From Photoswitch to Polycycle: Structure–Reactivity Relationships in the Photoinduced Isomerization–Cyclization of Amino(hetero)arylidene‐thiazolidine‐2,4‐diones

We report a comprehensive study of the photoinduced isomerization–cyclization cascade of o ‐amino(hetero)arylidenethiazolidine‐2,4‐diones (Het)Ar‐TZDs 1 , combining computational, synthetic, and kinetic investigations. Time‐dependent density functional theory (TD‐DFT) calculations were used to predict absorption maxima and to elucidate key structure‐property relationships, highlighting the dominant influence of substitution at position 4 of the aromatic ring of 1 . Guided by these results, a broad library of o ‐amino‐TZDs 1 was designed and synthesized, including heteroaryl analogues. Photochemical investigations demonstrated that the sequence is highly robust, overall, proceeding efficiently across a wide range of substrates and displaying a broad tolerance toward variations in their electronic properties or substitution patterns. A detailed kinetic analysis using ex situ and in situ 1 H NMR experiments was conducted and enabled a quantitative comparison of reaction efficiencies as a function of substituents on the aromatic ring, the TZD core, and the exocyclic double bond, across series 1(a) to 1(d) . Overall, this work establishes a fine structure‐reactivity framework for this photochemical cascade, identifying the key parameters governing its efficiency.