Ferrier reactions follow a mechanistic pathway whereby
Lewis acid
activation of a cyclic enol ether facilitates departure of an allylic
leaving group to form a glycosyl Ferrier cation. Attack on the Ferrier
cation provides a new acetal linkage concurrent with the transposition
of the alkene moiety. The idiosyncratic outcomes of Ferrier reactions
of seven-membered ring carbohydrate-based oxepines prompted an investigation
of its corresponding septanosyl Ferrier cation. Experiments that characterized
the ion, including gas-phase cryogenic IR spectroscopy matched with
density functional theory-calculated spectra of candidate cation structures,
as well as product analysis from solution-phase Ferrier reactions,
are reported here. Results from both approaches revealed an inclination
of the seven-membered ring cation to contract to five-membered ring
structures. Gas-phase IR spectra matched best to calculated spectra
of structures in which five-membered dioxolenium formation opened
the oxepine ring. In the solution phase, an attack on the ion by water
led to an acyclic enal that cyclized to aC-methylene-aldehydo
arabinofuranoside species. Attack by allyl trimethylsilane, on the
other hand, was diastereoselective and yielded aC-allyl septanoside.