The mechanism of an intramolecular Michael addition: a MNDO study

The 5-endo-trig cyclization reactions of α′-hydroxy α,β-unsaturated carbonyl compounds to form the 3-furanone ring system has been investigated by means of MNDO calculations. In acid medium, the activation energy for ring formation is reduced by protonation of the carbonyl oxygen and by electron-releasing β-substituents, both of which facilitate torsion about the α,β-CC bond. Rotation about the α,β-CC bond is essentially complete when the transition structure is achieved. In reverse ring opening, heterolytic rupture of the C—O bond is aided by prior enolization since induced polarization of the enol π system acts to stabilize the developing cationic centre at the β-carbon. In basic medium, ring closure is hindered by the combined effects of the intrinsic barrier to cyclization and solvation of the intermediate alkoxide. Ring opening in basic medium is prevented by prohibitively high intrinsic barriers except when a strongly electron-releasing group such as p-O(−)-phenyl is at the β-carbon.