New diterpenoids from the marine sponge Dactylospongia elegans

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Eleganstone A (1), a rare diterpene possessing a 5/6/4/5 fused tetracyclic ring skeleton, along with its probable biogenetic precursor (2), one new dolabellane diterpene (3), and one known dolabellane diterpene (4), have been isolated from the marine sponge Dactylospongia elegans. As a pair of Z/E isomers, 2 can convert into 4 by the light-induced isomerization. Structure elucidations of all compounds were performed by spectroscopic analysis, ECD calculations, and comparison with the literature. These compounds were tested for their antibacterial and cytotoxic activities. Compounds 1–4 exhibited weak antibacterial activity against Escherichisa coli, Bacillus subtilis, and Staphylococcus aureus with MIC values of 32–64 μg/mL.