Highly active asymmetric Diels-Alder reactions catalyzed by C₂-symmetric bipyrrolidines: catalyst recycling in water medium and insight into the catalytic mode

A new class of C₂-symmetric 3,3′-dialkoxy-2,2′-bipyrrolidines have been designed and synthesized for asymmetric organocatalytic Diels-Alder reactions of α,β-unsaturated aldehydes. The remarkable rate-accelerating effect for the cycloaddition reaction has been observed in aqueous medium. The catalyst 1c·2HClO₄ can be recovered and reused several times by simple extraction without significant loss of catalytic activity and stereoselectivity. The catalytic mode has been demonstrated by DFT calculation, NMR, and X-ray crystallographic studies for diiminium intermediate.