INTERMOLECULAR 1,3-DIPOLAR CYCLOADDITIONS OF MUNCHNONES WITH ACETYLENIC DIPOLAROPHILES - SORTING OUT THE REGIOSELECTIVITY

A series of 1,3-dipolar cycloadditions of munchnones with acetylenic d ipolarophiles was studied, wherein factors related to regioselectivity were investigated. The results from munchnones with electronically di vergent thioaryl substituents compared with others bearing alkyl subst ituents suggest that an unsymmetrical transition state structure, rath er than FMO perturbation, plays a significant role in regioselection. If eclipsing interactions preclude a highly unsymmetrical transition s tate, however, then minimizing steric interactions becomes important. A pair of complementarily substituted munchnones, differing only in th e position of isotopic labels, establishes an inherently symmetrical e lectronic nature of the mesionic heterocycle.