Determination of configuration at C-13 of (–)-ent-8β-hydroxy-labdan-15-oic acid and its biotransformation by Battus polydamas larvae

Battus polydamas (Linnaeus, 1758) is a specialist phytophagous insect and its larva feed on Aristolochiaceae species, of which were isolated, among other compounds, nitro derivatives, lignoids, and diterpenoids [1]. Species belonging to the Aristolochia genus contain insecticidal or deterrent compounds to the majority of insects, supporting the hypothesis that such compounds represent barriers to colonization by other non-adapted lepidopteran larvae [2]. Biotransformation studies by B. polydamas and other Aristolochiaceae-feeder have focused mainly on aristolochic acids, which have a 3,4-methylenedioxy-10-nitrophenanthrenic-1-acid carbon skeleton. In this study, B. polydamas larvae were fed at laboratory with leaves of Aristolochia giberti, and methanolic extracts of the leaves and feces were prepared by maceration. The chemical profiles of these extracts were compared by HPLC-PDA, NMR, LC-MS, and TLC. From both extracts a new labdane diterpene, (–)-ent-8β-hydroxy-labdan-15-oic acid (1), was isolated. The configuration at C-13 of 1 was established as R by the PGME method [3], whereas the absolute configuration of trans-decalin system was proposed on basis NMR spectroscopic data and in the negative signal of the optical activity observed. In addition, two new labdane diterpenes, (–)-ent-8β,18α-dihydroxy-labdan-15-oic acid (2) and (–)-ent-8β-hydroxy-labdan-15,18-dioic acid (3), possibly sequential oxidation products of 1, were isolated from feces extract and their structures were determined by spectroscopic analyses. The biotransformation of 1 into 2 and 3 and the excretion of these compounds in the feces suggest that 1 is detoxified by B. polydamas. The larvae also stored compound 3, but its ecological role is still unclear.

References:

[1] Lopes, L. M. X. et al. (2000) Res. Adv. in Phytochem. 2: 19 – 108.

[2] Nascimento, I. R. et al. (2003) Pest Manag. Sci. 60: 413 – 416.

[3] Yabuuchi, T., Kusumi, T. (2000)J. Org. Chem. 60: 397 – 404.