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We report a Br\u00f8nsted acid-catalyzed formal cycloaddition of N-substituted indoles with azabicyclobutane (ABB) ketones, providing a mild, transition-metal-free, and highly stereoselective route to aza-bicyclohexanes. Using pTSA\u00b7H2O in HFIP, the reaction proceeds at room temperature in up to 93% yield and 99:1 d.r. Mechanistic studies suggest cooperative activation by pTSA and HFIP via hydrogen-bond-assisted proton transfer. 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