Stereocontrolled Coupling between Aldehydes and Conjugated Alkenals Mediated by TiIII/H2O

In the presence of water, titanocene(III) complexes promote a stereoselective C-C bond-forming reaction that provides gamma-lactols by radical coupling between aldehydes and conjugated alkenals. The method is useful for both intermolecular reactions and cyclizations. The relative stereochemistry of the products can be predicted with confidence with the aid of model Ti-coordinated intermediates. The procedure can be carried out enantioselectively using chiral titanocene catalysts. [reaction: see text].