Origin of oxygen atoms in cantharidin biosynthesized by beetles

Abstract

Biosynthesis by blister beetles (Co1eoptera:Meloidae) of the defensive substance cantharidin (2), an apparent monoterpene, proceeds by unprecedented degradation of farnesol (l), a sesquiterpenoid precursor. To obtain chemical insight into this transformation, we examined the origin of the four oxygen atoms in 2. Labeling studies used adult male Epicauta pestifera that were exposed to either I80-enriched O2 or H20. Analysis of the mass spectrometric data of the resulting 2 indicated that the tetrahydrofuranyl oxygen atom and two, but not three, of the anhydride oxygen atoms are derived from 02, whereas the third anhydride oxygen atom comes from H20. Examination of maximally labeled 2 using mass spectrometry-mass spectrometry, which obviated complications owing to isotope dilution, revealed that the H20-derived oxygen atom is located in the anhydride ring in some molecules and in a carbonyl group in others, implicating intramolecular oxygen scrambling. Results indicate that 0,-derived incorporated oxygen atoms undergo no appreciable exchange with the medium. The possibility that 2 is a juvenile hormone metabolite is suggested. Insects not only are the largest and most diverse group of organisms on earth but also are evolutionarily distant from vertebrates, plants, and bacteria. For these reasons, the study of insect metabolism promises to reveal novel biological chemistry. For example, use by some insects of homomevalonate to synthesize their juvenile hormone,I a homosesquiterpenoid, is unique among all types of organisms studied. An understanding of the unique metabolic features of insects may reveal insights regarding biological chemistry and could provide the foundation for new approaches for the control of pest insects. Cantharidin (2), an apparent monoterpene that serves as a defensive substance in blister beetles,2 is formed by an unprecedented degradation of the CIS farnesyl skeleton. Following early experiment^,^ an extensive series of I4C radiolabeling studies4 by Schmid and his collaborators demonstrated that the carbon atoms in 2 are derived from farnesol (l), which itself is derived from (1) (a) Schooley, D. A,; Judy, K. J.; Bergot, B. J.; Hall, M. S.; Siddall, J. B. Proc. Natl. Acad. Sci. U.S.A. 1973, 70, 2921-2925. (b) Bergot, B. J.; Jamieson, G. C.; Ratcliff, M. A.; Schooley, D. A. Science (Washington, D. C. ) 1980, 210, 336-338. (2) Carrel, J. E.; Eisner, T . Science (Wushington, D . C.) 1974, 183, 155-751. (3) (a) Schlatter, C.; Waldner, E. E.; Schmid, H. Experientia 1968, 24, 994-995. (b) Guenther, H.; Ramstad, E.; Floss, H. G. J . Pharm. Sci. 1969, 58, 1274. (4) (a) Peter, M. G.; Waespe, H.-R.; Woggon, W.-D.; Schmid, H. Helu. Chim. Acto 1977,60, 1262-1272. (b) Woggon, W.-D.; Peter, M. G.; Schmid, H. Helu. Chim. Acta 1977.60, 2288-2294. (c) Peter, M. G.; Woggon, W.-D.; Schmid, H. Helv. Chim. Acta 1977, 60, 2756-2762. (d) Woggon, W.-D.; Hauffe, S. A,; Schmid, H. J. Chem. Soc., Chem. Commun. 1983, 212-274. mevalonate in the normal manner. Intriguingly, the two terminal methyl groups of 1 are almost completely randomized during its conversion into Z3** Tritium labeling experiments demonstrated that all of the hydrogen atoms in 2, with the exception of one attached to C-6, are derived from m e ~ a l o n a t e . ~ ~ . ~ Unfortunately, knowledge of the prigins of the carbon and hydrogen atoms in 2 fails to unveil chemical details regarding its biosynthesis beyond