diastereomers 11 and 13 as the final product. The rearrangement of 2 and 4 was monitored by following the change in signals in the benzyl proton region of the nmr spectrum.