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A new metal-free radical 5-exo-dig cyclization of phenol-linked 1,6-enynes with O2, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), and tBuONO is described. With this general method, carbonylated benzofurans can be accessed through incorporation of two oxygen atoms into the product from O2 and TEMPO through dioxygen activation and oxidative cleavage of the N-O bond, respectively.
Hu et al. (Wed,) studied this question.