N-Heterocyclic Carbene-Catalyzed Double Michael Addition: Stereoselective Synthesis of Spirofluorenes and Multisubstituted Indanes

The strong Bronsted basic character of N-heterocyclic carbenes (NHCs) has been used to promote the cascade double Michael addition between fluorenes and dienones. Under catalyst loadings of 1-5mol% of NHC, fluorene reacts with divinyl ketones (DVKs) to afford anti-spirofluorene compounds in high yields. However, when benzenedi(enones) were employed as Michael acceptors in the presence of 10mol% of NHC, fluorene undergoes a different inter- and intramolecular cascade double Michael addition, providing multi-substituted indanes in high yields with excellent diastereoselectivity.