Asymmetric diels-alder reactions of α,β-unsaturated aldehydes catalyzed by a diarylprolinol silyl ether salt in the presence of water

(Chemical Equation Presented) Water, water, everywhere: The title reaction predominantly affords the exo isomer with excellent enantioselectivity (see scheme; TMS=trimethylsilyl). The synthesis can be carried out without organic solvents and provides the products by distillation. Water increases the rate and enantioselectivity of the reaction.