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(A) 5-Exo-trig vs. 5-endo-trig cyclizations reactions. (B) Chiral ammonium ion-catalyzed 5-endo-trig intramolecular Michael reaction. (C) The lowest-energy, computed transition structure en route to 60 (B97D/6-31G(d)). The red dotted line represents a cation–π interaction; blue dotted lines represent hydrogen bonds. Grey and black dotted lines indicate breaking or forming bonds.
