Fig. 8: Stereochemical divergence in the cycloaddition of 2-imido- and 2-amido-1,3-dienes with electron-deficient dienophiles: Concerted mechanism vs stepwise pathway.

a Concerted exo-selective cycloaddition between 2-imido-1,3-dienes 6 and electron-poor dienophiles 3. b Concerted exo-selective intramolecular cycloaddition of 17. c Stepwise reaction pathway in squaramide-catalyzed reaction between 2-amido-1,3-dienes and electron-poor dienophiles 3.