Catalytic Enantioselective Hetero-[6+4] and -[6+2] Cycloadditions for the Construction of Condensed Polycyclic Pyrroles, Imidazoles, and Pyrazoles.

The development of the first chemo-, regio-, and stereoselective hetero-[6+4] and -[6+2] cycloadditions of heteroaromatic compounds via amino aza- and diazafulvenes is presented. Pyrroles, imidazoles, and pyrazoles substituted with a formyl group react with an aminocatalyst to generate an electron-rich hetero-6π-component that reacts in a chemo-, regio-, and stereoselective manner with electron-deficient dienes and olefins. For the hetero-[6+4] cycloaddition of the pyrrole system with dienes, a wide variation of both reaction partners is possible, providing attractive pyrrolo-azepine products in high yields and excellent enantioselectivities (99% ee). The hetero-[6+4] cycloaddition reaction concept is extended to include imidazoles and pyrazoles, giving imidazolo- and pyrazolo-azepines. The same activation concept is successfully employed to include hetero-[6+2] cycloadditions of the pyrrole system with nitroolefins, giving important pyrrolizidine-alkaloid scaffolds. Experimental NMR and mechanistic studies allowed for the identification of two different types of intermediates in the reaction. The first intermediate is the result of a rapid formation of an iminium ion, which generates a hetero-6π aminofulvene intermediate as a mixture of two isomers. Density functional theory calculations were used to determine the mechanism and sources of asymmetric induction in the hetero-[6+4] and -[6+2] cycloadditions. After formation of the reactive hetero-6π-components, a stepwise addition occurs with the diene or olefin, leading to a zwitterionic intermediate that undergoes cyclization to afford the cycloadduct, followed by eliminative catalyst release. The stereoselectivity is controlled by the second step, and computations elaborate on the various substrate and catalyst effects that alter the experimentally observed enantioselectivities. The computational studies provided a basis for improving the enantioselectivity of the hetero-[6+2] cycloaddition.

Organocatalytic [10+4] cycloadditions for the synthesis of functionalised benzo[a]azulenes.

A direct and mild strategy for the synthesis of benzo[a]azulenes based on an organocatalytic [10+4] cycloaddition reaction is described. The strategy enables a diversity-oriented approach for the synthesis of various poly-functionalised azulenes from easily accessible starting materials.

Catalytic Enantioselective [10+4]†Cycloadditions.

The first peri- and stereoselective [10+4] cycloaddition between catalytically generated amino isobenzofulvenes and electron-deficient dienes is described. The highly stereoselective catalytic [10+4] cycloaddition exhibits a broad scope with high yields, reflecting a robust higher-order cycloaddition. Experimental and computational investigations support a kinetic distribution of intermediate rotamers dictating the enantioselectivity, which relies heavily on additive effects.

Aminocatalyzed Synthesis of Enantioenriched Phenalene Skeletons through a Friedel-Crafts/Cyclization Strategy.

A series of enantioenriched phenalene-derived compounds were accessed by a Friedel-Crafts/cyclization strategy. Starting from α,ß-unsaturated aldehydes and 2-naphthol derivatives, high levels of enantioselectivity were obtained through iminium-enamine catalysis. The catalytic system composed of a diphenylprolinol silyl ether organocatalyst and triethylamine as a base was applied to a combination of diversely functionalized substrates. The obtained phenalene-derived architectures are promising building blocks for reaching natural products and exhibit fluorescence properties.

Asymmetric Synthesis of Fused Polycyclic Indazoles through Aminocatalyzed Aza-Michael Addition/Intramolecular Cyclization.

The first example of an asymmetric aminocatalyzed aza-Michael addition of 1H-indazole derivatives to α,ß-unsaturated aldehydes is described. The iminium/enamine cascade process lies at the heart of our strategy, leading to enantioenriched fused polycyclic indazole architectures. Variations on both the α,ß-unsaturated aldehydes and the indazole-7-carbaldehyde heterocycles were studied in order to broaden the scope of the transformation in synthetically interesting directions. The fused polycyclic indazoles exhibit fluorescence properties and can undergo synthetic transformations.