AB-INITIO TRANSITION STRUCTURES FOR HETERO-DIELS-ALDER CYCLOADDITIONSTO FURAN

Ab initio calculations at the RHF/3-21G and RHF/6-31G//RHF3-21G theor y levels provide data on the reactivity and stereoselectivity of heter o dienophile additions to furan. The calculated activation energies fo r the hetero Diels-Alder reactions are usually small, and the newly fo rmed bonds are shorter than in the corresponding all carbon Diels-Alde r reactions. The formation of the endo isomer is predicted to be energ etically more favorable due to n-pi repulsion between the heteroatom l one pair and the pi system of the furan ring. The activation energy fo r some hetero dienophiles is very low and the reaction is suggested to be of synthetic interest in the preparation of natural chiral compoun ds.