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In pursuit of potent pharmaceutical candidates and to further improve their chemical traits, small ring systems can serve as a potential starting point. Small ring units have the additional merit of loaded strain at their core, making them suitable reactants as they can capitalize on this intrinsic driving force. With the introduction of cyclobutenone as a strained precursor to ketene, the photocycloaddition with another strained unit, bicyclo1.1.0butane (BCB), enables the reactivity of both π-units in the transient ketene. This double strain-release driven 2π+2σ-photocycloaddition promotes the synthesis of diverse heterobicyclo2.1.1hexane units, a pharmaceutically relevant bioisostere. The effective reactivity under catalyst-free conditions with a high functional group tolerance defines its synthetic utility. Experimental mechanistic studies and density functional theory (DFT) calculations suggest that the 2π+2σ-photocycloaddition takes place via a triplet mechanism.
Dutta et al. (Tue,) studied this question.