UNPRECEDENTED INFLUENCE OF AZIDES AND THE EFFECT OF BULKY GROUPS ON ZINC-INDUCED REDUCTIONS OF DEOXY HALOGENO SUGARS

Zinc/silver-graphite induced ring-opening reactions of deoxy halogeno sugars by dealkoxyhalogenation may be suppressed by bulky protecting groups in the substrates, giving rise to simple dehalogenation products. Furanose derivatives are more sensitive to such steric effects than pyranose systems, although an X-ray analysis of 1,2-0-isopropylidene-3-0-benzyl-5-deoxy-5-iodo-alpha-D-xylofuranose 3 shows no evidence of steric hindrance by the C-3 substituent. Moreover, dehalogenation instead of reductive ring-opening reactions are observed in all cases in which organic azides are present in the reaction medium. This unprecedented effect of azides on zinc-induced reductions is equally effective both intra-and intermolecularly. This behaviour is best rationalized by assuming radical intermediates for dehalogenations, in contrast to transient organometallic species closely bound to the zinc surface when reductive ring-opening is observed. This mechanistic interpretation is supported by a labelling experiment.