New alkaloids from Formosan zoanthid Zoanthus kuroshio
Graphical abstract
Introduction
Marine invertebrates are proved to be rich sources of bioactive natural products, which are functionalized as chemical defense substances against other reef creatures. Diverse secondary metabolites are secreted directly by themselves or accumulated through the food chain. Zoanthid, a sessile benthos, which is easily found in the subtidal zone of subtropical or tropical area, is one kind of zooxanthellate-encrusting anemone with two rows of tentacles and appealing colors. In previous natural product studies, Zoanthus sp. (Cnidaria: Hexacorallia) was reported to have fatty hydrocarbons,1 steroids,2 ecdysones3 and alkaloids.4, 5, 6, 7, 8, 9, 10, 11 The unique alkaloids of Zoanthus sp. have aroused great interests of chemists and pharmacologists, because these type of alkaloids have been reported to demonstrate multiple biological activities such as human platelet aggregation inhibition,12 cytotoxic,13 anti-osteoporotic,14 anti-bacterial15 and anti-inflammatory activities.5 In addition, several groups have synthesized the complex functionalized ring system of zoanthamine alkaloids by Robinson annulation,16 Diels–Alder17 and Friedel–Crafts18 reactions. Zoanthus kuroshio, the most dominant species in western coast of Taiwan, which has a fluorescent-pink oral disc and brown tentacles, was collected and investigated for their secondary metabolites. Previously, two new types of alkaloids named kuroshines A (9) and B (10) were identified by our research group.10 In this paper, further isolation, structural elucidation, and in vitro cytotoxic data of isolated alkaloids (1–10) (Fig. 1) from the ethanolic extract of Formosan zoanthid Z. kuroshio are reported.
Section snippets
Results and discussion
The samples of Zoanthus kuroshio were collected in Kaohsiung seashore. The animal materials were lyophilized and then extracted with ethanol. After removing solvent, the residue was partitioned between H2O and EtOAc to yield an organic extract. Repeated HPLC chromatography of the EtOAc extract resulted in the isolation of seven new alkaloids (1–7), together with one known compounds, zoanthenamine (8). The structures of 1−8 were determined primarily through mass and spectroscopic analyses,
Conclusions
Kuroshines A, C‒G are new type of zoanthamine-type alkaloids with an octacyclic ring system. They can be easily identified by their unusual ether bridge between C-10 and C-28. Different oxidation statuses had resulted in the isolation of the new compounds 1–7. In summary, oxidation occurs at C-1, C-3, C-7, and C-11. It is note that the hydroxyl group attaching at C-1 or C-7 was first found in zoanthamine-type alkaloid. Our current study revealed that zoanthid has great potential to produce
General experimental procedures
Silica gel P60 (Silicycle) was used for column chromatography. The instrumentation for HPLC was composed of a Shimadzu LC-10AT pump and a Shimadzu SPD-20A UV–vis detector. UV spectra were obtained using a Jasco UV-530 ultraviolet spectrophotometer. IR spectra were obtained on a Perkin Elmer system 2000 FTIR spectrophotometer. Optical rotations were measured with a Jasco P-1020 digital polarimeter. NMR spectra were obtained by JEOL JNM ECS 400 MHz and Varian 600 MHz NMR. ESI-MS data were
Acknowledgements
We thank Dr. Wei-Chun Liu for her kind assistance. This work was supported by grants from Ministry of Science and Technology of Taiwan (MOST103-2628-B-037-001-MY3 award to Y.-B. C., 103-2320-B-037-005-MY2, awarded to F.-R.C. and NSC 103-2911-I-002-303, MOST 104-2911-I-002-302, MOST 103-2325-B-039-008, MOST 103-2325-B-039-007-CC1102-2320-B-037-012-MY2, awarded to Y.-C.W.), National Health Research Institutes (NHRI-EX103-10241BI), and in part from the grant from Chinese Medicine Research Center,
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