Abstract
A metal-free, TfOH (1.1 equiv)-mediated formal cycloisomerization of easily prepared allene-alkynylbenzenes to give pyrrolidines and cyclopentanes derivatives was developed. This reaction is initiated by the generation of allylic cation from allene, followed by alkyne's reaction with the allylic cation, to give a vinyl cation, which is finally intercepted by the triflate (TfO) anion. This cycloisomerization can be further tuned to become an acid-mediated intramolecular formal [4+3] cycloaddition by using 10 equiv of TfOH (The excess acid was used to promote the Friedel-Crafts reaction of the acid-mediated cycloisomerization products). The present system can also be applied to synthesized F-incorporated products by using HBF4 or Me3OBF4 as the fluoro source.
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