Cyclization of 1,4-dihydroxyanthraquinone with α,β-unsaturated aldehyde: a new strategy for the synthesis of cyclopentanoids

A cascade cyclization strategy has been developed for the synthesis of cyclopentane-fused anthraquinones through the condensation of 1,4-dihydroxyanthraquinone with α,β-unsaturated aldehydes in the presence of common inorganic base NaOH. These fused-tetracyclic anthraquinone products are formed in a single step from readily accessible starting materials under very mild conditions without the use of any expensive or complex reagents. This method represents an unprecedented example of a base-promoted protocol to access five-membered carbocyclic rings in a single step. Moreover, this chemistry also provides useful guidance for the preparation of 6,5-fused ring systems.