| Summary: | The methanol extract of a <em>Sinularia </em>sp., collected from Bowden Reef, Queensland, Australia, yielded ten natural products. These included the new nitrogenous diterpene (4<em>R</em>*,5<em>R</em>*,9<em>S</em>*,10<em>R</em>*,11<em>Z</em>)-4-methoxy-9-((dimethylamino)-methyl)-12,15-epoxy-11(13)-en-decahydronaphthalen-16-ol (<strong>1</strong>), and the new lobane, (1<em>R*</em>,2<em>R*</em>,4<em>S*</em>,15<em>E</em>)-loba-8,10,13(14),15(16)-tetraen-17,18-diol-17-acetate (<strong>2</strong>). Also isolated were two known cembranes, sarcophytol-B and (1<em>E</em>,3<em>E</em>,7<em>E</em>)-11,12-epoxycembratrien-15-ol, and six known lobanes, loba-8,10,13(15)-triene-16,17,18-triol, 14,18-epoxyloba-8,10,13(15)-trien-17-ol, lobatrientriol, lobatrienolide, 14,17-epoxyloba-8,10,13(15)-trien-18-ol-18-acetate and (17<em>R</em>)-loba-8,10,13(15)-trien-17,18-diol. Structures of the new compounds were elucidated through interpretation of spectra obtained after extensive NMR and MS investigations and comparison with literature values. The tumour cell growth inhibition potential of <strong>1</strong> and <strong>2</strong> along with loba-8,10,13(15)-triene-16,17,18-triol, 14,17-epoxyloba-8,10,13(15)-trien-18-ol-18-acetate, lobatrienolide, (1<em>E</em>,3<em>E</em>,7<em>E</em>)-11,12-epoxycembratrien-15-ol and sarcophytol-B were assessed against three human tumour cell lines (SF-268, MCF-7 and H460). The lobanes and cembranes tested demonstrated 50% growth inhibition in the range 6.8–18.5 µM, with no selectivity, whilst <strong>1</strong> was less active (GI<sub>50</sub> 70–175 µM).
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