Different Lanthanide Ions and the Pybox Substituents Induce the Reverse of the Sense of Induction in the Enantioselective Diels−Alder Reaction between Acryloyloxazolidinone and Cyclopentadiene

The Diels−Alder reaction of cyclopentadiene with acryloyl-1,3-oxazolidin-2-one is catalyzed by (R,R)-i-Pr- and (R,R)-Ph-pybox (5 and 6)/lanthanide(III) triflate complexes. The enantioselectivity of the reaction is influenced by 4 Å molecular sieves; moreover, the absolute sense of the enantioselection was found to be a function of the nature of the substituents on the chiral ligand (e.g., aromatic vs alkyl groups) as well as of the specific cation employed. In particular, seven lanthanide triflates were tested and a regular variation of the enantioselectivity as a function of the cation ionic radius was evidenced. The different sense of induction can be rationalized by assuming two competitive reacting complexes with different coordination number, which favor the attack on the opposite heterotopic faces of the coordinated dienophile.