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

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.