Rhodium-Catalyzed Intermolecular [4+2] Annulation of Cyclic Carbonyl Compounds

Significance

This work presents a simple method for constructing the unusual spirocyclic benzopyranones and isatin derivatives with excellent yields and enantioselectivities. The authors apparently experienced some difficulties during their preliminary explorations on the cross-[4+2] annulation of 2-alkynylbenzaldehyde with benzaldehyde or keto-ester substrates. Nevertheless, they identified the suitable phosphine ligand [(R,R)-10] for these reactions giving up to 80% ee, and learned that electron-deficient carbonyl substrates can be used in less excess to 2-alkynylbenzaldehyde to provide cross-annulation products in higher yields. Finally, when changing to cyclic dicarbonyl substrates, satisfactory results were obtained.