Tropical Marine Algae. II. The Structure Determination of New Halogenated Monoterpenes From Plocamium hamatum (Rhodophyta, Gigartinales, Plocamiaceae)

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Abstract
The structure determinations of new halogenated monoterpenes, isolated from collections of Plocamium hamatum J. Agardh , are reported. One collection of P. hamatum afforded (1R*,2R*,4S*,5R*,1′E)-1,2,4- trichloro-5-(2′-chloroethenyl)-1,5-dimethylcyclohexane (1) whose relative stereochemistry was determined by X-ray crystallography. Extensive use of two-dimensional carbon-hydrogen correlation experiments enabled unambiguous assignments of proton and carbon resonances to be made for this new compound. Complete carbon assignments for the related metabolite mertensene (2) are reported. A second collection of P. hamatum afforded three new halogenated monoterpenes. The relative and absolute stereochemistry of (2S,3E,5S,6S,7S)-1,5,7-tribromo-2,6,8-trichloro-2,6-dimethyloct-3-ene (3) was determined by n.m.r . and X-ray crystallography, while for (3R*,4S*,5E)-4,8-dibromo-3,7-dichloro-3,7-dimethylocta-1,5-diene (4) and (2R*,4S*,5R*,1*E)-4-bromo-2-chloro-5-(2′-chloroethenyl)-5-methyl-1-methylenecyclohexane (5), the structures and relative configurations were determined by n.m.r . spectroscopy.