Macrocylic bisbibenzyl natural products and their chemical synthesis

The macrocyclic bisbibenzyl family of natural products are commonly found in liverworts and other bryophytes, though the recent isolation of riccardin C from a primrose extract has demonstrated their existence in higher flowering plants. Each has a core comprising four aromatic rings and two ethano bridges, being derived in Nature from two molecules of lunularin. Sub-classes are distinguished by the connectivity between these lunularin units, while individual natural products are distinguished by the hydroxy- and/or alkoxy-substituents decorating the core structures. Further diversification results from halogenation and oxidation, which may lead to dimerization or the creation of additional rings. The review provides a timely update to a previous Natural Product Reports article by Keseru and Nogradi (Nat. Prod. Rep. 1995, 12, 69-75) and largely focuses on new additions to the family and the strategies used to effect their chemical synthesis.