Sarcomililate A, an Unusual Diterpenoid with Tricyclo[11.3.0.02,16]hexadecane Carbon Skeleton, and Its Potential Biogenetic Precursors from the Hainan Soft Coral Sarcophyton mililatensis

]hexadecane scaffold, along with two new cembranoids, sarcomililatols A and B (2 and 3), and two known related diterpenoids (4 and 5), was 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 (ECD = electronic circular dichroism) 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 lipopolysaccharide (LPS)-induced B lymphocytes proliferation.