SEMIEMPIRICAL 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 r ate-determining step appears to be the addition of a second nucleophil e to the enol of the ring-opened monolactone. The five-membered lacton es are found to be more reactive than their six-membered analogues. El ectron donor substituents (p-Me, p-MeO) increase and acceptor substitu ents (p-Cl, m-NO2) decrease the activation energy.