Synthesis of Chloro-Substituted 6H-Dibenzo[b,d]pyran-6-one Natural Products, Graphislactone G, and Palmariols A and B.

A palladium-mediated intramolecular aryl-aryl coupling reaction was applied to the total synthesis of the bioactive natural products, graphislactone G (1), and palmariols A (2) and B (3), which possess an unusual chloro-subsutituent on the 6H-dibenzo[b,d]pyran-6-one skeleton. Based on the transition state model of the coupling reaction, the mechanistic aspect for the regioselectivity of the aryl-aryl coupling reaction is also discussed.