A THEORETICAL-STUDY OF INTRAMOLECULAR DIELS-ALDER AND 1,3-DIPOLAR CYCLOADDITION STEREOSELECTIVITY USING ABINITIO METHODS, SEMIEMPIRICAL METHODS, AND A TANDEM QUANTUM MECHANIC MOLECULAR MECHANIC METHOD

Diels-Alder and nitrile oxide intramolecular cycloadditions were studied using several methods. The structures found using all methods are similar when the forming bonds lengths are constrained, but the stereochemical predictions are quite different. The experimental stereochemical differences found for the parent Diels-Alder reactions forming 6-5 and 6-6 systems are rationalized. When the addends are linked by three methylene groups (formation of a five-membered ring), the strain in the transition structure (TS) causes the addends to twist about the forming bonds, resulting in a skewed TS as compared to the intermolecular TS. However, when the addends are linked by four methylene groups (formation of a six-membered ring), there is little strain in the TS, and the addends do not twist.