Synthesis of Indole-Fused Dioxonanes via Brønsted Acid-Catalyzed (5 + 4) Cycloaddition

A Brønsted acid-catalyzed (5 + 4) cycloaddition between 3-alkynyl-2-indolylmethanols and <i>o</i>-hydroxybenzyl alcohols was established, which provided an efficient strategy for the synthesis of structurally diverse indole-fused dioxonanes (up to 86% yield). This work provides access to a new class of indole-fused 9-membered rings, simultaneously expanding the scope and utility of organocatalytic (5 + 4) cycloadditions.