Bs. Orlek et al., USE OF STERIC BUTTRESSES TO ENHANCE INTRAMOLECULAR CYCLOADDITIONS, Journal of the Chemical Society, Chemical Communications, (18), 1993, pp. 1412-1413
Examples are given illustrating the use of ortho-substituents on subst
ituted benzene rings as steric buttresses to limit the conformational
space available to a dipole and dipolarophile, forcing them into close
r proximity and hence to allow cycloadditions to proceed that otherwis
e do not.