Cyclobutanone Analogues of β‐Lactam Antibiotics: β‐Lactamase Inhibitors with Untapped Potential?

β‐Lactam antibiotics have been used for many years to treat bacterial infections. However the effective treatment of an increasing range of microbial infections is threatened by bacterial resistance to β‐lactams: the prolonged, widespread (and at times reckless) use of these drugs has spawned widespread resistance, which renders them ineffective against many bacterial strains. The cyclobutanone ring system is isosteric with β‐lactam: in cyclobutanone analogues, the eponymous cyclic amide is replaced with an all‐carbon ring, the amide N is substituted by a tertiary C?H α to a ketone. Cyclobutanone analogues of various β‐lactam antibiotics have been investigated over the last 35 years, initially as prospective antibiotics in their own right and inhibitors of the β‐lactamase enzymes that impart resistance to β‐lactams. More recently they have been tested as inhibitors of other serine proteases and as mechanistic probes of β‐lactam biosynthesis. Cyclobutanone analogues of the penam ring system are the first reversible inhibitors with moderate activity against all classes of β‐lactamase; other compounds from this family inhibit Streptomyces R61 dd ‐carboxypeptidase/transpeptidase, human neutrophil elastase and porcine pancreatic elastase. But has their potential as enzyme inhibitors been fully exploited? Challenges in synthesising diversely functionalised cyclobutanone derivatives mean that only a limited number have been made (with limited structural diversity) and evaluated. This review surveys the different synthetic approaches that have been taken to these compounds, the investigations made to evaluate their biological activity and prospects for future developments in this area.     

Diruthenium(I,I) Catalysts for the Formation of β‐ and γ‐Lactams via Carbenoid C?H Insertion of α‐Diazoacetamides

Intramolecular carbenoid C H insertion of five α‐diazoacetamides [N₂CH CONR₂, NR₂=NEt₂ ( 3a ), NBu₂ ( 3b ), N(i‐Pr)₂ ( 3c ), N(CH₂Ph)₂ ( 3d ), N(i‐Pr)(CH₂Ph) ( 3e )], was investigated using as catalysts dinuclear Ru(I,I) complexes of the type [Ru₂(μ‐L¹)₂(CO)₄L²₂], where L¹ is a bidentate bridging acetate, calix[4]arenedicarboxylate, saccharinate, pyridin‐2‐olate, or triazenide ligand, as well as [RuCl₂(p‐cymene)]₂. The Ru(I,I) complexes were found to be suitable catalysts for the carbenoid cyclization reactions, except in the case of 3a . With diazoamides 3b–e , [Ru₂(μ‐sac)₂(CO)₅]₂ (sac=saccharinate) and [Ru₂(μ‐6‐chloropyridin‐2‐olate)₂(CH₃CN)₂(CO)₄] are as effective as Rh₂(OAc)₄ under the same conditions, although some differences in the regioselectivity and chemoselectivity of the cyclization are observed. The carbenoid cyclization reactions yield γ‐lactams from diazoamides 3a and 3b , both a β‐ and a γ‐lactam from 3c , and a β‐lactam as well as a 3‐azabicyclo[5.3.0]deca‐5,7,9‐trien‐2‐one from 3d . With 3e , formation of γ‐lactam 21 and of bicyclic lactam 23 prevails.     

Organocatalytic Asymmetric C?S Bond Formation: Synthesis of α‐Methylene‐β‐mercapto Esters with Simple Alkyl Thiols

A highly efficient organocatalytic enantioselective C S bond formation reaction between simple alkyl thiols and Morita–Baylis–Hillman (MBH) carbonates is described. The optically active α‐methylene β‐mercapto esters could be obtained under mild reaction conditions with excellent enantioselectivities (up to 97% ee). The broad scope and simple operation make this methodology very practical.     

Enantioselective Fluorination of β‐Ketoamides Catalyzed by Ar‐BINMOL‐derived Salan?Copper Complex

A facile and powerful enantioselective construction of C F containing molecules was successfully developed through asymmetric fluorination of β‐ketoamides catalyzed by Ar‐BINMOL‐derived salan–Cu^II system (Ar‐BINMOL=1,1′‐Binaphthalene‐2‐α‐arylmethanol‐2′‐ol, Cu = copper). The present catalytic system exhibited excellent enantioselectivity and a broad substrate scope for indanone‐derived β‐ketoamides under mild conditions (up to 99 % ee and 99 % yields). Notably, the biomimetic salan‐copper complex was demonstrated for the first time to be a highly efficient catalyst in the fluorination of β‐ketoamides. Experimental results and mechanistic studies indicated that both excess amount of copper salt and electrophilic attack of cationic fluorine to activated methylene assisted by amide group on the β‐ketoamides were key factors for high yield and excellent enantioselectivity, respectively, in this enantioselective fluorination, which was controlled by the two‐point binding between the salan–copper complex with cyclic β‐ketoamides and hydrogen‐bonding activation of the electrophilic fluorinating reagent.

     

Palladium‐Catalyzed Decarboxylative Coupling of α‐ Oxocarboxylic Acids with C(sp2)?H of 2‐Aryloxypyridines

An efficient palladium‐catalyzed decarboxylative ortho‐acylation of 2‐aryloxypyridines with α‐oxocarboxylic acids is described. In this new transformation, the aromatic C(sp²) H bond was successfully acylated to give diverse aromatic ketones regioselectively in moderate to good yields. The pyridine group can be removed easily after the acylation to give the corresponding 2‐hydroxy aromatic ketones.     

α,β‐Unsaturated Aldehydes as Substrates for Asymmetric C?C Bond Forming Reactions with Thiamin Diphosphate (ThDP)‐Dependent Enzymes

The enzymes benzaldehyde lyase (BAL) from Pseudomonas fluorescens, benzoylformate decarboxylase (BFD) from Pseudomonas putida and pyruvate decarboxylase (PDC) from Saccharomyces cerevisiae provide different C C bond forming possibilities of α,β‐unsaturated aldehydes with aliphatic and aromatic aldehydes. Structure elucidation and determination of the absolute configuration of the products, which were obtained with high regio‐ and stereoselectivity were carried out. Selective 1,2‐reactivity with yields of 75% and >98% ee, for one single isomer ( A ) were obtained, by choosing the suitable enzyme in combination with the appropriate substrates. By varying enzymes or substrates the regioisomeric hydroxy ketones C , with up to >99% ee, can be obtained. The application of these new chiral building blocks in the synthesis of natural products or biological active substances is considerably facilitated by applying the different ThDP‐dependent enzymes as catalysts. Abbreviations: BAL, benzaldehyde lyase; BFD, benzoylformate decarboxylase; PDC, pyruvate decarboxylase; His, hexahistidine; 2‐HPP, 2‐hydroxy‐1‐phenylpropan‐1‐one; PAC, phenylacetylcarbinol; NTA, nitrilotriacetic acid; ThDP, thiamin diphosphate; wt, wild‐type.     

Iron(III)/Iodine‐Catalyzed C(sp2)?H Activation of α,β‐Unsaturated Aldehydes/Ketones with Amidines: Synthesis of 1,2,4,5‐Tetrasubstituted Imidazoles

An efficient methodology to access a library of 1,2,4,5‐tetrasubstituted imidazoles from a broad range of amidines and α,β‐unsaturated aldehydes/ketones via a C(sp²) H amination has been developed. This method represents a new and simple way to prepare highly substituted imidazoles from easily available starting materials, inexpensive catalysts, and with good functional group tolerance in good to excellent yields.

     

Mechanistic Switch via Subtle Ligand Modulation: Palladium‐Catalyzed Synthesis of α,β‐Substituted Styrenes via C?H Bond Functionalization

A new catalyst system able to efficiently perform the synthesis of styrenes via C H bond functionalization and a subtle ligand modification are described. The high level of activity achieved allows for the synthesis of highly functionalized α,β‐substituted styrenes, even the elusive E‐configured trisubstituted olefins, in a regio‐ and stereoselective manner. Mechanistic experiments allowed for the identification of the corresponding synthetic intermediates.     

Nickel‐Catalyzed α‐Benzylation of Arylacetonitriles via C?O Activation

Efficient Ni‐catalyzed direct cross‐couplings of benzylic alcohol derivatives with arylacetonitriles via C O activation are described. Various α‐benzylated arylacetonitriles including those with functional groups can be prepared under mild reaction conditions.

     

Copper Catalysts for Selective C?C Bond Cleavage of β‐O‐4 Lignin Model Compounds

The reactivity of homogeneous copper catalysts towards the selective C C bond cleavage of both phenolic and non‐phenolic arylglycerol β‐aryl ether lignin model compounds has been explored. Several copper precursors, nitrogen ligands, and solvents were evaluated in order to optimize the catalyst system. Using the optimized catalyst system, copper(I) trifluoromethanesulfonate [Cu(OTf)]/L/TEMPO (L=2,6‐lutidine, TEMPO=2,2,6,6‐tetramethyl‐piperidin‐1‐yl‐oxyl), aerobic oxidation of the non‐phenolic β‐O‐4 lignin model compound proceeded with good selectivity for C_α C_β bond cleavage, affording 3,5‐dimethoxybenzaldehyde as the major product. Aerobic oxidation of the corresponding phenolic β‐O‐4 lignin model proceeded with different selectivity, affording 2,6‐dimethoxybenzoquinone and α,β‐unsaturated aldehyde products resulting from cleavage of the C_α C_aryl bond. At low catalyst concentrations, however, a change in selectivity was observed as oxidation of the benzylic secondary alcohol predominated with both substrates.

     

C?H Functionalization of Enamides: Synthesis of β‐Amidovinyl Sulfones via Visible‐Light Photoredox Catalysis

A mild, practical and environmentally friendly strategy to prepare β‐amidovinyl sulfones has been developed based on the sulfonation of enamides or enecarbamates via visible‐light photoredox catalysis. Direct C H functionalizations of enamides or enecarbamates under the optimized conditions proceeded with a wide scope of substrates and remarkable selectivity to give functionalized vinyl sulfones with good to excellent yields.     

An Indium(III)‐Catalyzed Synthesis of 4,4‐Dichloro‐1‐aryl‐N‐alkyl‐1‐yn‐3‐amines via an Intermolecular C(sp2)?C(sp) Bond Formation

This paper demonstrates an indium(III) triflate‐catalyzed reaction of electron‐deficient α,α‐dichloroaldimines with terminal alkynes leading to a rapid and selective access to highly functionalized propargylic amines in good to excellent yields. The dichloromethylene moiety of the aldimine acts as an activating group to accomplish this transformation under very mild conditions.     

Should we be concerned about the dietary ω‐6:ω‐3 polyunsaturated fatty acid ratio?

Omega‐6 and ω‐3 polyunsaturated fatty acids compete at the active sites of enzymes responsible for Δ‐6 desaturation, incorporation into membrane phospholipids, release from membrane phospholipids and conversion into either prostaglandins or leukotrienes. Since prostaglandins are involved in the processes of platelet aggregation and inflammation then it has been suggested that ω‐6 and ω‐3 PUFA compete in vivo and that this has consequences for cell function and ultimately for health. It has been proposed that the dietary ω‐6: ω‐3 PUFA ratio should be used to measure this competition. Theoretical objections to the use of this ratio have recently been confirmed by hard scientific evidence showing that absolute amounts of ω‐6 and ω‐3 PUFA in the diet are more important than the ratio.     

Transition Metal‐Free Intermolecular α‐C?H Amination of Ethers at Room Temperature

We describe a new method for the intermolecular amination of the α‐C H bonds of ethers. A hypervalent iodine reagent was used as oxidant to enable the amination of cyclic and acyclic alkyl ethers with a wide range of amides, imides, and amines. The amination occurred at room temperature and without a transition metal catalyst. The method could be used to synthesize the anti‐cancer prodrug Tegafur and its analogues.

     

Iron‐Catalyzed Esterification of Benzyl C?H Bonds to Form α‐Keto Benzyl Esters

An atom‐economic and efficient non‐precious metal‐catalyzed esterification of benzyl C H bonds has been developed. A variety of α‐keto benzyl esters have been accessed in good yields through the reactions between benzyl derivatives and benzoylformic acids using iron trifluoride as a catalyst in the presence of di‐tert‐butyl peroxide under an inert atmosphere. This strategy provides a straightforward access to linearly expanded α‐keto benzyl esters. A plausible reaction mechanism is proposed.

     

Transition Metal‐Free α‐C(sp3)?H Bond Functionalization of Amines by Oxidative Cross Dehydrogenative Coupling Reaction: Simple and Direct Access to C‐4‐Alkylated 3,4‐Dihydroquinazoline Derivatives

A transition metal‐free catalytic system has been developed for the cross dehydrogenative coupling of amines with nitroalkanes under mild condition employing potassium iodide/tert‐butyl hydrogen peroxide catalytic system. This methodology was further extended for the construction of biologically important N‐heterocycles, namely, 3,4‐dihydroquinazoline derivatives. This novel strategy provides a simple, efficient, and direct access to 4‐alkyl‐3,4‐dihydroquinazoline derivatives.     

Highly Regioselective α‐Arylation of Coumarins via Palladium‐Catalyzed C?H Activation/Desulfitative Coupling

A novel regioselective α‐arylation of coumarins with readily available arenesulfonyl chlorides and sodium arenesulfinates via palladium‐catalyzed direct C H functionalizations under mild reaction conditions is described. This protocol presents an unexpected and highly regio‐controlled arylation of coumarins at C‐3 to construct interesting 3‐arylcoumarins with fascinating biological and fluorescent properties. The regioselectivity observed is in sharp contrast with that expected for the Heck reactions.

     

Are 1,4‐ and 1,5‐Disubstituted 1,2,3‐Triazoles Good Pharmacophoric Groups?

Over the last decade, 1,2,3‐triazoles have received increasing attention in medicinal chemistry thanks to the discovery of the highly useful and widely applicable 1,3‐dipolar cycloaddition reaction between azides and alkynes (click chemistry) catalyzed by copper salts and ruthenium complexes. After a decade of medicinal chemistry research on 1,2,3‐triazoles, we feel that the time is ripe to demonstrate the real ability of this heterocycle to participate in important and pivotal binding interactions with biological targets while maintaining a good pharmacokinetic profile. In this study, we retrieved and analyzed X‐ray crystal structures of complexes between 1,2,3‐triazoles and either proteins or DNA to understand the pharmacophoric role of the triazole. Furthermore, the metabolic stability, the capacity to inhibit cytochromes, and the contribution of 1,2,3‐triazoles to the overall aqueous solubility of compounds containing them have been analyzed. This information should furnish fresh insight for medicinal chemists in the design of novel bioactive molecules that contain the triazole nucleus.