Kinugasa Reaction: A Direct One-Pot Route to Highly Functionalized β-Lactams

Copper-Catalyzed Asymmetric Kinugasa/Michael Addition Cascade Reactions for the Synthesis of Chiral Spiro β-Lactams

A mild copper-catalyzed asymmetric Kinugasa/Michael addition cascade process is developed. The reaction of α, β-unsaturated ester-tethered propiolamides with nitrones provides an efficient protocol for the construction of functionalized chiral 2,6-diazaspiro[3.4]octane-1,5-dione products in satisfactory yields and with high enantio- and diastereoselectivities.

Stereoselective Kinugasa/Aldol Cyclization: Synthesis of Enantioenriched Spirocyclic β-Lactams

We report an enantioselective copper-catalyzed Kinugasa/aldol domino reaction. This strategy enables access to a range of spirocyclic β-lactam pyrrolidinones in a stereoselective fashion. Under mild reaction conditions, prochiral alkyne-tethered ketones are coupled with nitrones to enable the facile construction of two spirofused ring systems containing three continuous stereocenters with excellent enantioselectivity. Also disclosed are post-transformation modifications demonstrating potential downstream functionalization of the spirocyclic molecules.

Recent Applications on Dual-Catalysis for C–C and C–X Cross-Coupling Reactions

Coupling reactions stand amid the most significant reactions in synthetic organic chemistry. Of late, these coupling strategies are being viewed as a versatile synthetic tool for a wide range of organic transformations in many sectors of chemistry, ranging from indispensable synthetic scaffolds and natural products of biological significance to novel organic materials. Further, the use of dual-catalysis in accomplishing various interesting cross-coupling transformations is an emerging field in synthetic organic chemistry, owing to their high catalytic performance rather than the use of a single catalyst. In recent years, synthetic organic chemists have given considerable attention to hetero-dual catalysis; wherein these catalytic systems have been employed for the construction of versatile carbon–carbon [C(sp 3)–C(sp 3), C(sp 3)–C(sp 2), C(sp 2)–C(sp 2)] and carbon–heteroatom (C–N, C–O, C–P, C–S) bonds. Therefore, in this mini-review, we are emphasizing recently developed various cross-coupling reactions catalysed by transition-metal dual-catalysis (i.e., using palladium and copper catalysts, but omitting the reports on photoredox/metal catalysis).

1 Introduction

2 Cu/Pd-Catalysed Bond Formation

2.1 Pd/Cu-Catalysed C(sp 3)–C(sp 2) Bond Formation

2.2 Pd/Cu-Catalysed C(sp 2)–C(sp 2) Bond Formation

2.3 Pd/Cu-Catalysed C(sp)–C(sp 2) Bond Formation

2.4 Pd/Cu-Catalysed C(sp 3)–C(sp 3) Bond Formation

2.5 Pd/Cu-Catalysed C–X (X = B, N, P, S, Si) Bond Formation

3 Conclusion

Mechanism of the Kinugasa Reaction Revisited

The mechanism of the Kinugasa reaction, that is, the copper-catalyzed formation of β-lactams from nitrones and terminal alkynes, is re-evaluated by means of density functional theory calculations and in light of recent experimental findings. Different possible mechanistic scenarios are investigated using phenanthroline as a ligand and triethylamine as a base. The calculations confirm that after an initial two-step cycloaddition promoted by two copper ions, the resulting five-membered ring intermediate can undergo a fast and irreversible cycloreversion to generate an imine and a dicopper-ketenyl intermediate. From there, the reaction can proceed through a nucleophilic attack of a ketenyl copper intermediate on the imine and an intramolecular cyclization, rather than through the previously suggested (2 + 2) Staudinger synthesis.

Development of TsDPEN based imine-containing ligands for the copper-catalysed asymmetric Kinugasa reaction

A novel class of chiral N,N,N imine-containing ligands derived from TsDPEN (N-(p-tosyl)-1,2-diphenylethylene-1,2-diamine) has been developed and applied to the copper-catalyzed asymmetric Kinugasa reaction. The copper(ii) salt proved to be an efficient catalyst precursor, and it provides an efficient way to synthesize enantioenriched cis-β-lactam. The pathway is air-tolerant and easily manipulated, and the ligands are easy to synthesize. A working model is proposed in which the stereocontrolling step is the [2 + 2] cycloaddition between ketene and imine to explain the observed stereoselectivities.

A novel class of chiral N,N,N imine-containing ligands derived from TsDPEN has been developed and applied to the copper-catalyzed asymmetric Kinugasa reaction.

Transition-Metal-Free Multicomponent Approach to Stereoenriched Cyclopentyl-isoxazoles through C-C Bond Cleavage

An efficient multicomponent reaction for the synthesis of stereoenriched cyclopentyl-isoxazoles from camphor-derived α-oximes, alkynes, and MeOH is reported. Our method involved a series of cascade transformations, including the in situ generation of an IIII catalyst, which catalyzed the addition of MeOH to a sterically hindered ketone. Oxidation of the oxime, and rearrangement of the α-hydroxyiminium ion generated a nitrile oxide in situ, which, upon [3+2] cycloaddition reaction with an alkyne, delivered the regioselective product. This reaction was very selective for the syn-oxime. This multicomponent approach was also extended to the synthesis of a new glycoconjugate, camphoric ester-isoxazole C-galactoside.

Recent advances in β-lactam synthesis

During the past century, β-lactams have been identified as the core of penicillin and since then several strategies have been developed for their synthesis.

Kinugasa reactions in water: from green chemistry to bioorthogonal labelling

The Kinugasa reaction has become an efficient method for the direct synthesis of β-lactams from substituted nitrones and copper(I) acetylides. In recent years, the reaction scope has been expanded to include the use of water as the solvent, and with micelle-promoted [3+2] cycloadditions followed by rearrangement furnishing high yields of β-lactams. The high yields of stable products under aqueous conditions render the modified Kinugasa reaction amenable to metabolic labelling and bioorthogonal applications. Herein, the development of methods for use of the Kinugasa reaction in aqueous media is reviewed, with emphasis on its potential use as a bioorthogonal coupling strategy.