Benzoxonium ions rearrangement

The rearrangement of a benzoxonium intermediate can be convincingly demonstrated in the reaction of 3,5-0-benzylidene-1,2-0-isopropylidene-a-D-glucofuranose (120) with N-bromosuccini-mide, whereby the 6-bromo derivative 124 and the 3,6-anhydride 123 are obtained. It may be presumed that the benzoxonium ion 121 rearranges into the ion 122 through intramolecular attack by... 

The formation of 34 was explained by a rearrangement of 33 by way of a benzoxonium intermediate the ion is attacked at the less-hindered, C-5 position, to give a 5-deoxy-5-iodo derivative. The structure of 34 was confirmed by its reduction to 4-0-benzoyl-l,5-dideoxy-2,3-0-iso-propylidene-L-arabinitol. 

If a participating group, such as an ester or hydroxyl group, is present near to the acetal ring, a rearrangement of the benzoxonium intermediate can occur the rearranged ion can then be attacked intra-molecularly or externally by a nucleophile.145... 

---