Copper(II)-bisoxazoline-catalysed asymmetric Diels-Alder reactions of α-thioacrylates

A range of C-2-symmetric, chiral Cu-II. bisoxazoline complexes were tested as catalysts for the asymmetric Diels-Alder cycloaddition between cyclopentadiene and a range of alpha-sulfenylacrylates. The optimum acrylate was ethyl alpha-phenylthioacrylate and the optimum catalyst was the bisoxazoline derived from phenylalanine which, upon complexation with Cu(SbF6)(2), gave the cycloadducts in 92% yield, 88% de and >95% ee for the endo product. The a-phenylthio ester moiety was easily converted into a carbonyl group furnishing (1S,4S)-norbornenone with high enantioselectivity. Attempts to improve the diastereoselectivity by changing the bite angle of the ligand were unsuccessful. An attempt is made to rationalise our findings in terms of the transition-state structure of the ligand-metal-dienophile complex.