Iron(III)-Catalyzed Cyclization of Alkynyl Aldehyde Acetals: Experimental and Computational Studies

摘要

FeCl₃6 H₂O- and FeBr₃-catalyzed Prins cyclization/halogenation of alkynyl aldehyde acetals has been realized with acetyl chloride or bromide as halogen source in dichloromethane to afford 2-(1-halobenzylidene or alkylidene)-substituted five-membered carbo- and heterocycles, and thus provides an alternative route for vinylic CCl and CBr bond formation. Five- to eight-membered cyclic enones were efficiently synthesized by FeCl₃6 H₂O-catalyzed intramolecular cyclization of alkynyl aldehyde acetals in acetone under mild conditions. An oxocarbonium species generated in situ is proposed to initiate the reaction, and the target products are formed via vinylogous carbenium cation and oxete intermediates according to DFT calculations. Intermolecular reactions of alkynes and aldehyde acetals were also investigated with 20–40 mol % FeCl₃6 H₂O catalyst, and produced ,β-unsaturated enones and chlorinated indene derivatives. The present protocol has applications in the synthesis of carbo-, oxa- and azacycles.