Here we report a highly diastereoselective cyano group transfer radical cyclization reaction to construct 5/6-fused bicyclic ring systems that bear three contiguous and congested stereogenic elements, with 100% atom economy under catalyst-free and near-ultraviolet light irradiation conditions. Mechanistic investigations and density functional theory calculations suggest that the diastereoselectivity is governed by the conformational distribution of the triplet diradical intermediate and the rate of reverse intersystem crossing (RISC) before radical coupling.
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