exo-Methylene lactone group-containing compounds, such as (−)-xanthatin, are present in a large variety of biologically active natural products, including extracts of Xanthium strumarium (Cocklebur). These substances are reported to possess diverse functional
[Nicolaou; Lizos, Dimitrios E.; Kim, David W.; Schlawe, Daniel; De Noronha, Rita G.; Longbottom, Deborah A.; Rodriquez, Manuela; Bucci, Mariarosaria; Cirino, Giuseppe Journal of the American Chemical Society, 2006 , vol. 128, # 13 p. 4460 - 4470]