Electrophilic Spirocyclization of a 2-Biphenylacetylene via Conformational Fixing within a Hollow-Cage Host.

A 2-biphenylacetylene was fixed into a specific conformation within the confined cavity of a hollow cage, where it underwent a regioselective spirocyclization in the presence of an electrophile. A 5-endo-dig cyclization proceeded selectively in the cage, which stands in sharp contrast to the 6-endo-dig cyclization that normally occurs in common organic media. The folded conformation adopted by the substrate within the cage was examined by 1 H NMR spectroscopy and X-ray crystallographic analysis.