Calcium‐Catalyzed Regioselective Synthesis of Trisubstituted Allenes From <i>sec</i> ‐Propargyl Alcohols and Their Cyclizations

We describe here the first report on the alkaline earth-catalyzed regioselective synthesis of trisubstituted allenes from sec-propargyl alcohols (PAs) and active methyl ketones/electron-rich arenes. Unlike previous methods, the current approach does not require the protection of alcohol, heating, transition metal catalysts, or solvents. The reaction mechanism is proposed based on deuterium labelling, step-wise synthesis, and isolation of intermediates. The current protocol exhibits a broad substrate scope, high reaction yields, and enables gram-scale synthesis. Besides, we also demonstrated the one-pot cyclization of these allenes to obtain tetrahydroindoles and fused furans.