Application of the Moore rearrangement to the synthesis of 1,4-dioxygenated xanthones and efforts toward the total synthesis of lundurine B
A novel application of the Moore rearrangement was successfully developed and applied to the synthesis of 1,4-dioxygenated xanthones that would have been difficult to obtain otherwise. The 1,4-dioxygenated xanthone moiety is found in several naturally occurring, biologically active compounds. Several methods by which to obtain the 1,4-dioxygenated xanthone core have been reported; however, high step counts, low yields, and harsh reaction conditions preclude the use of these methods to complex xanthone natural products. Using the Moore rearrangement as a key step in the synthetic sequence has allowed us to prepare several xanthone natural products quickly and more efficiently than what is possible with the prior art.
Using the Martin group’s prior experience with the application of ring closing metathesis (RCM) to the field of alkaloid natural product synthesis, the preparation of lundurine B was undertaken. Key features of the proposed synthesis to lundurine B include the formation of a cyclopropane ring by the formation pyrazoline intermediate via [3+2] dipolar cycloaddition followed by dinitrogen extrusion. A second key step in the proposed sequence to lundurine B is a double RCM to form a five- and eight-membered ring in a single operation. While double RCM strategies have been applied to several elegant natural product syntheses, the formation of a five- and eight-membered ring in a single sequence has not been reported. Should the double RCM strategy prove successful for lundurine B, the conditions could in principle be applied to other structurally related natural products.