Formation and reactivity of new Nicholas-Ferrier pyranosidic cations: novel access to oxepanes via a 1,6-hydride shift/cyclization sequence.

Pyranosidic allylic (Ferrier) cations that share dicobalt hexacarbonyl propargyl (Nicholas) stabilization at C-1 display a remarkable reactivity leading to either substituted oxepanes or 3-C-branched pyranosides, depending on the substituent at O-6.