Transition-Metal-Free Amine Oxidation: A Chemoselective Strategy for the Late-Stage Formation of Lactams

Design, synthesis and biological evaluation of aloperine derivatives as potential anticancer agents

Modifications at different positions on the aloperine molecule were performed to improve its anticancer activity and develop anticancer drugs. The in vitro anticancer activities of 44 synthesized compounds were evaluated. The effect of modification positions on anticancer activity was discussed and a structure-activity relationship analysis was established. A novel series of compounds with modifications at the N12 position showed much higher cytotoxicity than aloperine. Among them, compound 22 displayed promising in vitro anticancer activity against PC9 cells with a median inhibitory concentration (IC50) of 1.43 μM. The mechanism studies indicated that compound 22 induced cell apoptosis and cell cycle arrest in PC9 cells. These results demonstrate the potential of aloperine thiourea derivatives in anticancer activity.

Copper-Nitroxyl-Catalyzed α-Oxygenation of Cyclic Secondary Amines Including Application to Late-Stage Functionalization

Cyclic secondary amines are prominent subunits in pharmaceutical compounds. Methods for direct functionalization of N-unprotected/unsubstituted piperidines and related heterocycles have limited precedent despite their potential to impact medicinal chemistry and organic synthesis. Herein, we report a Cu/nitroxyl co-catalyzed method for direct conversion of cyclic secondary amines to the corresponding lactams via aerobic dehydrogenation and oxidative coupling with water. The mild reaction conditions tolerate diverse functional groups, enabling application to molecules that cover broad chemical space. The method is showcased in selective functionalization of building blocks and complex molecules, including late-stage functionalization of bromodomain inhibitors.

Efficient and scalable synthesis of 3,4-dihydroisoquinolin-1(2H)-ones by benzylic oxidation of tetrahydroisoquinoline derivatives using cerium ammonium nitrate (CAN)

An efficient and scalable method for the synthesis of 3,4-dihydroisoquinolin-1(2H)-ones through benzylic oxidation of tetrahydroisoquinoline derivatives using a catalytic amount of cerium ammonium nitrate (CAN) and a stoichiometric amount of NaBrO3 as oxidants was developed. The reaction is significantly influenced by the substituent groups on the phenyl ring. While electron-withdrawing groups on the phenyl ring can lower the reactivities of the substrates, electron-donating groups on the phenyl ring can dramatically promote the oxidation rate.

Late-stage modification of bioactive compounds: Improving druggability through efficient molecular editing

Synthetic chemistry plays an indispensable role in drug discovery, contributing to hit compounds identification, lead compounds optimization, candidate drugs preparation, and so on. As Nobel Prize laureate James Black emphasized, "the most fruitful basis for the discovery of a new drug is to start with an old drug"1. Late-stage modification or functionalization of drugs, natural products and bioactive compounds have garnered significant interest due to its ability to introduce diverse elements into bioactive compounds promptly. Such modifications alter the chemical space and physiochemical properties of these compounds, ultimately influencing their potency and druggability. To enrich a toolbox of chemical modification methods for drug discovery, this review focuses on the incorporation of halogen, oxygen, and nitrogen-the ubiquitous elements in pharmacophore components of the marketed drugs-through late-stage modification in recent two decades, and discusses the state and challenges faced in these fields. We also emphasize that increasing cooperation between chemists and pharmacists may be conducive to the rapid discovery of new activities of the functionalized molecules. Ultimately, we hope this review would serve as a valuable resource, facilitating the application of late-stage modification in the construction of novel molecules and inspiring innovative concepts for designing and building new drugs.

Cyclometalated Ruthenium Catalyst Enables Selective Oxidation of N-Substituted Tetrahydroquinolines to Lactams

Herein, we report an unusual α-methylene oxidation of N-substituted tetrahydroquinoline to lactams using the cyclometalated Ru(II)-complex as a catalyst. Cyclic-α-methylene C-H bonds are selectively oxidized under the reaction conditions even in the presence of α-methyl or reactive α-methylene C-H bonds. This methodology is also useful in the late-stage functionalization of pharmaceuticals. Mechanistic study demonstrates that the high-valent Ru(VI)-cis-dioxo species plays an important role in controlling selectivity.

Reshuffle Bonds by Ball Milling: A Mechanochemical Protocol for Charge-Accelerated Aza-Claisen Rearrangements

This study presents the development of a mechanochemical protocol for a charge-accelerated aza-Claisen rearrangement. The protocol waives the use of commonly applied transition metals, ligands, or pyrophoric Lewis acids, e.g., AlMe₃. Based on (heterocyclic) tertiary allylamines and acyl chlorides, the desired tertiary amides were prepared in yields ranging from 17% to 84%. Moreover, the same protocol was applied for a Belluš-Claisen-type rearrangement resulting in the synthesis of a 9-membered lactam without further optimization.

Regiospecific α-methylene functionalisation of tertiary amines with alkynes via Au-catalysed concerted one-proton/two-electron transfer to O2

Regioselective transformations of tertiary amines, which are ubiquitously present in natural products and drugs, are important for the development of novel medicines. In particular, the oxidative α-C-H functionalisation of tertiary amines with nucleophiles via iminium cations is a promising approach because, theoretically, there is almost no limit to the type of amine and functionalisation. However, most of the reports on oxidative α-C-H functionalisations are limited to α-methyl-selective or non-selective reactions, despite the frequent appearance of α-methylene-substituted amines in pharmaceutical fields. Herein, we develop an unusual oxidative regiospecific α-methylene functionalisation of structurally diverse tertiary amines with alkynes to synthesise various propargylic amines using a catalyst comprising Zn salts and hydroxyapatite-supported Au nanoparticles. Thorough experimental investigations suggest that the unusual α-methylene regiospecificity is probably due to a concerted one-proton/two-electron transfer from amines to O₂ on the Au nanoparticle catalyst, which paves the way to other α-methylene-specific functionalisations.

Iron Trichloride-Mediated Cascade Reaction of Aminosugar Derivatives for the Synthesis of Fused Tetrahydroisoquinoline-Tetrahydrofuran Systems

A method to obtain tetrahydroisoquinolines (THIQs) fused to tetrahydrofuran rings from aminosugar derivatives has been developed. The procedure relies on a key deprotection of benzyl ethers followed by a double-cyclization sequence, using FeCl₃ as the sole reagent. This tandem reaction affords the construction of novel fused polycyclic heterocycles with total stereochemical control.

On drug discovery against infectious diseases and academic medicinal chemistry contributions

This perspective is an attempt to document the problems that medicinal chemists are facing in drug discovery. It is also trying to identify relevant/possible, research areas in which academics can have an impact and should thus be the subject of grant calls. Accordingly, it describes how hit discovery happens, how compounds to be screened are selected from available chemicals and the possible reasons for the recurrent paucity of useful/exploitable results reported. This is followed by the successful hit to lead stories leading to recent and original antibacterials which are, or about to be, used in human medicine. Then, illustrated considerations and suggestions are made on the possible inputs of academic medicinal chemists. This starts with the observation that discovering a “good” hit in the course of a screening campaign still rely on a lot of luck – which is within the reach of academics –, that the hit to lead process requires a lot of chemistry and that if public–private partnerships can be important throughout these stages, they are absolute requirements for clinical trials. Concerning suggestions to improve the current hit success rate, one academic input in organic chemistry would be to identify new and pertinent chemical space, design synthetic accesses to reach these and prepare the corresponding chemical libraries. Concerning hit to lead programs on a given target, if no new hits are available, previously reported leads along with new structural data can be pertinent starting points to design, prepare and assay original analogues. In conclusion, this text is an actual plea illustrating that, in many countries, academic research in medicinal chemistry should be more funded, especially in the therapeutic area neglected by the industry. At the least, such funds would provide the intensive to secure series of hopefully relevant chemical entities which appears to often lack when considering the results of academic as well as industrial screening campaigns.

A Facile Oxidation of Tertiary Amines to Lactams by Using Sodium Chlorite: Process Improvement by Precise pH Adjustment with CO2

By using cheap and innocuous sodium chlorite, a series of tertiary amines have been oxidized to the corresponding lactams with good selectivity and high yield. In this method, neither transition-metal catalyst nor oxidant was used. In the oxidation step, the pH of the sodium chlorite was precisely adjusted to pH around 6 using CO2, such pH is a compromise between oxidative properties, chemical stability, and unwanted precipitation. In addition, buffer salts are not necessary, which allows this oxidation reaction to be performed under safe and environmentally benign conditions.

Green chemistry meets medicinal chemistry: a perspective on modern metal-free late-stage functionalization reactions

The progress of drug discovery and development is paced by milestones reached in organic synthesis. In the last decade, the advent of late-stage functionalization (LSF) reactions has represented a valuable breakthrough. Recent literature has defined these reactions as the chemoselective modification of complex molecules by means of C-H functionalization or the manipulation of endogenous functional groups. Traditionally, these diversifications have been accomplished by organometallic means. However, the presence of metals carries disadvantages related to their cost, environmental hazard and health risks. Fundamentally, green chemistry directives can help minimize such hazards through the development of metal-free LSF methodologies. In this review, we expand the current discussion on metal-free LSF reactions by providing an overview of C(sp2)-H, and C(sp3)-H functionalizations, as well as the utilization of heteroatom-containing functional groups as chemical handles. Selected topics such as metal-free cross-dehydrogenative coupling (CDC) reactions, organocatalysis, electrochemistry and photochemistry are also discussed. By writing the first review on metal-free LSF methodologies, we aim to highlight current advances in the field with examples that reveal specific challenges and solutions, as well as future research opportunities.

δ-Lactones-A New Class of Compounds That Are Toxic to E. coli K12 and R2-R4 Strains

Lactones are among the well-known organic substances with a specific taste and smell. They are characterized by antibacterial, antiviral, anti-inflammatory, and anti-cancer properties. In recent years, among this group of compounds, new biologically active substances have been searched by modifying the main (leading) structure with new analogs with stronger or different responses that may have a toxic effect on the cells of pathogenic bacteria and constitute an alternative to commonly used antibiotics. A preliminary study of δ-lactone derivatives as new potential candidates for antibacterial drugs was conducted. Particular emphasis was placed on the selection of the structure of lactones with the highest biological activity, especially those with fluorine in their structure as a substituent in terms of action on bacterial lipopolysaccharide (LPS) in the model strains of Escherichia coli K12 (without LPS in its structure) and R2–R4 (LPS of different lengths in its structure). In the presented studies, on the basis of the conducted MIC and MBC tests, it was shown that the antibacterial (toxic) activity of lactones depends on their structure and the length of the bacterial LPS in the membrane of specific strains. Moreover, oxidative damage of bacterial DNA isolated from bacteria after modification with newly synthesized compounds after application of the repair enzyme Fpg glycosylase was analyzed. The analyzed damage values were compared with the modification with appropriate antibiotics: ciprofloxacin, bleomycin, and cloxacillin. The presented research clearly shows that lactone derivatives can be potential candidates as substitutes for drugs, e.g., the analyzed antibiotics. Their chemical and biological activity is related to coumarin derivatives and the corresponding δ-lactone groups in the structure of the substituent. The observed results are of particular importance in the case of increasing bacterial resistance to various drugs and antibiotics, especially in nosocomial infections and neoplasms, and in the era of a microbial pandemic.

α-Oxygenation of N-aryl/alkyl heterocyclic compounds via ruthenium photocatalysis

We herein report ruthenium(iii) photocatalyzed oxidation of N-aryl tertiary amines to the corresponding amides.

Molecular iodine mediated oxidative cleavage of the C–N bond of aryl and heteroaryl (dimethylamino)methyl groups into aldehydes

An efficient iodine mediated conversion of aryl or heteroaryl (dimethylamino)methyl compunds to aryl or heteroaryl aldehydes is achieved via cleavage of C-N bond.

Revisiting applications of molecular iodine in organic synthesis

Molecular iodine contributes significantly to organic transformations in synthetic organic chemistry. It works effectively due to its mild Lewis acidic character, ability as an oxidizing agent, good moisture stability, and easy availability.

Mercury-Free Automated Synthesis of Guanidinium Backbone Oligonucleotides

A new method for synthesizing deoxynucleic guanidine (DNG) oligonucleotides that uses iodine as a mild and inexpensive coupling reagent is reported. This method eliminates the need for the toxic mercury salts and pungent thiophenol historically used in methods aimed at preparing DNG oligonucleotides. This coupling strategy was readily translated to a standard MerMade 12 oligonucleotide synthesizer with coupling yields of 95% and has enabled the synthesis of a 20-mer DNG oligonucleotide, the longest DNG strand to date, in addition to mixed DNA-DNG sequences with 3-9 DNG inserts. Importantly, DNG oligonucleotides exhibit robust unaided cellular uptake as compared to unmodified oligonucleotides without apparent cellular toxicity. Taken together, these findings should greatly increase the accessibility of cationic backbone modifications and assist in the development of oligonucleotide-based drugs.

Transition-Metal-Free Deconstructive Lactamization of Piperidines

One of the major challenges in organic synthesis is the activation or deconstructive functionalization of unreactive C(sp³ )-C(sp³ ) bonds, which requires using transition or precious metal catalysts. We present here an alternative: the deconstructive lactamization of piperidines without using transition metal catalysts. To this end, we use 3-alkoxyamino-2-piperidones, which were prepared from piperidines through a dual C(sp³ )-H oxidation, as transitory intermediates. Experimental and theoretical studies confirm that this unprecedented lactamization occurs in a tandem manner involving an oxidative deamination of 3-alkoxyamino-2-piperidones to 3-keto-2-piperidones, followed by a regioselective Baeyer-Villiger oxidation to give N-carboxyanhydride intermediates, which finally undergo a spontaneous and concerted decarboxylative intramolecular translactamization.

Pd-catalyzed asymmetric allylic substitution cascade using α-(pyridin-1-yl)-acetamides formed in situ as nucleophiles

A Pd-catalyzed asymmetric allylic substitution cascade reaction, using α-(pyridin-1-yl)-acetamides (formed in situ) as nucleophiles, has been developed, generating chiral piperidine-containing amino acid derivatives via a one-pot procedure in high yields and with up to 96% ee. The products can be easily converted into potential bioactive compounds, unnatural chiral amino acids and dipeptides.

Mechanistic Study of Ruthenium-Catalyzed C-H Hydroxylation Reveals an Unexpected Pathway for Catalyst Arrest

We have recently disclosed [(dtbpy)₂RuCl₂] as an effective precatalyst for chemoselective C-H hydroxylation of C(sp³)-H bonds and have noted a marked disparity in reaction performance between 4,4'-di- tert-butyl-2,2'-bipyridine (dtbpy)- and 2,2'-bipyridine (bpy)-derived complexes. A desire to understand the origin of this difference and to further advance this catalytic method has motivated the comprehensive mechanistic investigation described herein. Details of this reaction have been unveiled through evaluation of ligand structure-activity relationships, electrochemical and kinetic studies, and pressurized sample infusion high-resolution mass spectrometry (PSI-MS). Salient findings from this investigation include the identification of more than one active oxidant and three disparate mechanisms for catalyst decomposition/arrest. Catalyst efficiency, as measured by turnover number, has a strong inverse correlation with the rate and extent of ligand dissociation, which is dependent on the identity of bipyridyl 4,4'-substituent groups. Dissociated bipyridyl ligand is oxidized to mono- and bis- N-oxide species under the reaction conditions, the former of which is found to act as a potent catalyst poison, yielding a catalytically inactive tris-ligated [Ru(dtbpy)₂(dtbpy N-oxide)]²⁺ complex. Insights gained through this work highlight the power of PSI-MS for studies of complex reaction processes and are guiding ongoing efforts to develop high-performance, next-generation catalyst systems for C-H hydroxylation.

Bioinspired Oxidative Cyclization of the Geissoschizine Skeleton for the Total Synthesis of (-)-17-nor-Excelsinidine

We report the first total synthesis of (-)-17-nor-excelsinidine, a zwitterionic monoterpene indole alkaloid that displays an unusual N4-C16 connection. Inspired by the postulated biosynthesis, we explored an oxidative coupling approach from the geissoschizine framework to forge the key ammonium-acetate connection. Two strategies allowed us to achieve this goal, namely an intramolecular nucleophilic substitution on a 16-chlorolactam with the N4 nitrogen atom or a direct I₂ -mediated N4-C16 oxidative coupling from the enolate of geissoschizine.

Oxidative β-C-H sulfonylation of cyclic amines

A new and efficient process to access β-functionalisation of cyclic amines via a mild oxidative β-sulfonylation.

A transition metal-free strategy for the dehydrogenative β-sulfonylation of tertiary cyclic amines is described. N-Iodosuccinimide facilitates regioselective oxidative sulfonylation at C–H bonds positioned β to the nitrogen atom of tertiary amines, installing enaminyl sulfone functionality in cyclic systems. Mild reaction conditions, broad functional group tolerance and a wide substrate scope are demonstrated. The nucleophilic character of the enaminyl sulfone is harnessed, demonstrating potential application for scaffold diversification.

Metal-Free Thermal Activation of Molecular Oxygen Enabled Direct α-CH2-Oxygenation of Free Amines

Direct oxidation of α-CH₂ group of free amines is hard to achieve due to the higher reactivity of amine moiety. Therefore, oxidation of amines involves the use of sophisticated metallic reagents/catalyst in the presence or absence of hazardous oxidants under sensitive reaction conditions. A novel method for direct C-H oxygenation of aliphatic amines through a metal-free activation of molecular oxygen has been developed. Both activated and unactivated free amines were oxygenated efficiently to provide a wide variety of amides (primary, secondary) and lactams under operationally simple conditions without the aid of metallic reagents and toxic oxidants. The method has been applied to the synthesis of highly functionalized amide-containing medicinal drugs, such as O-Me-alibendol and -buclosamide.