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. 2006 Jul 24;341(10):1748-52.
doi: 10.1016/j.carres.2006.02.024. Epub 2006 Mar 13.

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

David Crich  1 , Qingjia Yao, Albert A Bowers
Affiliations

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

David Crich et al. Carbohydr Res. .

Abstract

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.

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Figures

Scheme 1
Scheme 1
Radical and ionic pathways for benzylidene fragmentation.
Scheme 2
Scheme 2
Reaction of 8 with N-bromosuccinimide.
Scheme 3
Scheme 3
Mechanism for the formation of 3-bromo-3-deoxyguloside 11.
See this image and copyright information in PMC

References

    1. Hanessian S. Carbohydr Res. 1966;2:86–88.
    1. Failla DL, Hullar TL, Siskin SB. J Chem Soc, Chem Commun. 1966:716–717.
    1. Hanessian S, Plessas NR. J Org Chem. 1969;34:1035–1044.
    1. Hanessian S, Plessas NR. J Org Chem. 1969;34:1045–1053.
    1. Hullar TL, Siskin SB. J Org Chem. 1970;35:225–227. - PubMed

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