The torquoselectivity of the 4 pi electrocyclic ring-opening reaction of 2-azetines can be controlled by the Bronsted acidity of the catalyst and the polarity of the solvent. DFT calculations provided insight into the mechanism of this remarkable switch. Anti and syn stereoisomers of alpha,beta-unsaturated amidines were selectively synthesized from ynamides and aldimines in the presence of Tf(2)NH and CSA, respectively.