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Structure determination of two unusual C25 steroids with bicyclo[4.4.1]A/B rings from Penicillium decumbens by NMR spectroscopy

The genus Penicillium, despite the large numbers of natural products identified, still proves to be rich in structurally unique and biologically active secondary metabolites. A number of fascinating secondary metabolites with antibacterial, antifungal, cytotoxic, and free radical scavenging activities, including citrinadins, scalusamides, perinadine A, and C25 steroids, have been previously isolated from this genus. Our recent investigation of a Penicillium decumbens strain from a limestone soil led to the isolation of cyclopenicillone with a unique 2,5-dimethylcyclopent-2-enone carbon skeleton. It demonstrated a dose-dependent inhibition against LPS-induced NO production in RAW264.7 macrophages. As part of our ongoing effort to study the chemical and biological diversity of this strain, we report herein the isolation and structure elucidation of two unusual C25 steroids with bicyclo[4.4.1]A/B rings (Fig. 1), along with 12 known structurally related compounds. To our knowledge, C25 steroids with bicyclo[4.4.1]A/B rings from natural sources are extremely rare. The extensive application of 1D and 2D NMR techniques, high-resolution ESI-MS (HRESIMS) spectrometry, and circular dichroism spectral analysis were performed to characterize their structures and establish the complete H and C resonance assignments.