On the regioselectivity of the Hanessian-Hullar reaction in 4,6-O-benzylidene protected galactopyranosides.

The N-bromosuccinimide mediated fragmentation of methyl 4,6-O-benzylidene-beta-D-galactopyranoside results in the formation of methyl 4-O-benzoyl-6-bromo-6-deoxy-beta-D-galactopyranoside and methyl 4-O-benzoyl-3-bromo-3-deoxy-beta-D-gulopyranoside, as opposed to the methyl 6-O-benzoyl-3-bromo-3-deoxy-beta-D-gulopyranoside originally reported. The kinetic methyl 4-O-benzoyl-6-bromo-6-deoxy-beta-D-galactopyranoside rearranges to the thermodynamic methyl 4-O-benzoyl-3-bromo-3-deoxy-beta-D-gulopyranoside under the reaction conditions, likely via a 3,6-anhydro galactopyranoside. The NBS-mediated cleavage of 4,6-O-benzylidene acetals in the galactopyranoside series is therefore shown to conform to the regiochemistry observed in the corresponding gluco- and mannopyranoside series with preferential cleavage of the C6-O6 bond by an ionic mechanism.