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 C-2-symmetric 3,3'-dialkoxy-2,2'-bipyrrolidines have been designed and synthesized for asymmetric organocatalytic Diels-Alder reactions of alpha,beta-unsaturated aldehydes. The remarkable rate-accelerating effect for the cycloaddition reaction has been observed in aqueous medium. The catalyst 1c center dot 2HClO(4) 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. (C) 2010 Elsevier Ltd. All rights reserved.