Advances in the Organocatalytic Asymmetric Mannich Reaction of Six‐Membered Unsaturated Heterocycles: Methodology and Application

Enantioselective synthesis of α-(3-pyrrolyl)methanamines with an aza-tetrasubstituted center under metal-free conditions

Construction of a chiral methanamine unit at the C3 position of pyrrole is highly desirable; nevertheless, it remains challenging due to its intrinsic electronic properties. Herein, we present an operationally straightforward and direct asymmetric approach for accessing α-(3-pyrrolyl)methanamines under benign organocatalytic conditions for the first time. The one-pot transformation proceeds smoothly through an amine-catalyzed direct Mannich reaction of succinaldehyde with various endo-cyclic imines, followed by a Paal-Knorr cyclization with a primary amine. Several N-H/alkyl/Ar α-(3-pyrrolyl)methanamines with an aza-tetrasubstituted center have been synthesized with good yields and excellent enantioselectivity.

Enantioselective Synthesis of Biphenyl-Bridged ϵ-Sultams by Organocatalytic Mannich Reactions of Cyclic N-Sulfonylimines with Unactivated Ketones

A highly enantioselective Mannich reaction of biphenyl-bridged seven-membered cyclic N-sulfonylimines with methyl alkyl ketones is disclosed in this study. The reaction was performed under organocatalysis by using a quinine-derived primary amine as the catalyst in combination with a Brønsted acid as the co-catalyst. High yields (up to 89 %) and excellent enantioselectivities (up to 97 % ee) were observed. For methyl alkyl ketones containing a larger alkyl substituent, specific regioselective addition to the C=N bond is favored at the methyl group. On the contrary, ketones containing a smaller alkyl substituent or hydroxyacetone substrates gave major syn selective Mannich products at the methylene group.

Asymmetric Acyclic 1,3-Difunctionalization of Vinyl Carbenes via Site-Selective Vinylogous Mannich-Type Interception of Oxonium Ylides

A novel and highly stereoselective acyclic 1,3-difunctionalization of vinyl metal carbene species has been developed via Rh(II)/chiral phosphoric acid co-catalyzed three-component reactions of vinyldiazoacetates with alcohols and imines. This innovative approach features excellent regio-, diastereo-, and enantioselectivities, demonstrating a broad scope and functional group compatibility. Notably, this is the first example of three-component asymmetric acyclic 1,3-difunctionalization with in situ-formed vinyl metal carbenes.

Indoles Oxidative Ring-Opening/Cyclization Cascade with the 1,2-Diaminoarenes: Direct Synthesis of 2-Aryl-3-(2-aminoaryl)quinoxalines

A mild oxidative sequential tandem reaction was developed to rapidly generate 2-aryl-3-(2-aminoaryl) quinoxalines. This method exploited 2-substituted indoles as substrate to form quinoxalines in a one-pot reaction. The key to this tandem reaction was the formation of 3-iodoindoles, which underwent Kornblum-type oxidation with DMSO to generate active imine 2-substitued 3H-indol-3-ones. The active imines were captured in situ by 1,2-diaminobenzenes to construct diverse quinoxalines. The transformation can be accomplished at room temperature with excellent functional group tolerance.

Catalytic stereoselective Mannich-type reactions for construction of fluorinated compounds

For its unique role in developing and designing new bioactive materials and healthcare products, fluoro-organic compounds have attracted remarkable interest. Along with ever-increasing demand for a wider availability of fluorine-containing structural units, a large diversity of methods has been introduced to incorporate fluorine atoms specially in a stereoselective fashion. Among them, catalytic Mannich reaction can proceed with a broad variety of reactants and open clear paths for the synthesis of versatile amine synthons in the synthesis of natural product and pharmaceutical molecules. This review provides an overview of the employment of catalytic asymmetric Mannich reactions in the synthesis of fluorine-containing amine compounds and highlights the conceivable distinct mechanisms.

α-C-H Bond Functionalization of Unprotected Alicyclic Amines: Lewis-Acid-Promoted Addition of Enolates to Transient Imines

Enolizable cyclic imines, obtained in situ from their corresponding lithium amides by oxidation with simple ketone oxidants, are readily alkylated with a range of enolates to provide mono- and polycyclic β-aminoketones in a single operation, including the natural product (±)-myrtine. Nitrile anions also serve as competent nucleophiles in these transformations, which are promoted by BF3 etherate. β-Aminoesters derived from ester enolates can be converted to the corresponding β-lactams.

Diversification of Unprotected Alicyclic Amines by C-H Bond Functionalization: Decarboxylative Alkylation of Transient Imines

Despite extensive efforts by many practitioners in the field, methods for the direct α-C-H bond functionalization of unprotected alicyclic amines remain rare. A new advance in this area utilizes N-lithiated alicyclic amines. These readily accessible intermediates are converted to transient imines through the action of a simple ketone oxidant, followed by alkylation with a β-ketoacid under mild conditions to provide valuable β-amino ketones with unprecedented ease. Regioselective α'-alkylation is achieved for substrates with existing α-substituents. The method is further applicable to the convenient one-pot synthesis of polycyclic dihydroquinolones through the incorporation of a SN Ar step.

Mannich-type allylic C–H functionalization of enol silyl ethers under photoredox–thiol hybrid catalysis

The synergy of an Ir-based photosensitizer with mild oxidizing ability and a thiol catalyst enables efficient allylic C–H functionalization of enol silyl ethers with imines under visible light irradiation.

C2-Symmetric 1,2-Diphenylethane-1,2-diamine-Derived Primary-Tertiary Diamine-Catalyzed Asymmetric Mannich Addition of Cyclic N-Sulfonyl Trifluoromethylated Ketimines

A simple chiral primary-tertiary diamine derived from C₂-symmetric 1,2-diphenylethane-1,2-diamine as the organocatalyst in combination with the trifluoroacetic acid additive for the asymmetric Mannich reaction of cyclic N-sulfonyl trifluoromethylated ketimines and methyl ketones afforded the desired product with high enantioselectivity (73-96% ee). The reactions proceeded well for a variety of different substituted cyclic N-sulfonyl trifluoromethyl ketimines and various alkyl methyl ketones, providing access to diverse enantioenriched benzo-fused cyclic sulfamidate N-heterocycles bearing a trifluoromethylated α-tetrasubstituted carbon stereocenter. This study also investigated the diastereoselective reduction of the carbonyl group and ring cleavage reduction of the sulfamidate group of the corresponding Mannich product.

Catalytic Asymmetric Access to Noncanonical Chiral α-Amino Acids from Cyclic Iminoglyoxylates and Enamides

Here we describe an enantioselective Mannich reaction of cyclic iminoglyoxylates with enamides by virtue of chiral phosphoric acid catalysis in a one-pot manner. The wide substrate scope, mild reaction conditions, and constantly excellent enantioselectivities (>95% ee in most cases) render this protocol highly practical for the rapid construction of valuable noncanonical chiral α-amino-acid building blocks.

Deconjugated butenolide: a versatile building block for asymmetric catalysis

Deconjugated butenolides have emerged as a popular synthon for the enantioselective synthesis of γ-lactones. This review provides a comprehensive overview on the catalytic asymmetric reactions of deconjugated butenolides reported till date.