Plocoralides A–C, polyhalogenated monoterpenes from the marine alga Plocamium corallorhiza
Graphical abstract
Organic extracts of Plocamium corallorhiza contain three new polyhalogenated monoterpenes, plocoralides A–C (1–3). Compounds 1–3 show moderate activity towards esophageal cancer cells.
Introduction
Red algae (Rhodophyta) of the genera Plocamium, Chondrococcus and Ochtodes produce a wealth of biologically active linear and cyclic polyhalogenated monoterpenes (Faulkner, 2002, Blunt et al., 2003). These metabolites exhibit a range of biological activities including antifeedant effects on reef herbivores (Paul et al., 1980, Paul et al., 1988, Sakata et al., 1991), antimicrobial (König et al., 1999), insecticidal (San-Martin et al., 1991, Argandona et al., 2002), antitubercular (König et al., 2000) and anticancer (Fuller et al., 1992, Fuller et al., 1994) activities. As part of a programme to investigate the chemistry of South African marine algae we have studied the non-polar constituents from Plocamium corallorhiza (Turner) Hooker & Harvey (Plocamiaceae, Plocamiales). P. corallorhiza is common and abundant in South Africa, often dominating large areas of rock in the subtidal fringe and shallow subtidal due to its masses of spreading coralloid attachment structures (hence the species name, meaning ‘coralloid roots’; Stegenga et al., 1997). This seaweed has also been reported from Namibia, Mozambique, St. Paul and Amsterdam Islands in the southern Indian Ocean, and Madagascar (Guiry and Nic Dhonncha, 2003). In South Africa, P. corallorhiza is harvested as feed for cultivated abalone (Anderson et al., 2003).
In the present work, we report on the isolation, structure elucidation and biological activity of three new ocimene-type polyhalogenated metabolites, plocoralides A–C (1–3) as well as three known compounds (4–6) from an organic extract of P. corallorhiza. The structures of compounds 1–3 are based on a regular isoprenoid carbon skeleton and are characterized by a rare 1,1-dichloro-2-ene moiety.
Section snippets
Results and discussion
Specimens of P. corallorhiza were collected from the intertidal zone at Kalk Bay near Cape Town, South Africa and immediately frozen. The partially thawed sample was extracted sequentially with MeOH and CH2Cl2. Fractionation of the CH2Cl2 extract by silica gel column chromatography (hexane–EtOAc) gave several fractions which were further purified by semi-preparative normal phase HPLC to yield compounds 1–6.
A molecular formula of C10H14Br2Cl2 was deduced from HREIMS and 13C NMR data for
General
Optical rotations were measured on a Perkin–Elmer 141 polarimeter. IR spectra were obtained as films on KBr disks using a Perkin–Elmer Spectrum 2000 FT-IR spectrometer. The 1H (400 MHz), 13C (100 MHz), DEPT-135, HMQC, HMBC, COSY-90 NMR spectra were all recorded on a Bruker Avance 400 NMR spectrometer using standard pulse sequences. Spectra were referenced to residual protonated solvent resonances (CHCl3 δH 7.25, δC 77.0). Compounds were purified using a Spectra-Physics IsoChrom LC HPLC system,
Acknowledgments
The authors thank Mr. Aubrey Sonemann (Chemistry Department, Rhodes University) for recording low-resolution mass spectra and the South African Department of Environmental Affairs and Tourism for a marine research permit. This research was supported by grants from Rhodes University and the South African National Research Foundation (REDIBA Program).
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