Triflic Acid‐Catalyzed Enynes Cyclization: A New Strategy beyond Electrophilic π‐Activation

The cyclization of enynes, catalyzed by a transition metal, represents a powerful tool to construct an array of cyclic compounds through electrophilic π‐activation. In this paper, we disclose a new and efficient strategy for enynes cyclization catalyzed by triflic acid. The salient features of this transformation includes a broad substrate scope, metal free synthesis, open flask and mild conditions, good yields, ease of operation, low catalyst loading, and easy scale‐up to gram scale. A preliminary mechanism study demonstrated that the activation model of the reaction was σ‐activation, which is different from the transition‐metal‐catalyzed enynes cyclization. Our strategy affords a complementary method to the traditional strategies, which use transition‐metal catalysts.