Photochemical Transformation of Rearranged Pregnane Steroids from the Soft Coral Scleronephthya gracillimum and Their Antibacterial Properties.

Three uncommon rearranged pregnane steroids, namely, sclerogroids A-C (1-3), along with seven known related ones 4-10, have been isolated from the soft coral Scleronephthya gracillimum collected off Ximao Island. The photoreactions of four α,β-unsaturated ketosteroids (8, 11-13) have been successfully performed, leading to the generation of two novel carbon skeleton pregnane steroids with an unprecedented 5/3/6/6/5 pentacyclic ring scaffold, namely, sclerogranes A (8b) and B (11f), together with a series of rearranged steroids and oxidation products. The structures of the new compounds were elucidated by a combination of spectroscopic analyses, time dependent density functional theory (TDDFT)-electronic circular dichroism (ECD) calculations, and comparison of the NMR data with those of known analogues. Bioassays demonstrated that compounds 1-4 and 8a exhibit significant antibacterial activity against various strains of vancomycin-resistant enterococci, as well as other pathogenic bacteria in humans and fish with minimum inhibitory concentration (MIC) values ranging from 1.9 to 29.6 μg/mL. In addition, compound 11c showed antineoplastic activity against HT-29 with an IC50 value of 27.24 ± 2.87 μM, while compounds 8d and 11f exhibited anti-inflammatory activity by inhibiting the release of NO in RAW264.7 cells with IC50 values of 15.4 ± 0.53 and 7.56 ± 0.93 μM, respectively.