Aminocatalytic [8+2] Cycloaddition Reactions toward Chiral Cyclazines.

An efficient and exceptionally stereoselective synthesis of chiral cycl[3.2.2]azines has been realized by means of the rational design and utilization of novel (E)-3-benzylidene-3H-pyrrolizines in iminium-ion-catalyzed [8+2] cycloaddition reactions. The presented protocol allows for the incorporation of diverse enals, including cinnamaldehydes, enolizable aldehydes, and substrates of extended conjugation. The obtained products contain both an electron-rich alkenyl pyrrole moiety and an electron-deficient carbaldehyde substituent, and both moieties can undergo selective transformations with retention of the stereochemical information established in the [8+2] cycloaddition.

Enantioselective Construction of the Cycl[3.2.2]azine Core via Organocatalytic [12 + 2] Cycloadditions.

The first enantioselective [12 + 2] cycloaddition has been developed for the construction of a chiral cycl[3.2.2]azine core, a tricyclic moiety with a central ring-junction nitrogen atom, by an operationally simple one-step organocatalytic process. The reaction concept builds upon aminocatalytically generated 12π-components derived from 5H-benzo[a]pyrrolizine-3-carbaldehydes reacting with different electron-deficient 2π-components and affording the complex scaffold of benzo[a]cycl[3.2.2]azine (indolizino[3,4,5-ab]isoindole) with excellent enantio- and diastereoselectivity in good yields. The developed reaction is robust toward diverse substituent patterns and has been extended to different classes of electron-deficient 2π-components by minor variations in reaction setup. The obtained [12 + 2] cycloadducts are electron-deficient in nature, and their reaction with nucleophiles have been demonstrated. The enantioselective [12 + 2] cycloaddition with α,ß-unsaturated aldehydes as the electron-deficient 2π-components relies upon an unconventional, simple aminocatalyst. In order to understand the high stereoselectivity of the [12 + 2] cycloaddition for this simple catalyst, combined experimental and computational investigations were performed. The investigations point to activation of both the 5H-benzo[a]pyrrolizine-3-carbaldehyde and the α,ß-unsaturated aldehyde by the aminocatalyst and that the reaction proceeds by a stepwise cycloaddition, where especially the ring-closure is crucial for the stereochemical outcome. For other electron-deficient 2π-components, such as α,ß-unsaturated ketoesters and nitroolefins, a more sterically demanding aminocatalyst has been applied and the corresponding [12 + 2] cycloadducts are obtained with excellent stereoselectivity. The [12 + 2] cycloaddition with vinyl sulfones afforded fully unsaturated systems, which display photoluminescence properties and for which quantum yields have been evaluated.