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.