Visible-light-promoted [3 + 2] cycloaddition for the synthesis of spirooxindoles under external photocatalyst-free conditions

A visible-light-promoted [3 + 2] cycloaddition of 3-alkenylindole-2-ones with 2-mercaptoimidazoles for the construction of diverse imidazo[2,1-b]thiazole fused 3,3′-spirooxindoles has been developed. Mechanism studies have shown that this transformation is initiated through visible-light excitation of 3-alkenylindole-2-ones, rather than an electron donor-acceptor (EDA) complexes-promoted process. In summary, we have successfully developed a method for simultaneously constructing C−N and C−S bonds under visible-light conditions, and synthesized over 40 examples of new nitrogen- and sulfur-containing fused heterocyclic compounds with potential biological activity with a yield of up to 99 %. This method exhibits simple operation, mild reaction conditions, high atom economy, good functional group tolerance and does not require the use of any transition metals and external photocatalysts. Gram-scale reaction further demonstrates the practicality of this protocol.