A regioselective spirocyclization of a 2-biphenyl acetylene was achieved by virtue of a spatial constraint in a self-assembled molecular cage. The unusual, folded conformation in the cavity alters the selectivity of the electrophilic cyclization from 6-endo-dig to 5-endo-dig to form a hindered spiro compound. NBS= N-bromosuccinimide; NIS= N-iodosuccinimide.