Sarcomililate A, an Unusual Diterpenoid with tricyclo[11.3.0.0 2,16 ]hexadecane Carbon Skeleton, and Its Potential Biogenetic Precursors from the Hainan Soft Coral Sarcophyton mililatensis.

A novel diterpenoid, sarcomililate A (1), possessing a previously undescribed tricyclo[11.3.0.02,16 ]hexadecane scaffold, along with two new cembranoids, sarcomililatols A and B (2 and 3), two known related diterpenoids (4 and 5) were isolated from the Hainan soft coral Sarcophyton mililatensis. The complete chemical structure including absolute configuration (AC) of 1 was unambiguously determined by a combination of residual dipolar coupling (RDC) based NMR analysis, TDDFT-ECD calculation, and Snatzke's method. The AC of 1 was further confirmed by a comparison with 2, whose AC was determined by anomalous X-ray diffraction. A plausible biogenetic relationship of 1-3 was further proposed. All the reported compounds exhibited interesting inhibitory effects on the ConA-induced T lymphocytes and/or LPS-induced B lymphocytes proliferation.