Extraction of Plocamium mertensii and an unclassified Plocamium species found off the Western Australian coast yielded (1R,2S,4S,1'E)-2-bromo-1-chloro-4-(2'-chloroethenyl)-1-methyl-5-methylenecyclohexane(3) and (1R,2S,4R,5R,1'E)-4-bromo-1,2-dichloro-5-(2'-chloroethenyl)-1,5-dimethylcyclohexane (4) respectively. The structures and absolute configurations were established by single-crystal X-ray crystallographic structure determination.

Two new monocyclic sesquiterpenes have been isolated from the green alga Caulerpa flexilis (Lamx) var. muelleri (Sond.) (Wom.).

lR,4S)-(-)-Calamenene has been synthesized from dihydroxyserrulatic acid of known absolute configuration. (1R,4S)-(-)-8-Methoxycalamenene is shown to be the enantiomer of a calamenene isolated from the gorgonian Subergorgia hicksoni.

The isolation and structural elucidation of three new cembrene diterpenes from Eremophila species is reported. (1R,3Z,7E,11Z)-15- Hydroxycembra-3,7,11-trien-19-oic acid (3) and (1R,3Z,8ξ,11Z)-15- hydroxycembra-3,11-dien-19-oic acid (4) were isolated from E. dempsteri and (1R,3S,4R,7E,11Z)-3,15-epoxy-19-oxocembra-7,11-dien-18-ol as the malonate half ester derivative (15) was isolated from E. Platycalyx and a variety of E. Fraseri. In addition the preparation of (1R,3Z,7Z,11Z)- cembra-3,7,11,15-tetraene (13; all- cis-cembrene-A ) is reported.

The structure of a new tricyclic diterpene (1) isolated from an unclassified Higginsia sp. has been determined by X-ray crystallographic methods. The structures of two related co-metabolites have been deduced from a comparison of their spectroscopic properties with those of (1).