Semi-empirical AM1 and PM3 molecular orbital calculations on the mechanism of the hydrolysis of unsaturated lactones: substituted (E )-5,5′-diphenylbifuranylidenediones and 3,7-diphenylpyrano[4,3-c]pyran-1,5-diones

Semi-empirical molecular orbital (AM1 and PM3) calculations including solvent effects by the SCRF model on the mechanism of the addition of nucleophiles to unsaturated five- and six-membered bislactones of the Pechmann dye type indicate a similar mechanism for both systems. The rate-determining step appears to be the addition of a second nucleophile to the enol of the ring-opened monolactone. The five-membered lactones are found to be more reactive than their six-membered analogues. Electron donor substituents (p-Me, p-MeO) increase and acceptor substituents (p-Cl, m-NO2) decrease the activation energy.