Unraveling Solvent and Irradiation Effects in the Photochemistry of α‐Santonin

Despite nearly two centuries of investigation, the photochemistry of α‐santonin continues to reveal unexpected reactivity. In this study, we systematically explored the behavior of α‐santonin under UVC (254 nm) and UVA (390 and 456 nm) irradiation in protic and aprotic solvents, uncovering a marked dependence of product distribution on both irradiation wavelength and solvent environment. While irradiation at 254 nm in aprotic solvents selectively afforded lumi ‐santonin ( 1 ), irradiation at 390 nm promoted further rearrangement to mazdasantonin ( 4 ). In protic media, solvent polarity and steric hindrance strongly influenced the reaction outcome, leading to photosantonic acid esters and lactone derivatives. Notably, irradiation in secondary and tertiary alcohols at 390 nm provided two previously undescribed lumi ‐derivatives. Mechanistic investigations suggest the involvement of competing diradical and zwitterionic pathways. These results demonstrate that modern wavelength‐controlled photochemistry can still uncover unexplored transformations even within classical photochemical systems such as α‐santonin.