Complexity-Building Exhaustive Dearomatization of Benzenoid Aromatics within an ESIPT-Initiated Three-Step Photochemical Cascade.

Dearomative cycloadditions offer rapid access to complex 3D molecular architectures, commonly via a sp-to-sp rehybridization of two atoms of an aromatic ring. Here we report that the 6e π-system of a benzenoid aromatic pendant could be exhaustively depleted within a single photochemical cascade. An implementation of this approach involves the initial dearomative [4+2] cycloaddition of the Excited State Intramolecular Proton Transfer (ESIPT)-generated azaxylylene, followed by two consecutive [2+2] cycloadditions of auxiliary π moieties strategically positioned in the photoprecursor. Such photochemical cascade fully dearomatizes the benzenoid aromatic ring, saturating all six sp atoms to yield a complex sp-rich scaffold with high control of its 3D molecular shape, rendering it a robust platform for rapid systematic mapping of underexplored chemical space. Significant growth of molecular complexity-starting with a modular synthesis of photoprecursors from readily available building blocks-is quantified by Böttcher score calculations.