Organocatalytic asymmetric synthesis of dihydrocarbazoles via a formal [4+2] cycloaddition of in situ generated o-quinodimethanes with enals

Shaping Chirality via Stereoselective, Organocatalytic [4+2] Cycloadditions involving Heterocyclic ortho-Quinodimethanes

Polycyclic compounds bearing a complex heterocyclic core such as an aromatic heterocycle "fused" with one or more functionalized rings, are widespread leading molecules in the domain of synthetic organic chemistry and pharmaceuticals. Although many synthetic methodologies have been devised to access achiral, fused heteroaromatic scaffolds, or related chiral variants adorned with out-of-cycle stereogenic elements, equally efficient strategies to afford chiral heterocycles featuring in-cycle stereocenters, exist to a lesser extent and presently represent a growing field of investigation. The mild, organocatalytic generation of elusive ortho-quinodimethane intermediates (oQDMs), derived from suitable heteroaromatic carbonyl- or carbonyl-like pronucleophiles has recently proved successful in the synthesis of such peculiar chiral architectures via stereoselective [4+2] cycloadditions. This review provides an overview of the most important advances attained in this field over the last decade.

Copper-catalyzed multicomponent reactions for the efficient synthesis of diverse spirotetrahydrocarbazoles

In the presence of copper sulfate, three- or four-component reactions of 2-methylindole, aromatic aldehydes and various cyclic dienophiles in refluxing toluene afforded diverse spirotetrahydrocarbazoles. This reaction is an important development of the Levy reaction by using 2-methylindole to replace ethyl indole-2-acetate and successfully provides facile access to important polysubstituted spiro[carbazole-3,3'-indolines], spiro[carbazole-2,3'-indolines], spiro[carbazole-3,5'-pyrimidines] and spiro[carbazole-3,1'-cycloalkanes] in satisfactory yields and with high diastereoselectivity.

Multicomponent Reaction for Diastereoselective Synthesis of Spiro[carbazole-3,4'-pyrazoles] and Spiro[carbazole-3,4'-thiazoles]

In the presence of copper sulfate, the three-component reaction of aromatic aldehydes, ethylindole-3-acetate and 4-arylidene-5-methyl-2-phenylpyrazol-3-ones, in refluxing toluene afforded spiro[carbazole-3,4'-pyrazoles] in good yields with high diastereoselectivity. More importantly, the similar CuSO₄ promoted the four-component reaction of two molecular aromatic aldehydes with ethylindole-3-acetate and 5-methyl-2-phenyl-pyrazol-3-one resulted in 2,4-diarylspiro[carbazole-3,4'-pyrazoles] in satisfactory yields. Additionally, CuSO₄ promoted the four-component reaction of two molecular aromatic aldehydes, ethylindole-3-acetate and 2-phenylthiazol-4-one, in refluxing toluene gave 2,4-diarylspiro[carbazole-3,4'-thiazoles] with diastereomeric ratios in the range of 3:1 to 20:1.

Highly enantioselective tandem cycloisomerization/Diels-Alder reaction of 2-(1-alkynyl)-2-alken-1-ones and enals: dual catalysis with platinum and amines

Herein, we disclosed a highly efficient strategy of enantioselective synthesis of 2,3-furan-fused carbocycles bearing three-contiguous stereocenters. This transformation is catalyzed by dual catalysis of PtCl4/chiral amines via tandem dehydrogenative annulation/Diels-Alder reaction of 2-(1-alkynyl)-2-alken-1-ones and enals. The in situ generation of the furan-based ortho-quinodimethane intermediates and the iminium activation of enals are crucial to this transformation.

Advances in organocatalytic asymmetric reactions of vinylindoles: powerful access to enantioenriched indole derivatives

This review summarizes advances in vinylindole-based organocatalytic asymmetric reactions since 2008 and includes the applications of some methodologies in the total synthesis of natural products, points out remaining challenges in this research area.

Enantioselective [4+2] Annulation to the Concise Synthesis of Chiral Dihydrocarbazoles

A highly efficient phosphine-catalyzed enantioselective [4 + 2] annulation of allenoates with 3-nitroindoles or 3-nitrobenzothiophenes has been developed. The protocol represents a unique dearomatization–aromatization process to access functionalized dihydrocarbazoles or dihydrodibenzothiophenes with high optical purity (up to 97% ee) under mild reaction conditions. The synthetic utility of the highly enantioselective [4 + 2] annulation enables a concise synthesis of analgesic agent.

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High regio-, chemo-, and enantioselectivity

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Broad substrate scope

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Dearomatization/aromatization steps proceed under mild conditions

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Concise synthesis of chiral analgesic agent

Recent advances in the application of Diels–Alder reactions involving o-quinodimethanes, aza-o-quinone methides and o-quinone methides in natural product total synthesis

This review summarizes recent advances in Diels–Alder reactions involving o-QDMs, o-QMs and aza-o-QMs. The power and potential of this strategy in organic synthesis and natural product total synthesis is highlighted.

Organocatalytic asymmetric synthesis of tetrahydrocarbazoles via an inverse-electron-demand Diels–Alder reaction of 2,3-indole-dienes with enals

A novel 2,3-indole-diene synthon was designed and first applied to an inverse-electron-demand Diels–Alder reaction to construct tetrahydrocarbazoles with excellent stereoselectivities.