Expanstines A–D: four unusual isoprenoid epoxycyclohexenones generated by Penicillium expansum YJ-15 fermentation and photopromotion

Two new isoprenoid epoxycyclohexenones, expanstines A–B ( 1–2 ), and two novel isoprenoid epoxycyclohexenones, expanstines C–D ( 3–4 ), featuring an unusual oxetane ring were isolated from Penicillium expansum YJ-15. Their structures were initially investigated in detail by NMR spectroscopy, HRESIMS, XRD and ECD experiments. Expanstines A–D were derived under UV and visible light from 5 and 6 , which plausibly underwent a rare intramolecular photoene reaction and a [2 + 2] Paternò–Büchi photoaddition respectively. Both 3 and 4 exhibited potent antibacterial activities against B. subtilis , with an MIC of 0.25 μg mL −1 .