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 dipolarophiles was studied, wherein factors related to regioselectivity were investigated. The results from munchnones with electronically divergent thioaryl substituents compared with others bearing alkyl substituents suggest that an unsymmetrical transition state structure, rather than FMO perturbation, plays a significant role in regioselection. If eclipsing interactions preclude a highly unsymmetrical transition state, however, then minimizing steric interactions becomes important. A pair of complementarily substituted munchnones, differing only in the position of isotopic labels, establishes an inherently symmetrical electronic nature of the mesionic heterocycle.