On the origin of changes in topicity observed in Diels-Alder reactions catalyzed by Ti-TADDOLates

New C-2 symmetric TADDOLs containing different groups at the 2-position of the dioxolane ring have been prepared. The Ti catalysts derived from these have been studied in the Diels-Alder reaction of cyclopentadiene and (E)-2-butenoyl-1,3-oxazolidin-2-one. Substituents at the C-2 position of the dioxolane ring can play an important role in determining the selectivity as well as the nature of the major isomer. This effect is more important for TADDOLs containing bulky aromatic groups such as 3,5-dimethylphenyl- or 1-naphthyl at the alpha -positions. Experimental evidence supports the hypothesis that pi-pi. interactions between aromatic groups at the C-2 and the ones at the a-positions are critical in this respect. (C) 2001 Elsevier Science Ltd. All rights reserved.