A theoretical study of the stereoselectivities of the Diels-Alder addition of cyclopentadiene to ethyl-(S)-lactyl acrylate catalyzed by aluminium chloride

We have studied the AlCl3-catalyzed cycloaddition of ethyl(S)-lactyl acrylate (ELA) and cyclopentadiene (CPD) by MP2 and DFT (B3LYP) methods using the 6-31G(d,p) basis set, with or without solvent effects. Four types of dienophile-Lewis acid (LA) complexes were considered - two types of ELA(.)AlCl(3), ELA(.)2AlCl(3) and ELA-AlCl2+. The stereochemical predictions of these models were compared with available experimental data. The participation of lactate-bound ELA(.)AlCl(3) complexes is insignificant because of their relatively high TS energies. The diastereofacial selectivities exhibited by the chelated ELA(.)AlCl(2)(+) complex do not agree with experimental data, suggesting that this type of complex is not very probable, even with ligands having an appropriate carbonyl orientation. Our results show the dominating role of ELA-2AlCl(3) in the catalyzed reaction, in agreement with experimental data. The observed diastereofacial stereoselectivities are due to a delicate equilibrium of electrostatic and steric interactions in the different transition states between the two LA groups and between these two groups and the cyclopentadiene moiety. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006.