Plocamium hamatum and its monoterpenes: chemical and biological investigations of the tropical marine red alga

The polyhalogenated monoterpene content of six samples of the tropical marine red alga Plocamium hamatum, collected from the southern, central and northern regions of The Great Barrier Reef, Australia, was assessed. In all but two of the samples, the polyhalogenated monoterpene content was shown to differ markedly. In total, eleven previously reported compounds were isolated and characterised (1-11). Compound 2 was obtained for the first time as a pure natural product. For compound 4 a single crystal X-ray crystallographic analysis was undertaken which established its absolute configuration as (1S,2S,4R,5R,1'E)-2-bromo-1- bromomethyl-1,4-dichloro-5-(2'-chloroethenyl)-5-methylcyclohexa ne. Complete and unambiguous 1H and 13C NMR data are reported for 2 and 4. For 6-8, some prior 13C NMR assignments are revised. The biological activities of compounds 2-8 and 11 were assessed and indicated 4 to have potent antialgal activity towards Chlorella fusca in an agar diffusion bioassay, as well as being moderately antitubercular and cytotoxic. Compound 6 demonstrated moderate cytotoxicity.