Penitol A and Penicitols E–I: Citrinin Derivatives from Penicillium citrinum and the Structure Revision of Previously Proposed Analogues

Penitol A (1), a new citrinin derivative with a rare tricyclic spiro skeleton, was isolated from a coral-derived strain of the fungus Penicillium citrinum. In addition, penicitols E–I (2–6), five new citrinin analogues, were coisolated. Their structures were determined by an analysis of 1D/2D NMR and HRESIMS data, statistical DP4+ analyses based on DFT-GIAO NMR calculations, quantum chemistry ECD calculations, and a single-crystal X-ray diffraction study. The structures of penicitol A (7) and two related synthetic intermediates were revised. Biological evaluation results revealed that penitol A (1) exhibited cytotoxic activity against K562 tumor cells, with an IC50 value of 8.8 μM. A proposed route of formation of compounds 1–7 was reported.