EMPIRICAL FORCE-FIELD MODELS FOR THE TRANSITION-STATES OF INTRAMOLECULAR DIELS-ALDER REACTIONS BASED UPON ABINITIO TRANSITION STRUCTURES

A quantitative model based upon Allinger's MM2 force field has been devised to calculate the diastereoselectivity of intramolecular Diels-Alder (IDA) reactions. The parameters for the modified MM2 force field were derived whenever possible from ab initio calculations on the intermolecular transition structures for the Diels-Alder reactions of butadiene plus ethylene, acrolein, and acrolein coordinated to BH3. The force field reproduces the ab initio 3-21G transition structures for the intramolecular Diels-Alder reactions of 1,3,8-nonatriene and 1,3,9-decatriene. The force field was developed for both thermal and acid-catalyzed reactions and provides insight into the origins of the diastereoselectivity in the IDA cycloaddition for a wide variety of nonatrienes and decatrienes. The flexibility of the transition structure and the conformational effects due to the chain connecting the two reacting moieties were shown tote of the greatest importance in determining the stereochemical outcome of these reactions. The use of the parameters in the new MM3 force field was tested.