Haliclomeroterpenes A-D: Four Meroterpenes with Unprecedented Skeletons from the Marine Sponge Haliclona sp

Four novel meroterpenes, haliclomeroterpenes A-D (1-4), were isolated from the marine sponge Haliclona sp. Their structures with absolute configurations were elucidated by spectroscopic methods, quantum chemical calculations, and X-ray diffraction. Compounds 1 and 2 are meroterpenes bearing a bicyclo[4.4.0]-1-decene coupled tetrahydrofuro[3,4-b]furanone with a C-4-spiro-fused 6/6/5/5 tetracyclic skeleton. Compound 3 is a meroterpene possessing a bicyclo[4.4.0]-1-decene coupled furanone with a C-4-spiro-fused 6/6/5 tricyclic skeleton. Compound 4 represents the unusual skeleton of furanone[3,2-b]bicyclo[4.4.0]-1-decene. The plausible biogenetic pathways of 1-4 were also proposed. The formation of all four of these compounds was proposed to arise from the condensation of cadinene sesquiterpene and 2,4-dimethyl-3-oxopentanoic acid or propionic acid, respectively. Notably, compounds 1 and 2 showed moderate inhibitory activities against the K562 cell line with IC₅₀ values of 17.23 and 27.91 μM, respectively. In addition, compound 2 showed weak anti-NO activities with IC₅₀ values of 40.22 μM.