Synthesis of C-mannopyranosylphloroacetophenone derivatives and their anomerization

The reaction of 2,3,4-tri-O-benzyl-α-l-rhamnopyranosyl fluoride (6-deoxy-2,3,4-tri-O-benzyl-α-l-mannopyranosyl fluoride) with 2,4-dibenzylphloroacetophenone, in the presence of boron trifluoride·diethyl etherate, afforded both the 3-C-α-l- and the 3-C-β-l-rhamnopyranosylphloroacetophenone derivatives. The 3-C-α-l-rhamnoside was produced as a major product, while the 3-C-β-l-rhamnoside was produced as a minor product via anomerization of the 3-C-α-l-rhamnoside. Alternatively, the reaction of 2,3,4,6-tetra-O-benzyl-α-d-mannopyranosyl fluoride with 2,4-di-benzylphloroacetophenone afforded both the 3-C-α-d- and the 3-C-β-d-mannnopyranosylphloroacetophenone derivatives under identical conditions. The 3-C-β-d-mannoside was produced as a major product via anomerization of the 3-C-α-d-mannoside during the reaction. These differences in composition result apparently from the magnitude of the 1,3-diaxial interactions between the C-3 and C-5 positions in these sugar moieties.