Preparation of β-lactams by [2+2] cycloaddition of ketenes and imines

Synthesis and Antibacterial Activity of Novel Phosphonated CF3-β-lactams

A new series of C-3 phosphonated 4-CF3-β-lactams was stereoselectively synthesized from corresponding 4-CF3-β-lactams, applying two different protocols for phosphonate group incorporation. The first method involved the direct incorporation of a phosphonate (V) moiety at the C-3 position although it was limited by steric hindrance. The second approach enabled the incorporation of a less bulky phosphonite (III), which was subsequently oxidized to the corresponding phosphonate (V). The synthesized β-lactam ring features both fluorinated and phosphonate substituents, which are known for their biological significance, such as enhancing membrane permeability, improving binding interactions, and inhibiting enzymes. Considering these properties, along with the inherent antibacterial potential of β-lactams, we evaluated the antibacterial activity of selected C-3 phosphonated 4-CF3-β-lactams against four bacterial strains (Staphylococcus aureus (S. aureus), methicillin-resistant Staphylococcus aureus (MRSA), Neisseria gonorrheae, Escherichia coli (E. coli)). Applying the disk diffusion method, MIC measurements, and β-lactamase inhibition assays, compounds 11 and 16 emerged as the most promising candidates in this preliminary antibacterial evaluation.

Diverse Annulations of Alkyl(phenyl)phosphinic Chlorides and Imines

The reactivity of alkyl(aryl)phosphinic chlorides and imines was investigated in the presence of a base. The results indicated that allyl/benzyl(aryl)phosphinic chlorides gave rise to 1,2-azaphosphines (δ-phostims), while nonbenzylic alkyl(aryl)phosphinic chlorides produced the corresponding β-phosphinolactams (β-phostims) in their reaction with cyclic dibenzo[b,f][1,4]oxazepines, whereas steric electron-rich arylmethyl(aryl)phosphinic chlorides generated [2 + 2 + 2] annuloadducts in some cases. The current investigation provides useful insight into the reactivity of phosphinic chlorides and imines.

Forging structural complexity: diastereoselective synthesis of densely substituted β-lactams with dual functional handles for enhanced core modifications

The preparation of the β-lactam motif containing both C-Br and N-O bonds as functional handles remains an unmet synthetic challenge. Described herein is a novel and highly diastereoselective NBS-mediated cyclization of N-alkoxy α,β-unsaturated silyl imino ethers to furnish nearly three dozen α-bromo N-alkoxy β-lactams. The reaction gives rapid and convenient access to structurally diverse monocyclic, spirocyclic and fused β-lactams in moderate to good yields. The two functional handles were shown to be useful for the further elaboration of the β-lactam core.

Recent advances in the application of [2 + 2] cycloaddition in the chemical synthesis of cyclobutane-containing natural products

Cyclobutanes are distributed widely in a large class of natural products featuring diverse pharmaceutical activities and intricate structural frameworks. The [2 + 2] cycloaddition is unequivocally the primary and most commonly used method for synthesizing cyclobutanes. In this review, we have summarized the application of the [2 + 2] cycloaddition with different reaction mechanisms in the chemical synthesis of selected cyclobutane-containing natural products over the past decade.

Synthesis of β-Lactams through Carbonylation of Diazo Compounds Followed by the [2 + 2] Cycloaddition Reaction

Reporting an efficient method for the synthesis of β-lactams by the carbonylation of diazo compounds, using [Co2(CO)8] to corresponding ketenes, followed by [2 + 2] cycloaddition with imines. The newly developed strategy was successfully applied to electronically and structurally diverse substrates to produce the corresponding β-lactams under mild reaction conditions. Fourier transform infrared spectroscopy was employed to monitor ketene formation and the transformation of ketene into β-lactam. All the products were fully characterized by using various analytical and spectroscopic techniques.

Imines with rare α-heteroatom substituted amine components generated in situ via the Staudinger/aza-Wittig tandem reaction and their application in multicomponent reactions

A series of structurally diverse α-heteroatom substituted methyl azides (XCH2N3, where X = phthalimidoyl, benzotriazolyl, arylsulfanyl, aryloxy, alkoxy) have been prepared and evaluated for the in situ generation of imines via the Staudinger/aza-Wittig tandem reaction with aldehydes and triphenylphosphine. The obtained imines were successfully introduced into four types of multicomponent reactions: the Staudinger β-lactam synthesis with diazo carbonyl compounds, the Castagnoli-Cushman reaction with cyclic anhydrides, and the Ugi and azido-Ugi reactions with isocyanides and carboxylic acids or TMS-azides. These transformations allowed the preparation of four-to-seven-membered lactams, acyclic bisamides and 5-(aminomethyl)-1-alkyltetrazoles with a complex and previously poorly accessible periphery. Moreover, it was demonstrated that phthalimide derivatives can be deprotected to afford medicinally relevant N-aminomethyl lactams.

The Role of Surface Complexes in Ketene Formation from Fatty Acids via Pyrolysis over Silica: from Platform Molecules to Waste Biomass

Fatty acids (FA) are the main constituents of lipids and oil crop waste, considered to be a promising 2G biomass that can be converted into ketenes via catalytic pyrolysis. Ketenes are appraised as promising synthons for the pharmaceutical, polymer, and chemical industries. Progress in the thermal conversion of short- and long-chain fatty acids into ketenes requires a deep understanding of their interaction mechanisms with the nanoscale oxide catalysts. In this work, the interactions of fatty acids with silica are investigated using a wide range of experimental and computational techniques (TPD MS, DFT, FTIR, in situ IR, equilibrium adsorption, and thermogravimetry). The adsorption isotherms of linear and branched fatty acids C1-C6 on the silica surface from aqueous solution have been obtained. The relative quantities of different types of surface complexes, as well as kinetic parameters of their decomposition, were calculated. The formation of surface complexes with a coordination bond between the carbonyl oxygens and silicon atoms in the surface-active center, which becomes pentacoordinate, was confirmed by DFT calculations, in good agreement with the IR feature at ∼1680 cm 1. Interestingly, ketenes release relate to these complexes' decomposition as confirmed by the thermal evolution of the absorption band (1680 cm-1) synchronously with the TPD peak of the ketene molecular ion. The established regularities of the ketenezation are also observed for the silica-induced pyrolysis of glyceryl trimyristate and real waste, rapeseed meals.

Recent synthetic strategies of spiro-azetidin-2-one, -pyrrolidine, -indol(one) and -pyran derivatives-a review

Spiro-heterocycles have received special attention in medicinal chemistry because of their promising biological activity. Over the years, many synthetic methodologies have been established for the construction of spirocyclic compounds. Spiro heterocycles such as spiro-azetidin-2-one, -pyrrolidine, -indol(one) and -pyran derivatives have been found to exhibit diversified biological and pharmacological activity in addition to their therapeutic properties. In view of these facts, we decided in this review to present representative synthetic approaches of the aforementioned spiro heterocycles, especially in the past 20 years.

Lewis Acid Catalyzed Formal (3+2)-Cycloaddition of Bicyclo[1.1.0]butanes with Ketenes

Design, synthesis and application of benzene bioisosteres have attracted a lot of attention in the past 20 years. Recently, bicyclo[2.1.1]hexanes have emerged as highly attractive bioisosteres for ortho- and meta-substituted benzenes. Herein we report a mild, scalable and transition-metal-free protocol for the construction of highly substituted bicyclo[2.1.1]hexan-2-ones through Lewis acid catalyzed (3+2)-cycloaddition of bicyclo[1.1.0]-butane ketones with disubstituted ketenes. The reaction shows high functional group tolerance as documented by the successful preparation of various 3-alkyl-3-aryl as well as 3,3-bisalkyl bicyclo[2.1.1]hexan-2-ones (26 examples, up to 89 % yield). Postfunctionalization of the exocyclic ketone moiety is also demonstrated.

Photochemical Access to Substituted β-Lactams and β-Lactones via the Zimmerman-O'Connell-Griffin Rearrangement

A photomediated protocol giving facile access to substituted β-lactam and β-lactones is presented. The method realizes, for the first time, the use of the Zimmerman-O'Connell-Griffin (ZOG) rearrangement in [2 + 2] cycloaddition. Products are obtained in high yield with excellent diastereoselectivity under visible light irradiation and under mild reaction conditions.

Synthesis of functionalized 3-aryl-3H-benzofuranone derivatives from aryl acetate via [3 + 2] annulation of 1,4-dihydroxy-2-naphthoic acid ester

A novel [3 + 2] cascade reaction of aryl acetate with 1,4-dihydroxy-2-naphthoic acid ester by cooperative catalysis of isothiourea and vanadium is developed. The key step of the reaction is that the C1-ammonium enolates generated from aryl acetate will cycloadduct to 1,4-naphthoquinone intermediates. 3-Aryl-3H-benzofuranone derivatives can be prepared in medium to good yields under mild conditions.

One-pot synthesis of cyclic-aminotropiminium carboxylate derivatives with DNA binding and anticancer properties

Tropolone, a nonbenzenoid aromatic molecule, is a constituent of troponoid natural products possessing a wide range of bioactivities, including anticancer. This report describes the one-pot synthesis and mechanistic studies of fifteen fluorescent C^aryl-N^alkyl-substituted cyclic-aminotroponiminium carboxylate (cATC) derivatives by unusual cycloaddition and rearrangement reactions. Herein, the biochemical studies of four cATC derivatives reveal a non-intercalative binding affinity with DNA duplex. In vitro/in vivo studies show strong anti-tumor activity in three cATC derivatives. These derivatives enter the cells and localize to the nucleus and cytoplasm, which are easily traceable due to their inherent fluorescence properties. These three cATC derivatives reduce the proliferation and migration of HeLa cells more than the non-cancer cell line. They induce p38-p53-mediated apoptosis and inhibit EMT. In xenograft-based mouse models, these cATC derivatives reduce tumor size. Overall, this study reports the synthesis of DNA binding fluorescent C^aryl-N^alkyl-cyclic-aminotroponiminium derivatives which show anti-tumor activity with the minimum side effect.

In situ generation of imines by the Staudinger/aza-Wittig tandem reaction combined with thermally induced Wolff rearrangement for one-pot three-component β-lactam synthesis

A new efficient protocol for diastereoselective three-component one-pot lactam synthesis involving the in situ generation of imines via the Staudinger/aza-Wittig tandem reaction combined with the Wolff-rearrangement and ketene-imine cycloaddition was developed to produce a series of 24 novel structurally diverse β-lactam- or 1,3-oxazine-products. It was shown that this synthesis can be performed both as a two step-procedure and true MCR with simultaneous loading of all reactants. The intramolecular version of the 1^st step provided facile access to seven-membered cyclic imines, which allowed further preparation of a series of rare tricyclic β-lactams. For the intermolecular version of the 1^st step (acyclic imine generation), it was shown that the outcome of the synthesis is different from that using pre-synthesized and isolated imines. Additionally, this is the first example of the implementation of the Staudinger/aza-Wittig tandem reaction for the preparation of four-membered heterocycles.

Palladium-Catalyzed Chemodivergent Carbonylation of ortho-Bromoarylimine to Biisoindolinones and Spiroisoindolinones

We herein report a palladium-catalyzed carbonylative cyclization reaction of ortho-bromoarylimines that allows for the chemodivergent synthesis of functionalized biisoindolinones and spirocyclic isoindolinones. Either product could be selectively obtained by switching the reaction temperatures and ligands, and the biisoindolinone products could be afforded facilely with catalyst loadings as low as 0.05 mol %. Further transformation of the biisoindolinone product is also described, which represents a novel and concise approach to the biisoindoline diamine ligand.

1,4-Aryl migration in ketene-derived enolates by a polar-radical-crossover cascade

The arylation of carboxylic acid derivatives via Smiles rearrangement has gained great interest in recent years. Both radical and ionic approaches, as well as radical-polar crossover concepts, have been developed. In contrast, a reversed polar-radical crossover approach remains underexplored. Here we report a simple, efficient and scalable method for the preparation of sterically hindered and valuable α-quaternary amides via a polar-radical crossover-enolate oxidation-aryl migration pathway. A variety of easily accessible N-alkyl and N-arylsulfonamides are reacted with disubstituted ketenes to give the corresponding amide enolates, which undergo upon single electron transfer oxidation, a 1,4-aryl migration, desulfonylation, hydrogen atom transfer cascade to provide α-quaternary amides in good to excellent yields. Various mono- and di-substituted heteroatom-containing and polycyclic arenes engage in the aryl migration reaction. Functional group tolerance is excellent and substrates as well as reagents are readily available rendering the method broadly applicable.

The α-arylation of amides via aryl migration has attracted considerable interest in recent years. Here, the authors report a method for the preparation of bulky α-quaternary amides via a polar-radical crossover enolate oxidation-aryl migration cascade.

Oxindole synthesis via polar-radical crossover of ketene-derived amide enolates in a formal [3 + 2] cycloaddition

Herein we introduce a simple, efficient and transition-metal free method for the preparation of valuable and sterically hindered 3,3-disubstituted oxindoles via polar-radical crossover of ketene derived amide enolates. Various easily accessible N-alkyl and N-arylanilines are added to disubstituted ketenes and the resulting amide enolates undergo upon single electron transfer oxidation a homolytic aromatic substitution (HAS) to provide 3,3-disubstituted oxindoles in good to excellent yields. A variety of substituted anilines and a 3-amino pyridine engage in this oxidative formal [3 + 2] cycloaddition and cyclic ketenes provide spirooxindoles. Both substrates and reagents are readily available and tolerance to functional groups is broad.

Diastereoselective formation of β-lactams via a three-component reaction

A highly diastereoselective three-component reaction of N-hydroxyanilines, diazo compounds and cyclobutenones to form densely functionalized β-lactams has been realized.

Synthesis of β-Phosphinolactams from Phosphenes and Imines

Various cis-β-phosphinolactams are synthesized stereoselectively for the first time from imines and diazo(aryl)methyl(diaryl)phosphine oxides under microwave irradiation. Diazo(aryl)methyl(diaryl)phosphine oxides first undergo the Wolf rearrangement to generate phosphenes. Imines nucleophilically attack the phosphenes followed by an intramolecular nucleophilic addition via less steric transition states to give final cis-β-phosphinolactams. C-Styrylimines generally give rise to β-phosphinolactams in higher yields than C-arylimines. The stereoselectivity and proposed mechanism are rationalized by DFT theoretical calculation.

Photoredox Catalytic Phosphite-Mediated Deoxygenation of α-Diketones Enables Wolff Rearrangement and Staudinger Synthesis of β-Lactams

A novel visible-light-driven catalytic activation of C=O bonds by exploiting the photoredox chemistry of 1,3,2-dioxaphospholes, readily accessible from α-diketones and trialkyl phosphites, is reported. This mild and environmentally friendly strategy provides an unprecedented and efficient access to the Wolff rearrangement reaction which traditionally entails α-diazoketones as precursors. The resulting ketenes could be precisely trapped by alcohols/thiols to give α-aryl (thio)acetates and by imines to afford the valuable β-lactams in up to 99 % yields.

Visible-Light-Induced Cycloaddition of α-Ketoacylsilanes with Imines: Facile Access to β-Lactams

We report the synthesis of β-lactams from α-ketoacylsilanes and imines, which proceeds via a formal [2+2] photochemical cycloaddition with in situ generation of siloxyketene. This mild and operationally simple reaction proceeds in an atom-economic fashion with broad substrate scope, including aldimines, ketimines, hydrazones, and fused nitrogen heterocycles, affording a variety of important β-lactams with satisfactory diastereoselectivities in most cases. This reaction also features good functional-group tolerance, facile scalability and product diversification. Experimental and computational studies suggest that α-ketoacylsilanes can serve as photochemical precursors by engaging in a 1,3 silicon shift to the distal carbonyl group.

The facile and efficient synthesis of novel monocyclic cis-β-lactam conjugates with a 1-methyl-1H-imidazole-2-thiol nucleus

We developed a facile and efficient synthesis of novel monocyclic cis-β-lactam conjugates with a 1-methyl-1H-imidazole-2-thiol nucleus through a ketene-imine [2+2] cycloaddition reaction of acyl chloride and different Schiff bases.

Transition metal-free and solvent-free calcium carbide promotes the formation of β-keto sulfoxide from acyl chloride and DMSO

A transition metal-free, solvent-free, cheap and readily available calcium carbide promoted one-pot reaction of acid chloride with dimethyl sulfoxide produced β-keto sulfoxide directly.

Flow Chemistry for Cycloaddition Reactions

Continuous flow reactors form part of a rapidly growing research area that has changed the way synthetic chemistry is performed not only in academia but also at the industrial level. This Review highlights the most recent advances in cycloaddition reactions performed in flow systems. Cycloadditions are atom-efficient transformations for the synthesis of carbo- and heterocycles, involved in the construction of challenging skeletons of complex molecules. The main advantages of translating these processes into flow include using intensified conditions, safer handling of hazardous reagents and gases, easy tuning of reaction conditions, and straightforward scaling up. These benefits are especially important in cycloadditions such as the copper(I)-catalyzed azide alkyne cycloaddition (CuAAC), Diels-Alder reaction, ozonolysis and [2+2] photocycloadditions. Some of these transformations are key reactions in the industrial synthesis of pharmaceuticals.

A Comprehensive Review on C-3 Functionalization of β-Lactams

BACKGROUND

A lot of advancement has been made in the area of β-lactams in recent times. Most of the research is targeted towards the synthesis of novel β-lactams, their functionalization and exploring their biological potential. The C-3 functionalization of β-lactams has continued to attract considerable interest of the scientific community due to their utility as versatile intermediates in organic synthesis and their therapeutic applications. This has led to the significant increase in efforts towards developing efficient and economic strategies for C-3 functionalized β-lactams.

OBJECTIVE

The present review aims to highlight recent advancement made in C-3 functionalization of β - lactams.

CONCLUSION

To summarize, functionalization of β-lactams at C-3 is an essential aspect of β-lactam chemistry in order to improve/modify its synthetic utility as well as biological potential. The C-3 carbocation equivalent method has emerged as an important and convenient strategy for C-3 functionalization of β-lactam heterocycles which provides a wide range of β-lactams viz. 3-alkylated β-lactams, 3-aryl/heteroarylated β-lactams, 3- alkoxylated β-lactams. On the other hand, base mediated functionalization of β-lactams via carbanion intermediate is another useful approach but their scope is limited by the requirement of stringent reaction conditions. In addition to this, organometallic reagent mediated α-alkylation of 3-halo/3-keto-β-lactams also emerged as interesting methods for the synthesis of functionalized β-lactams having good yields and diastereoselectivities.

Microwave-assisted periselective annulation of triarylphosphenes with aldehydes and ketones

The reaction of diazo(aryl)methyl(diaryl)phosphine oxides with aldehydes and ketones generates benzo-δ-phosphinolactones in low to good yields with 1,1-diarylalk-1-enes as byproducts under microwave irradiation. Diazo(aryl)methyl(diaryl)phosphine oxides first undergo a Wolff rearrangement to form diaryl(aryl)phosphenes, which further react with aldehydes and ketones to afford benzo-δ-phosphinolactones and β-phosphinolactones. The latter are unstable under heating and fragment into the corresponding 1,1-diarylalk-1-enes and arylphosphine dioxides under reaction conditions. The arylphosphine dioxides become arylphosphonic acids during workup. The periselectivity in the annulation shows that the reaction of diaryl(aryl)phosphenes with most aldehydes and ketones favors phosphene phenyl participation in (4 + 2) annulation over (2 + 2) annulation.

Diastereoselectivity Switch in the Pd(0)/InI-Mediated Reactions of β-Lactams with Aldehydes: An Entry into Nonracemic Semiprotected (3E)-2,6-Enediols

The switch from N-methylimidazole (N-MI) to the Et₃N N-ligand efficiently alters diastereoselectivity in Pd(0)/InI-mediated allylations of aldehydes with β-lactam-derived organoindiums. As a result, (3E)-selective allylations and crotylations of a variety of aliphatic and (hetero)aromatic aldehydes with differently substituted chiral ε-amido-allylindiums were developed. Depending on the relative β-lactam configuration, the reactions occur under kinetic or thermodynamic control, with effective remote 1,5- or 1,4,5-asymmetric induction to afford a diversity of previously unavailable (3E)-2,5-syn-2,6-anti-, (3E)-2,5-anti-2,6-anti-, and (3E)-2,5-anti-2,6-syn-substituted enediols, amino alcohols, and homoallylic alcohols in modest to high yields and with moderate to excellent diastereoselectivity. The effect of the N-ligand as well as β-lactam and aldehyde structures on the yield and stereoselectivity was investigated.

[2 + 1 + 1] Assembly of spiro β-lactams by Rh(ii)-catalyzed reaction of diazocarbonyl compounds with azirines/isoxazoles

A domino synthesis of dispiro-fused N-vinyl β-lactams from diazo-Meldrum's acid and 2H-azirines or 5-alkoxyisoxazoles via the "2-azabuta-1,3-diene formation/Staudinger ketene-imine cycloaddition" sequence is described. The Rh2(Piv)4-catalyzed formation of the 2-azabuta-1,3-diene intermediates in the first stage of the reaction sequence proceeds via addition of the rhodium carbenoid to the azirines/isoxazoles and provides the first example of the generation of iminium-type ylides from diazo-Meldrum's acid. This methodology was extended to monospiro-β-lactams, which were synthesized in two steps using acyclic α-diazocarbonyl compounds in the stage of the formation of 2-azabuta-1,3-dienes. The method allows for the rapid assemblage of the carbon part of the azetidin-2-one system from diazo compounds only and affords spiro- and dispiro-β-lactams in moderate yields. A bromine atom in the 2-bromoalkenyl moiety of the synthesized β-lactams can be easily substituted by hydrogen under catalytic hydrogenation conditions or by 2-pyridyl-substituent via the Stille cross-coupling reaction.

Remote 1,5-Stereoselectivity Control by an N-Ligand Switch in the Pd(0)/InI-Promoted Reactions of 4-Ethynyl-β-lactams with Aldehydes

Configurationally stable ε-amido-propargylindiums, generated in situ from N-Ts-4-ethynylazetidin-2-ones in the presence of InI, catalytic amounts of Pd(PPh₃)₄, and N-methylimidazole or pyridine ligand react with unusual regioselectivity with aromatic and aliphatic aldehydes to afford 2,6- syn- or 2,6- anti-allenediols with excellent central-to-axial chirality transfer and useful levels of acyclic remote 1,5-stereocontrol.

Exploring the generation and use of acylketenes with continuous flow processes

The generation and use of acyl ketenes under continuous flow reaction conditions is reported. Several reaction classes of these reactive intermediates have been studied. Under zero headspace conditions, a ketone exchange process is possible between volatile ketones. The process can be readily scaled to deliver gram quantities of product.

Acyclic Remote 1,5- and 1,4,5-Stereocontrol in the Catalytic Stereoselective Reactions of β-Lactams with Aldehydes: The Effect of the N-Methylimidazole Ligand

The application of the N-methylimidazole ( N-MI) ligand in the Pd(0)/InI-promoted allylations of aldehydes with β-lactam-derived organoindiums enables the reaction of azetidin-2-ones with diversely substituted allyl moieties, inert under previously reported conditions. As a result, allylations and crotylations of a variety of aromatic and aliphatic aldehydes with previously unavailable chiral ε-amido-allylindiums bearing α-, β-, or γ-substituted allyl fragments were developed. The reactions occur under thermodynamic control with a highly efficient remote 1,5- or 1,4,5-stereocontrol to afford a diversity of (3 Z)-2,5- anti-2,6- syn- or (3 Z)-2,5- syn-2,6- anti-substituted enediols, aminoalcohols, and homoallylic alcohols in moderate to high yields and with an excellent diastereoselectivity. A detailed study on the effect of the β-lactam and aldehyde structures and chirality on the yield and stereochemistry in the products was carried out.

3-Homoacyl coumarin: an all carbon 1,3-dipole for enantioselective concerted (3+2) cycloaddition

A new type of all-carbon 1,3-dipole precursor, 3-homoacylcoumarin, was employed for the stereoselective (3+2) cycloaddition with indandione alkylidenes to generate a series of coumarin/indandione-fused spirocyclopentanes bearing four contiguous stereogenic centers. While two reaction pathways progressed simultaneously, detailed mechanistic investigation revealed that the highly efficient stereoselective concerted route dominated the extremely slow stepwise pathway.