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 diox olane ring can play an important role in determining the selectivity as wel l as the nature of the major isomer. This effect is more important for TADD OLs containing bulky aromatic groups such as 3,5-dimethylphenyl- or 1-napht hyl 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 Lt d. All rights reserved.