Caucanolides A-F, Unusual Antiplasmodial Constituents from a Colombian Collection of the Gorgonian Coral Pseudopterogorgia bipinnata Claudia A. Ospina, † Abimael D. Rodrı ´guez,* ,† Juan A. Sa ´ nchez, ‡ Eduardo Ortega-Barria, § Todd L. Capson, ⊥ and Alejandro M. S. Mayer | Department of Chemistry, University of Puerto Rico, P. O. Box 23346, U.P.R. Station, San Juan, Puerto Rico 00931-3346, Departamento de Ciencias Biolo ´ gicas, Universidad de los Andes, P. O. Box 4976, Bogota ´ , Colombia, Instituto de Investigaciones Cientı ´ficas Avanzadas y Servicios de Alta Tecnologı ´a, Centro de Estudios Biome ´ dicos, Clayton, Building 175, P. O. Box 7250, Panama 5, Republic of Panama, Smithsonian Tropical Research Institute, P. O. Box 2072, Balboa, Anco ´ n, Panama, and Department of Pharmacology, Chicago College of Osteopathic Medicine, Midwestern University, 555 31st Street, Downers Grove, Illinois 60515 Received July 5, 2005 Six new diterpenoids, caucanolides A-F(1-6), have been isolated from extracts of the gorgonian octocoral Pseudopterogorgia bipinnata collected near the Colombian Southwestern Caribbean Sea. The structures of 1-6 were elucidated by comprehensive analysis of spectroscopic data. The caucanolides showed in vitro antiplasmodial activity against the malaria parasite, Plasmodium falciparum. In addition to possessing structures based on novel carbon skeletons, one of these metabolites, caucanolide B (2), constitutes the only example from nature of a secondary metabolite possessing the N 1 ,N 1 -dimethyl-N 2 - acylformamidine functionality. The chemical diversity of gorgonian secondary metabo- lites reported to date has shown that these marine organ- isms represent an excellent resource for the discovery of novel pharmacologically active agents. 1 In particular, gor- gonian octocorals of the genus Pseudopterogorgia are a rich source of unusual biologically active diterpenoids, sesquit- erpenes, and polyhydroxylated steroids with diverse struc- tures. 2 In a continuation of our search for biologically active compounds from the genus Pseudopterogorgia, we have examined the gorgonian coral Pseudopterogorgia bipinnata (Gorgoniidae) collected from Providencia (Old Providence) Island located in the Southwestern Caribbean Sea during March 2002. 3 A sample of the organic extract of this animal was included in an initial screening carried out as part of an effort in the discovery of new antimalarial agents through a collaborative agreement with the Panama In- ternational Cooperative Biodiversity Group (ICBG) pro- gram. 4 In the present study, this extract was found to be active in inhibiting the growth of Plasmodium falciparum and, thus, merited further chemical investigation. Results and Discussion The 1:1 CHCl 3 /MeOH extract of the gorgonian coral P. bipinnata was subjected to our standard solvent partition- ing scheme, 3b and the hexane and CHCl 3 extracts were purified by a combination of gel filtration chromatography on Bio-Beads SX-3 (toluene), silica gel chromatography eluting with hexane/EtOAc mixtures, and HPLC to afford known compounds kallolide A acetate, 5 kallolide C, 5 bipin- napterolide A, 3d gersemolide, 6 pinnatin B, 3c bipinnatolide F 3d (for their structures see the Supporting Information), and the new bilactone diterpenoids caucanolides A-F(1- 6). 7 The structures were elucidated on the basis of spec- troscopic data including HRMS measurements and long- range 1 H- 13 C correlations. The relative stereochemistries were determined by a combination of analysis of NOESY data together with 1 H- 1 H coupling constants of 1-6. However, since the new metabolites are “open-chain” conformationally flexible systems, and no rigorous molec- ular mechanics/dynamics calculations were performed to establish the dominant conformations of 1-6, the stereo- chemical assignments should be considered tentative, rather than definitive. Notwithstanding, in support of our proposed configurations, helpful stereochemical analogies were drawn to known relatives of the pseudopterane and cembrane classes of diterpenes that were co-isolated during this investigation. Caucanolide A (1) (yield 0.11% on dry gorgonian weight basis) was obtained as a viscous optically active oil, [R] D -42.0° (c 0.7, CHCl 3 ). While no pseudomolecular ion peak was detected in the HRFABMS, an intense [M + H - H 2 ] + fragment ion at m/z 373.1650 appropriate for a molecular formula of C 21 H 26 O 6 (calcd 373.1651 for C 21 H 25 O 6 ) was detected instead, requiring nine sites of unsaturation. Its IR spectrum showed prominent absorption bands at 3083, 1764, 1754, 1661, and 1626 cm -1 , indicative of olefin and R,-unsaturated carbonyl functionalities, and the UV spec- trum showed absorptions at λ max (ǫ) 204 (26 200) and 246 (15 500) nm, suggesting the presence of R,-butenolide moieties in compound 1. The 1 H NMR spectrum of compound 1 indicated the presence of two trisubstituted olefins [δ 7.20 (br d, 1H) and 6.77 (br q, 1H)]; an isopropenyl group [δ 5.05 (br s, 1H), 4.90 (br s, 1H), and 1.76 (s, 3H)]; a methoxyl group [δ 3.21 (s, 3H)]; and a vinyl methyl group [δ 1.99 (d, 3H)]. Furthermore, its 1 H NMR spectrum revealed the presence of an isolated pair of mutually coupled methylene groups [δ 2.47 (br t, 2H) and 2.25 (br t, 2H)]; two adjacent sp 3 methines [δ 5.25 (dd, 1H) and 2.78 (d, 1H)]; and an R-substituted-,-dimethyl-R,-unsaturated aldehyde [δ 10.10 (s, 1H), 2.20 (s, 3H), and 2.02 (s, 3H)]. The 13 C NMR and DEPT NMR spectra indicated the presence of eight quaternary carbons, five methyls, three methylenes, and five methine carbons (Table 1). Further, the 13 C NMR spectrum of compound 1 indicated the * To whom correspondence should be addressed. Tel: +787-764-0000, ext. 4799. Fax: +787-751-0625. E-mail: arodrig@cnnet.upr.edu. † University of Puerto Rico. ‡ Universidad de los Andes. § Instituto de Investigaciones Cientı ´ficas. ⊥ Smithsonian Tropical Research Institute. | Midwestern University. 1519 J. Nat. Prod. 2005, 68, 1519-1526 10.1021/np050239z CCC: $30.25 © 2005 American Chemical Society and American Society of Pharmacognosy Published on Web 09/27/2005