An efficient catalyst for highly enantioselective exo-Diels–Alder reaction between alkenoyl-1,3-oxazolidin-2-ones and cyclopentadiene

The typically endo-selective Diels–Alder reactions of cyclopentadiene with acryloyl- and (E)-crotonoyl-oxazolidin-2-ones have been studied to find exo-selective catalysts. Whereas bis(oxazoline)-based catalysts promote high degree of endo-selectivity (with high ee), those derived from 2,6-bis[(4′R,5′R)-diphenyl-1′,3′-oxazolin-2′-yl]pyridine (pybox) and the triflates of EuIII, LaIII and CeIV induce different stereoselectivity. Not only the exo products are obtained with EuIII in more than 50% yield, but the enantioselectivity is excellent (more than 99% ee). The absolute configuration of the previously unknown exo cycloadduct was unambiguously determined. A stereochemical model is proposed for the activated substrate–catalyst complex which suggests that the excellent efficiency of diphenyl-substituted pybox, compared to that of the corresponding 4-phenyl-substituted ligand, is due to the substituent in the 5-position, suitably placed to blind the Re-face of the coordinated reagent.