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.
Crich
David
D
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607-7061, USA. dcrich@uic.edu
Yao
Qingjia
Q
Bowers
Albert A
AA
D001597:Benzylidene Compounds / Q000737:chemistry*; D001974:Bromosuccinimide / Q000737:chemistry; D005690:Galactose / Q000737:chemistry*; D019906:Nuclear Magnetic Resonance, Biomolecular; D009904:Optical Rotation
D001597:Benzylidene Compounds; D001974:Bromosuccinimide; D005690:Galactose