Halide trapping of the Nazarov intermediate in strained polycyclic systems: a new interrupted Nazarov reaction

1,4-Dien-3-ones encased within bridged bicyclic frameworks undergo efficient Nazarov electrocyclization upon treatment with TiCl4, but the resulting cyclopentenyl cations are trapped by chloride in preference to deprotonation. In contrast to the usual eliminative pathway, which destroys one of the stereocenters formed during electrocyclization, this process preserves both new centers and generates an additional one at the site of chloride trapping. Examples involving skeletal rearrangements and other Lewis acids are also discussed.