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A Bro̷nsted acid-catalyzed 5 + 1 annulation of 2-(1 H -indol-2-yl)phenols, a class of indole-based 1,5-CO-synthons, with aldehydes has been established. By this approach, various indole-fused dihydrochromanes were synthesized in moderate to good yields (up to 99%) with excellent regioselectivities. Control experiments not only showed the H-bonding interaction between the NH group of the substrates─and the acid catalyst is vital to realize this transformation─but also indicated that an indolylmethanol-type intermediate was possibly formed via the first step of nucleophilic addition of indole C3-position to aldehydes during the reaction process. This reaction represents the first Bro̷nsted acid-catalyzed regioselective 5 + 1 annulation of 2-(1 H -indol-2-yl)phenols and provides a novel strategy for constructing biologically intriguing indole-fused dihydrochromane skeletons in a highly regioselective manner.
Liu et al. (2024) studied this question.