TADDOL-TiCl2 catalyzed Diels-Alder reactions: Unexpected influence of the substituents in the 2-position of the dioxolane ring on the stereoselectivity

Abstract

Several α,α,α′,α′-tetra(3,5-dimethylphenyl)-1,3-dioxolane-4,5-dimethanols have been synthesized, and their TiCl2 derivatives used as catalysts in the reaction of cyclopentadiene with (E)-2-butenoyl-1,3-oxazolidin-2-one. The substituents in the 2-position of the dioxolane ring exert a decisive influence on the extent and direction of the asymmetric induction and, consequently any possible enantiomer of the cycloadducts can be preferentially obtained by changing these substituents. Molecular mechanics calculations of the structures and relative energies of the possible dienophile-catalyst complexes offer a tentative explanation for the results described.