Process Development for the Synthesis of Monocyclic β-Lactam Core 17

Electronic origins of the stereochemistry in β-lactam formed through the Staudinger reaction catalyzed by a nucleophile

This paper evaluates the electronic effects of molecular substituents on the stereoselectivity of the umpolung Staudinger catalytic reaction. This is especially important because experimental studies on constructing the β-lactam ring, a core structure of most antibiotics, through catalyzed Staudinger reactions have been massively progressing over the last century. Yet, there is a necessity for an in-depth understanding of the reaction mechanisms to help chemists, working on the well-established discoveries, improve these to optimize the stereoselectivity and yield of synthetic methods. Access to practical and effective advancements in forming optically pure β-lactam is paramount in the field of medical chemistry. This paper specifically investigates how changing the N-protecting group in the imine fragment can switch the stereoselectivity of the PPY-catalyzed Staudinger reaction. To do so, we employed the density functional theory (DFT) for geometry optimization and electronic analysis at the B3LYP/6-31G(d) level of theory to examine and compare the role of N-tosyl (N-Ts) and N-triflyl (N-Tf) imine on the mechanism pathways, i.e., imine-first or ketene-first, and stereochemistry of the reaction, i.e., cis or trans β-lactam. Our results show that the reaction mechanism pathway cannot be simply switched from ketene-first to imine-first by changing the substituent on the imine nitrogen atom, which is contrary to the reported experimental results, and both imines go through the ketene-first mechanism with different stereochemistries, which is cis selective for imine-Ts and trans selective for imine-Tf. Based on electronic analyses, the reversal in diastereoselectivity in the N-triflyl imine system could be attributed to the charge transfers and electron-density distribution over the transition states. Therefore, the cis/trans selectivity of the PPY-catalyzed Staudinger reaction could be effectively controlled by the electronic characteristics of the molecular substituents in the reactants. A N-protecting group in imine with a more electron-withdrawing nature seems to accelerate the stereo-determining step, ring closure, and increase the stabilization charge transfers in the transition state, leading to a preference for trans β-lactam formation. It seems that using a N-substituent with a higher electron-withdrawing nature can initially activate the imine by the nucleophilic catalyst in competition with ketene, i.e., imine-first versus ketene-first. These results can provide an insight into select proper substituents for the fragments to synthesis β-lactam with a desired stereochemistry. Also, a comprehensive comparison was performed between calculations with and without dispersion.

Synthesis of α-CF3 Amides via Palladium-Catalyzed Carbonylation of 2-Bromo-3,3,3-trifluoropropene

Amide compounds are important organic compounds, which play an important role in biomedical chemistry, materials science, life science, and other fields. The synthesis of α-CF₃ amides, especially compounds containing 3-(trifluoromethyl)-1,3,4,5-tetrahydro-2H-benzo[b][1,4]diazepine-2-one, has long been a challenge due to the tensile properties and instability of the rings. Here, we report an example of palladium-catalyzed carbonylation of CF₃-containing olefin to form α-CF₃ acrylamide. By controlling the ligands, we can get different amide compounds as products. This method has good substrate adaptability and functional group tolerance.

Synthesis of 3-Amino-4-substituted Monocyclic ß-Lactams—Important Structural Motifs in Medicinal Chemistry

Monocyclic ß-lactams (azetidin-2-ones) exhibit a wide range of biological activities, the most important of which are antibacterial, anticancer, and cholesterol absorption inhibitory activities. The synthesis of decorated monocyclic ß-lactams is challenging because their ring is highly constrained and consequently reactive, which is also an important determinant of their biological activity. We present the optimized synthesis of orthogonally protected 3-amino-4-substituted monocyclic ß-lactams. Among several possible synthetic approaches, Staudinger cycloaddition proved to be the most promising method for initial ring formation, yielding monocyclic ß-lactams with different substituents at the C-4 position, a phthalimido-protected 3-amino group, and a (dimethoxy)benzyl protected ring nitrogen. Challenging deprotection methods were then investigated. Oxidative cleavage with cerium ammonium nitrate and ammonia-free Birch reduction was found to be most effective for selective removal of ring nitrogen protection. Hydrazine hydrate was used for deprotection of the phthalimido group, and the procedure had to be modified by the addition of HCl in the case of aromatic substituents at the C-4 position. The presented methods and the synthesized 3-amino-4-substituted monocyclic ß-lactam derivatives are an important step toward new ß-lactams with potential pharmacological activities.

Optimization of TopoIV Potency, ADMET Properties, and hERG Inhibition of 5-Amino-1,3-dioxane-Linked Novel Bacterial Topoisomerase Inhibitors: Identification of a Lead with In Vivo Efficacy against MRSA

Novel bacterial topoisomerase inhibitors (NBTIs) are among the most promising new antibiotics in preclinical/clinical development. We previously reported dioxane-linked NBTIs with potent antistaphylococcal activity and reduced hERG inhibition, a key safety liability. Herein, polarity-focused optimization enabled the delineation of clear structure-property relationships for both microsomal metabolic stability and hERG inhibition, resulting in the identification of lead compound 79. This molecule demonstrates potent antibacterial activity against diverse Gram-positive pathogens, inhibition of both DNA gyrase and topoisomerase IV, a low frequency of resistance, a favorable in vitro cardiovascular safety profile, and in vivo efficacy in a murine model of methicillin-resistant Staphylococcus aureus infection.

A general strategy for the synthesis of α-trifluoromethyl- and α-perfluoroalkyl-β-lactams via palladium-catalyzed carbonylation

β-Lactam compounds play a key role in medicinal chemistry, specifically as the most important class of antibiotics. Here, we report a novel one-step approach for the synthesis of α-(trifluoromethyl)-β-lactams and related products from fluorinated olefins, anilines and CO. Utilization of an advanced palladium catalyst system with the Ruphos ligand allows for selective cycloaminocarbonylations to give diverse fluorinated β-lactams in high yields.

One-Pot Construction of Diverse β-Lactam Scaffolds via the Green Oxidation of Amines and Its Application to the Diastereoselective Synthesis of β-Amino Acids

In this study, a simple one-pot construction of β-lactam scaffolds was successfully achieved via 4,6-dihydroxysalicylic acid-catalyzed organocatalytic oxidation of amines to imines using molecular oxygen. Although some imines are highly unstable and difficult to isolate by conventional methods, the organocatalytic oxidation of amines described herein, followed by their direct reaction with acyl chlorides in the presence of a base, afforded a series of new β-lactam derivatives with excellent cis selectivity, which could not be synthesized and isolated by previously reported methods. Thus, this one-pot protocol will be one of the powerful methods applicable to the synthesis of various potential drug candidates and functional molecules. Furthermore, the subsequent hydrolysis of these β-lactams successfully afforded the corresponding β-amino acids as almost single diastereomers in up to 99% yields.

Novel bacterial topoisomerase inhibitors derived from isomannide

A series of Novel Bacterial Topoisomerase Inhibitors (NBTIs) employing a linker derived from isomannide were synthesized and evaluated. Reduced hERG inhibition was observed compared to structure-matched analogues with different linkers, and compound 6 showed minimal proarrhythmic potential using an in vitro panel of cardiac ion channels. Compound 6 also displayed excellent activity against fluoroquinolone-resistant MRSA (MIC₉₀ = 2 μg/mL) and other Gram-positive pathogens.

Monobactams: A Unique Natural Scaffold of Four-Membered Ring Skeleton, Recent Development to Clinically Overcome Infections by Multidrug- Resistant Microbes

Background:

Monobactam antibiotics have been testified to demonstrate significant antibacterial activity especially the treatment of infections by superbug microbes. Recently, research has been focused on the structural modifications, and new generation of this privileged natural scaffold.

Objective:

Efforts have been made to discover the structure-antibacterial relationship of monbactams in order to avoid the aimless work involving the ongoing generated analogues. This review aims to summarize the current knowledge and development of monobactams as a broad-spectrum antibacterial scaffolds. The recent structural modifications that expand the activity, especially in the infections by resistant-strains, combinational therapies and dosing, as well as the possibility of crosshypersensitivity/ reactivity/tolerability with penicillins and cephalosporins will also be summarized and inferred. Different approaches will be covered with emphasis on chemical methods and Structure- Activity Relationship (SAR), in addition to the proposed mechanisms of action. Clinical investigation of monobactams tackling various aspects will not be missed in this review.

Conclusion:

The conclusion includes the novels approaches, that could be followed to design new research projects and reduce the pitfalls in the future development of monobactams.

Total Synthesis of (-)-Himeradine A

(-)-Himeradine A is a complex lycopodium alkaloid with seven rings and ten stereogenic centers that shows anticancer activity against lymphoma L1210 cells. A total synthesis has been developed that builds off prior work on (+)-fastigiatine. A 2,4,6-trisubstitited piperidine ring forms the core of the quinolizidine segment, and was prepared by diastereoselective reduction of a pyridine and classic resolution of an intermediate. The remaining secondary amine was introduced with a catalyst-controlled Overman rearrangement. The piperidine segment was coupled in a B-alkyl Suzuki reaction with a bicyclic bromoenone, which was a key intermediate for the synthesis of (+)-fastigiatine. The final transformation featured a transannular Mannich reaction and cyclization to complete the quinolizidine. Five bonds and four new rings were generated in this one-pot procedure. (-)-Himeradine A was prepared in 17 steps in the longest linear sequence.

1,3-Dioxane-Linked Bacterial Topoisomerase Inhibitors with Enhanced Antibacterial Activity and Reduced hERG Inhibition

The development of new therapies to treat methicillin-resistant Staphylococcus aureus (MRSA) is needed to counteract the significant threat that MRSA presents to human health. Novel inhibitors of DNA gyrase and topoisomerase IV (TopoIV) constitute one highly promising approach, but continued optimization is required to realize the full potential of this class of antibiotics. Herein, we report further studies on a series of dioxane-linked derivatives, demonstrating improved antistaphylococcal activity and reduced hERG inhibition. A subseries of analogues also possesses enhanced inhibition of the secondary target, TopoIV.