Heterodiene syntheses—XIII: The stereochemical consequences of electronic and steric interactions in the transition state of the cycloaddition between 4-arylidene-5-pyrazolones and vinylethers

Abstract

The competition between exo and endo transition states in the cycloaddition of 4-arylidene-5-pyrazolones and vinylethers is rationalized in terms of steric and electronic interactions. The steric interactions depend mainly upon the requirements of the substituent in position 3 of the starting pyrazolone. The electronic factors are rationalized in terms of secondary non-bonding interactions between the HOMO of the vinylether which acts as donor and the LUMO of the pyrazolone which acts as acceptor. An E configuration of the pyrazolone is suggested as “reacting” species.