Asymmetric synthesis of pyrrolo[1,2-a]indoles via organocatalytic [3 + 2] annulation of substituted 2-vinylindoles with azlactones

Deracemization of Atropisomeric Biaryls Enabled by Copper Catalysis

Atropisomeric biaryls have found crucial applications in versatile chiral catalysts as well as in ligands for transition metals. Herein, we have developed an efficient crystallization-induced deracemization (CID) method to access chiral biaryls from their racemates with a chiral ammonium salt under copper catalysis including BINOL, NOBIN, and BINAM derivatives. After being significantly accelerated by its bidentate diamine ligand, the copper catalyst exhibits high efficiency and selectivity in racemizing biaryl skeletons, and the cocrystal complex would be enantioselectively formed together with chiral ammonium salt, which on acid-quenching would directly deliver chiral biaryl without further chromatographic purification. This CID process is easily scalable, and the chiral ammonium salt was nicely recoverable. Ligand effect studies showed that bulky alkyl substitution was an indispensable element to ensure efficient racemization, which probably proceeds via a radical-cation intermediate and further allows axial rotation by forming a delocalized radical.

Catalytic asymmetric de novo construction of 4-pyrrolin-2-ones via intermolecular formal [3+2] cycloaddition of azoalkenes with azlactones

The chiral phosphoric acid-catalyzed asymmetric intermolecular formal [3+2] cycloaddition of azoalkenes with azlactones has been established. This convergent protocol leads to a facile and enantioselective de novo construction of a wide range of fully substituted 4-pyrrolin-2-ones bearing a fully substituted carbon atom in good yields and with excellent enantioselectivities (26 examples, 72-95% yields and 87-99% ee).

Highly diastereoselective synthesis of benzothiazolo[3,2-a]pyridines via [4 + 2] annulation reaction of 2-vinylbenzothiazoles and azlactones

An efficient AgOTf-catalyzed [4 + 2] annulation reaction of 2-vinylbenzothiazoles and azlactones was successfully performed under mild reaction conditions. With this approach, a series of novel benzothiazolo[3,2-a]pyridine derivatives was readily obtained in good to excellent yields (68-96%), with high diastereoselectivities and tolerating quite a broad scope of substituents. By using chiral phosphoric acid catalyst, the desired products were obtained in high enantioselectivities, up to -94%. This methodology provides a rapid and useful method for constructing fused benzothiazole derivatives.

Tetrahydroxydiboron and Nickel Chloride Cocatalyzed Rapid Radical Cyclization toward Pyrrolizidine and Indolizidine Alkaloids

A novel tetrahydroxydiboron and nickel chloride cocatalyzed radical cyclization cascade with a broad substrate scope and an ultrashort reaction time was developed. The mechanistic investigation indicated that the reaction might involve a homocleavage of tetrahydroxydiboron and nickel hydride intermediates. This approach enables the simple and efficient synthesis of a series of heteropolycycles.

Application of 3-Alkyl-2-vinylindoles in Catalytic Asymmetric Dearomative (2+3) Cycloadditions

The first application of 3-alkyl-2-vinylindoles in catalytic asymmetric dearomative cycloadditions was established by chiral phosphoric acid (CPA)-catalyzed (2+3) cycloaddition with azoalkenes, leading to the generation of chiral pyrroloindolines bearing two tetrasubstituted stereogenic centers in good yields (61-96%) and excellent stereoselectivities (all >95:5 dr, 86-99% ee). This reaction has realized the first enantioselective dearomative cycloaddition of 3-alkyl-2-vinylindoles, which brings a new reactivity to this class of vinylindoles and will enrich the chemistry of 3-alkyl-2-vinylindoles. In addition, this approach has provided a useful strategy for the construction of enantioenriched pyrroloindoline skeletons bearing two tetrasubstituted stereogenic centers. More importantly, the bioassay of these chiral pyrroloindolines has revealed that some compounds exhibit strong anti-cancer activity against Hela and MCF-7 cell lines, which will be helpful for discovering anti-cancer drug candidates.

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.

Copper(i)-catalyzed asymmetric [3 + 3] annulation involving aziridines to construct tetrahydro-β-carbolines

A copper-catalyzed asymmetric [3 + 3] annulation of 2-vinylindoles with 2-aryl aziridines has been described, providing direct access to enantioenriched tetrahydro-β-carbolines.