Halogenation, of glycals

In polar solvents, traw -addition products 23 (a-D-manno) and 26 -D-gluco) were predominantly obtkned in nearly equal amounts through chloronium ions 21 and 22. 

In polar solvents, the formation of oxycarbenium ions instead of syn ion pairs will be favored and the attack by bromide ions from below the molecular plane (kinetic anomeric effect) should be easier when bromine at C-2 is above (intermediate 2 in scheme 1) higher proportions of an a-trans addition product are therefore expected whatever the protecting groups are. 

The results obtained in electrophilic additions of bromine (and other electrophiles) to 3,4,6-tri-O-benzyl-D-glucal 29 have been recently reexamined [21]. 

Radical debromination was then performed with tributyltin hydride and catalytic amounts of azobis(isobutyronitrile) (AIBN). 

To explain the good stereoselectivity of the glycosylation reaction, the authors postulated that the gluco bromonium ion (below-plane attack) is destabilized by the reverse anomeric effect . This effect, which was put forward [23] to explain the preference of anomeric cationic substituents for the equatorial position on a pyr-anose ring, was recently shown [24] to be not consistent with theories of molecular 

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