Catalytic asymmetric synthesis of trans-configured beta-lactones: cooperation of Lewis acid and ion pair catalysis.

The development of the first trans-selective catalytic asymmetric [2+2] cyclocondensation of acyl halides with aliphatic aldehydes furnishing 3,4-disubstituted beta-lactones is described. This work made use of a new strategy within the context of asymmetric dual activation catalysis: it combines the concepts of Lewis acid and organic aprotic ion pair catalysis in a single catalyst system. The methodology could also be applied to aromatic aldehydes and offers broad applicability (29 examples). The utility was further demonstrated by nucleophilic ring-opening reactions that provide highly enantiomerically enriched anti-aldol products.