| The green alga Caulerpa racemosa belonging to the genus of Caulerpa (Chlorophyta, Siphonales, Caulerpaceae), an established rich source of secondary metabolites with novel structures and special bioactivities, are found throughout the world, mostly in tropical and subtropical waters. C. racemosa is a widely occurring marine plant in China, particularly along the coast of Dongshan (Fujian), Taiwan, Hainan, and Guangdong. During the course of our ongoing search for the biologically active substances from Chinese marine organisms, we carried out a chemical and bioactive investigations on the seaweed C. racemosa collected from the Zhanjiang coastline, Guangdong Province, China, in June 2010, since no phytochemical investigation has been done previously on this Chinese species.This research was financially supported by the National Natural Science Foundation of China (No. 20862013).From C. racemosa,54 compounds, including 12 diterpenoids,5 vitamin E derivatives,7 alkaloids,15 steroids, prenylated aromatics, and 13 fatty acids, were isolated by repeated chromatography on normal-and reversed-phase silica gel columns, Sephadex LH-20, and final purification by RP-HPLC. Their structures and relative stereochemistry were elucidated on the basis of extensive spectroscopic analysis, such as 1D-and 2D-NMR, EI-MS, ESI-MS, UV, IR, and optical rotation, and chemical methods, and by comparison of the data with those reported data in the literature. Among these 54 compounds,12 new compounds, including four compounds possessing unprecedented carbon skeletons, were identified, and 3 metabolites,1-acetate-7,11,15-trimethyl-3-methylene-1,2-hexadecanediol (10), 7-oxabicyclo [4.1.0] hept-3-ene-2,5-dione (15), (8E)-heptadecen-7-one (51), were isolated from a natural source for the first time. Additionally,19 known metabolites (13,16,25-27,32,34,36-45,49,50) firstly found in the genus of Caulerpa and 8 known compounds (1,2,5,6,9,11,14,24) from C. racemosa for the first time, were described.These 12 new metabolites from the alga are classified into five types:3 diterpenes (racemosols A-C (3,4,12),4 alkaloids (racemosins A-C (18-20) and caulerchlorin (23)), one steroid ((23E)-β-sitost-23-ene-28-one (35)), one polyacetylenic fatty acid ((8E,12Z,15Z)-10-hydroxy-8,12,15-octadecatrien-4,6-diynoic acid (52)), one vitamin E derivatives (racemosol D (17)), and 2 prenylated aromatics (caulerterpenoids A (53) and B (54)). Racemosins A-C (18-20), three alkaloids with unprecedented carbon skeleton, possess novel bisindole unit and rearranged/degraded eight-membered cyclic ring of caulerpin (21). Interestingly, caulerterpenoids A (53) and B (54) represent the first examples of prenylated dimethyl benzenes from a natural source, of which 54 is a molecule with a new carbon skeleton. More interestingly, compound 52, a fatty acid with a conjugated polyacetylene group, was isolated from marine algae for the first time. Fatty acids with a conjugated polyacetylene group are commonly typical properties of terrestrial plants and marine sponges. In addition, the co-ocurrence of five structural types of metabolites revealed the presence of a variety of extremely complicated biosynthetic pathways in C. racemosa. In the bioscreening experiments in vitro, only monomethyl caulerpinate (22) and caulerchlorin (23) had a moderate/weak antifungal activity against Cryptococcus neoformans (32609) with MIC80 values of 16 and 64μg mL-1, respectively. A further study to acquire more information concerning pharmacogical activities of these compounds is in progress. |
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