Zn(OTf)2-Catalyzed Cyclization of Proparyl Alcohols with Anilines, Phenols, and Amides for Synthesis of Indoles, Benzofurans, and Oxazoles through Different Annulation Mechanisms

Zn(OTf)2 (10 mol %) catalyzed the cyclization of propargyl alcohols with PhXH (X = O, NH) in hot toluene (100 οC) without additive and gave indole and benzofuran products with different structures. In such transformations, α-carbonyl intermediates A and C were isolated as reaction intermediates. The 1,2-nitrogen shift in the formation of indole is catalyzed by Zn(OTf)2, and its mechanism has been elucidated. This catalytic cyclization is also applicable to the synthesis of oxazoles through the cyclization of propargyl alcohols and amides without a 1,2-nitrogen shift.