Sesquiterpenes from Laurencia similis

One new sesquiterpene, (4E)-1-bromo-5-[(1'S*,3'R*)-3'-bromo-2',2'-dimethyl-6'-methylenecyclohexyl]-3-methylpent-4-ene-2,3-diol (1), and fifteen known sesquiterpenes, isopalisol (2), luzonensol (3), palisadin B (4), aplysistatin (5), palisadin A (6), 4-hydroxyl-palisudin C (7), 5-acetoxypalisadin B (8), 10-hydroxyaristolan-9-one (9), aristol-8-en-1-one (10), aristolan-9-en-1-one (11), aristolan-1(10)-en-9-one (12), aristolan-1(10)-en-9-ol (13), aristolan-1(10),8-diene (14), aristolan-1,9-diene (15) and aristofone (16), were isolated from a sample of marine red alga Laurencia similis. Their structures were established by detailed NMR spectroscopic analysis and comparison with literature data. Compounds 2-9, and 16 were isolated for the first time from this species. All these metabolites were submitted for a cytotoxicity assay against the tumor cell line BEL7402 (human liver adenocarcinoma), but all of them were found inactive (IC50 > 10 μg/mL).


Introduction
Red algae of the genus Laurencia have proved to be a rich source of secondary metabolites, mainly sesquiterpenes, C 15 -acetogenins, and a few di-and triterpenes. More than 300 have been characterized from some 40 species collected in various parts of the world since the 1960s [1]. Many of these OPEN ACCESS metabolites have been reported to possess a variety of biological activities, such as antimicrobial [2], antifeedant [3], anthelmintic [4,5], and cytotoxic activities [6,7]. Within the context of our program to systematically assess the chemical and biological diversity of marine algae distributed along the Chinese coast, further investigations of the chemical constituents of the red alga L. similis, collected from Sanya Bay, Hainan Province, led to the isolation and identification of one new sesquiterpene, which is described in this paper, together with 15 known sesquiterpenes.

Results and Discussion
The dried and powdered alga L. similis was extracted with 95% EtOH at room temperature. The concentrated extracts were partitioned between H 2 O and EtOAc. The EtOAc-soluble fraction was purified by a combination of silica gel and Sephadex LH-20 column chromatography, as well as preparative TLC procedures, to yield compounds 1-16.
Compound 1 was isolated as colorless oil, and showed IR absorptions for hydroxy functional group (3410 cm -1 5 Hz, H-4); δ C : 128.5 (C-5), 136.6 (C-4)], and no any other multiple bonds signals. Thus, it required 1 to be monocyclic. The NMR data (Table  1) demonstrated the presence of three methyls, three methylenes, three methines, and two quaternary carbons in addition to the four olefinic carbons. Comparison of the NMR data of 1 with the Compound 2 suggested that it had the same cyclohexane ring portion. This portion could be connected by HMBC data, including from H 3 -8' and H 3 -9' to C-1', C-2' and C-3', from H 2 -7' to C-1' and C-5'. The third spin system was also established by the HMBC data, including between H 3 -6 and C-2, C-3 and C-4, between H-4 and C-3 and C-1'. Furthermore, the connectivity of C-1' and C-5 was also established by the HMBC data from H-1' to C-4, C-5, C-2', C-6', C-7' and C-8'. Thus, the planar structure of 1 was clearly established.
Compounds 1-16 were evaluated for the cytotoxicity against a human liver adenocarcinoma (BEL7402) cell line. However, all of them were found to be inactive (IC 50 >10 μg/mL).

Alga material
The

Cytotoxicity Assay
Cytotoxic assay toward the human liver adenocarcinoma (BEL7402) cell line was carried out as previously reported [18].