Active Molecular Gripper as a Macrocycle Synthesizer

A confined space preorganizes substrates, which substantially changes their chemical reactivity and selectivity; however, the performance as a reaction vessel is hampered by insensitivity to environmental changes. Here, we show a dynamic confined space formed by substrate grasping of an amphiphilic host with branched aromatic arms as an active molecular gripper capable of performing substrate grasping, macrocyclization, and product release acting as a macrocycle synthesizer. The confined reaction space is formed by the substrate grasping of the molecular gripper, which is further stabilized by gel formation. Confining a linear substrate in the closed form of the gripper triggers a spontaneous ring-forming reaction to release a macrocycle product by opening. The consecutive open-closed switching enables repetitive tasks to be performed with remarkable working efficiency.

Macrocycles and macrocyclization in anticancer drug discovery: Important pieces of the puzzle

Increasing disease-related proteins have been identified as novel therapeutic targets. Macrocycles are emerging as potential solutions, bridging the gap between conventional small molecules and biomacromolecules in drug discovery. Inspired by successful macrocyclic drugs of natural origins, macrocycles are attracting more attention for enhanced binding affinity and target selectivity. Due to the conformation constraint and structure preorganization, macrocycles can reach bioactive conformations more easily than parent acyclic compounds. Also, rational macrocyclization combined with sequent structural modification will help improve oral bioavailability and combat drug resistance. This review introduces various strategies to enhance membrane permeability in macrocyclization and subsequent modification, such as N-methylation, intramolecular hydrogen bonding modulation, isomerization, and reversible bicyclization. Several case studies highlight macrocyclic inhibitors targeting kinases, HDAC, and protein-protein interactions. Finally, some macrocyclic agents targeting tumor microenvironments are illustrated.

The Recent Advances in Iron-Catalyzed C(sp3 )-H Functionalization

The use of iron as a core metal in catalysis has become a research topic of interest over the last few decades. The reasons are clear. Iron is the most abundant transition metal on Earth's crust and it is widely distributed across the world. It has been extracted and processed since the dawn of civilization. All these features render iron a noncontaminant, biocompatible, nontoxic, and inexpensive metal and therefore it constitutes the perfect candidate to replace noble metals (rhodium, palladium, platinum, iridium, etc.). Moreover, direct C-H functionalization is one of the most efficient strategies by which to introduce new functional groups into small organic molecules. The majority of organic compounds contain C(sp3 )-H bonds. Given the enormous importance of organic molecules in so many aspects of existence, the utilization and bioactivity of C(sp3 )-H bonds are of the utmost importance. This review sheds light on the substrate scope, selectivity, benefits, and limitations of iron catalysts for direct C(sp3 )-H bond activations. An overview of the use of iron catalysis in C(sp3 )-H activation protocols is summarized herein up to 2022.

Iron-Catalyzed Alkoxyl Radical-Induced C-C Bond Cleavage/gem-Difluoroalkylation Cascade

An inexpensive iron-catalyzed alkoxyl radical-induced C-C bond cleavage/gem-difluoroalkylation cascade is presented. Regulated by the structure of alkoxyl radical precursors, fluorinated distal diketones were synthesized through a ring-opening strategy and difluoroalkylated medium-sized lactones and macrolactones were constructed via a ring-expansion strategy. Both protocols proceeded under mild and redox neutral conditions with a broad substrate scope and good functional group compatibility.

A cheap metal catalyzed ring expansion/cross-coupling cascade: a new route to functionalized medium-sized and macrolactones

An efficient alkoxyl radical-triggered ring expansion/cross-coupling cascade was developed under cheap metal catalysis. Through the metal-catalyzed radical relay strategy, a wide range of medium-sized lactones (9-11 membered) and macrolactones (12, 13, 15, 18, and 19-membered) were constructed in moderate to good yields, along with diverse functional groups including CN, N3, SCN, and X groups installed concurrently. Density functional theory (DFT) calculations revealed that reductive elimination of the cycloalkyl-Cu(iii) species is a more favorable reaction pathway for the cross-coupling step. Based on the results of experiments and DFT, a Cu(i)/Cu(ii)/Cu(iii) catalytic cycle is proposed for this tandem reaction.

Biomimetic Synthesis of Chejuenolides A-C by a Cryptic Lactone-Based Macrocyclization: Stereochemical Implications in Biosynthesis

A hypothetical Mannich macrocyclization in the biosynthesis of chejuenolides A-C served as the basis for the synthetic design herein. Using a lactone-based linear precursor constructed via a tactic sequence of aldol-Julia-aldol reactions on a gram scale, the biomimetic total synthesis and structural validation of chejuenolides A-C were successfully achieved for the first time. The β-oxo-δ-lactone unit in the macrocyclized adducts was fragile and readily converted to a series of C2/C18-diastereoisomers via a decarboxylation and protonation pathway. Stereochemical identification of the biosynthetic precursor (O3P2) confirmed structural adherence to the given macrocycles and previously clarified lankacidins. Moreover, the stereovariants of the linear precursor designed for the macrocyclization event highlighted the unparalleled impact of using this biomimetic approach to determine the stereoselectivity in the proposed enzymatic reaction by reviving the lost or unstable intermediate.

The Synthesis of 2,2-BIS(1-INDOL-3-YL)Acenaphthylene-1(2)-Ones Using Nanocatalysis: Fluorescent Sensing for Cu2+ Ions

2,2-bis(1H-indol-3-yl)acenaphthylene-1(2H)-ones were synthesised by the reaction of acenaphthenequinone and 2 equivalents of indole using Fe3O4@SiO2@Si-Pr-NH-CH2CH2NH2 as the basic magnetic nanocatalyst, assembled under greener and sustainable conditions in high purity and yields. Furthermore, the photoluminescence properties of 2,2-bis(2-methyl-1H-indol-3-yl)acenaphthylene-1(2H)-one were exploited for the sensing of copper ions in the mixed solvent systems comprising H2O and CH3CN in excitation wavelength at 410 nm with a detection limit of 9.5 ∙ 10–6 M.

Asymmetric Synthesis of Chiral Aza-macrodiolides via Iridium-Catalyzed Cascade Allylation/Macrolactonization

Iridium-catalyzed cascade allylation/macrolactonization between vinylethylene carbonate (VEC) and isatoic anhydride derivatives was successfully developed, readily generating a wide range of C₂-symmetric chiral macrodiolides bearing 14-membered rings in moderate to good yields with excellent diastereoselectivities and enantioselectivities (generally 99% ee). Control experiments revealed that racemic VEC as the precursor of electrophilic iridium-π-allyl species underwent kinetic resolution process. This expedient protocol features easily available substrates, excellent stereoselective control, and high step economy.

Planar Chirality: A Mine for Catalysis and Structure Discovery

Planar chirality is one of the most fascinating expressions of chirality, which is exploited by nature to lock three-dimensional chiral conformations and, more recently, by chemists to create new chiral reagents, catalysts, and functional organic materials. Nevertheless, the shortage of procedures able to induce and secure asymmetry during the generation of these unique chiral entities has dissuaded chemists from exploiting their structural properties. This Minireview intends to illustrate the limited but remarkable catalytic methods that have been reported for the production of planar chirality in strained molecules and serve as a source of inspiration for the development of new unconventional procedures, which are expected to appear in the near future.

Divergent, C-C Bond Forming Macrocyclizations Using Modular Sulfonylhydrazone and Derived Substrates

A divergent approach to C-C bond forming macrocycle construction is described. Modular sulfonylhydrazone and derived pyridotriazole substrates with three key building blocks have been constructed and cyclized to afford diverse macrocyclic frameworks. Broad substrate scope and functional group tolerance have been demonstrated. In addition, site-selective postfunctionalization allowed for further diversification of macrocyclic cores.

Hydrogenation of Secondary Amides using Phosphane Oxide and Frustrated Lewis Pair Catalysis

The metal‐free catalytic hydrogenation of secondary carboxylic acid amides is developed. The reduction is realized by two new catalytic reactions. First, the amide is converted into the imidoyl chloride by triphosgene (CO(OCCl₃)₂) using novel phosphorus(V) catalysts. Second, the in situ generated imidoyl chlorides are hydrogenated in high yields by an FLP‐catalyst. Mechanistic and quantum mechanical calculations support an autoinduced catalytic cycle for the hydrogenation with chloride acting as unusual Lewis base for FLP‐mediated H₂‐activation.

Chirality-matched catalyst-controlled macrocyclization reactions

Macrocycles, formally defined as compounds that contain a ring with 12 or more atoms, continue to attract great interest due to their important applications in physical, pharmacological, and environmental sciences. In syntheses of macrocyclic compounds, promoting intramolecular over intermolecular reactions in the ring-closing step is often a key challenge. Furthermore, syntheses of macrocycles with stereogenic elements confer an additional challenge, while access to such macrocycles are of great interest. Herein, we report the remarkable effect peptide-based catalysts can have in promoting efficient macrocyclization reactions. We show that the chirality of the catalyst is essential for promoting favorable, matched transition-state relationships that favor macrocyclization of substrates with preexisting stereogenic elements; curiously, the chirality of the catalyst is essential for successful reactions, even though no new static (i.e., not "dynamic") stereogenic elements are created. Control experiments involving either achiral variants of the catalyst or the enantiomeric form of the catalyst fail to deliver the macrocycles in significant quantity in head-to-head comparisons. The generality of the phenomenon, demonstrated here with a number of substrates, stimulates analogies to enzymatic catalysts that produce naturally occurring macrocycles, presumably through related, catalyst-defined peripheral interactions with their acyclic substrates.

The Fruit of Gold: Biomimicry in the Syntheses of Lankacidins

Natural products are constructed by organisms in impressive ways through various highly selective enzyme-catalyzed chemical reactions. Over the past century, there has been considerable interest in understanding and emulating the underlying biosynthetic logic for the target molecule. The successful implementation of a biomimetic strategy usually has some uniquely valuable benefits over other abiotic routes in total synthesis by (1) corroborating the chemical feasibility of a given biogenetic hypothesis and further unraveling some insightful implications for future biosynthetic studies and (2) providing remarkably more concise access to not only the original synthetic target but also diversified biogenetically related congeners, which may result in either the structural reassignment of previously disclosed natural products or the anticipation of undiscovered natural products. However, for the devised essential biomimetic transformation, fine-tuning the optimization of the substrates and the reaction conditions can sometimes be painstakingly challenging. Turning to nature for inspiration can provide additional impetus for methodological innovations.Previously used as oral veterinary drugs, lankacidins have potential as next-generation antibiotics to tackle the problems caused by multidrug-resistant bacteria with novel modes of action (MoAs). The hypersensitive and densely functionalized lactonic core within this family of macrocyclic polyketides poses a formidable challenge for chemical total synthesis and derivatization. In this account, we summarized the evolution of a unified biomimetic approach toward 10 lankacidin antibiotics and their linear biosynthetic intermediates in the longest linear 7-12 steps from readily available starting materials. Our endeavor commenced with an intermolecular bioinspired amido sulfone-based Mannich reaction approach to assemble 2 advanced fragments under mild biphasic organocatalytic conditions. It successfully gave rise to stereodivergent access to 4 C2/C18-isomeric lankacyclinols but failed to efficiently deliver lactone-containing congeners through Stille macrocyclization. Facilitated by the thermolysis chemistry of N,O-acetal to generate the requisite N-acyl-1-azahexatriene species, we realized the projected Mannich macrocyclization and eight macrocyclic lankacidins can be produced by orchestrated desilylative manipulations. In this process, we were able to perform structural reassignments of isolankacidinol (7 to 50) and isolankacyclinol (104 to 83) and, for the first time, elucidate the natural occurrence of 2,18-bis-epi-lankacyclinol (84). Moreover, the inability of the current biomimetic route to cofurnish the reported structure of 2,18-seco-lankacidinol A (15) triggered a proposed structural revision that is rooted in reconsidered biogenesis and was confirmed by a divergent synthesis that enabled us to identify the correct isomer (116). Finally, the modular, diversity-oriented design also provided streamlined entries to acyclic 2,18-seco-lankacidinol B (120) and the biosynthetic intermediate LC-KA05 (17) together with its C7-O-deacetylated congeners in all C4/C5-stereochemical variations (18, 127-129), culminating in a need for structural revision to the six-membered lactonic segment in LC-KA05-2. The selection and execution of biomimetic strategies in lankacidin total synthesis give rise to all the previously mentioned advantages at the current stage. The modular-based, late-stage diversified complex construction offers an exceptionally high level of synthetic flexibility for future synthetic forays toward newly isolated or chemically modified congeners within the lankacidin family.

Practical Enzymatic Production of Carbocycles

The Pd-catalyzed carbon-carbon bond formation pioneered by Heck in 1969 has dominated medicinal chemistry development for the ensuing fifty years. As the demand for more complex three-dimensional active pharmaceuticals continues to increase, preparative enzyme-mediated assembly, by virtue of its exquisite selectivity and sustainable nature, is poised to provide a practical and affordable alternative for accessing such compounds. In this minireview, we summarize recent state-of-the-art developments in practical enzyme-mediated assembly of carbocycles. When appropriate, background information on the enzymatic transformation is provided and challenges and/or limitations are also highlighted.

Macrolactonization Reactions Driven by a Pentafluorobenzoyl Group*

Macrolactones constitute a privileged class of natural and synthetic products with a broad range of applications in the fine chemicals and pharmaceutical industry. Despite all the progress made towards their synthesis, notably from seco-acids, a macrolactonization promoter system that is effective, selective, flexible, readily available, and, insofar as possible, compatible with manifold functional groups is still lacking. Herein, we describe a strategy that relies on the formation of a mixed anhydride incorporating a pentafluorophenyl group which, due to its high electronic activation enables a convenient access to macrolactones, macrodiolides and esters with a broad versatility. Kinetic studies and DFT computations were performed to rationalize the reactivity of the pentafluorophenyl group in macrolactonization reactions.

Five Step Total Synthesis of Lythranidine

A concise synthesis of the alkaloid lythranidine is reported. The strategy exploits the target's local C₂ symmetry by adopting a two directional synthetic approach, first in an acyclic environment, then in a cyclic system and finally in a bridged macrocyclic domain. The latter phase of the synthesis, which installs all four stereocenters, involves a thermodynamically controlled, twofold intermolecular/transannular aza-Michael addition and a twofold hydride reduction. The synthesis is one third of the length of the most step-economic previous approach, providing access to gram quantities of the natural product. The broad-spectrum nature of the synthesis is demonstrated through the preparation of three diastereomeric analogues of the natural product.

Heck macrocyclization in natural product total synthesis

Covering: 1981-2020 Heck macrocyclization is a logical extension of the award-winning Mizoroki-Heck reaction. Through covalent linking of two otherwise discrete coupling partners, the resultant chimeric substrate is transformed into a large ring with enhanced rigidity and unique functional group disposition. Pioneered in the early 1980s, this methodology has evolved into a competent option for creating diverse macrocycles. Despite its growing influence, hitherto no systematic survey has ever appeared in the literature. The present review delineates the state-of-the-art of Heck macrocyclization in the context of natural product synthesis. Sixteen selected cases, each examined from a different perspective, coalesce into the view that the title reaction is a viable tool for synthesis-enabled macrocycle research.

Catalytic enantioselective synthesis of macrodiolides and their application in chiral recognition

New types of C₂-symmetric chiral macrodiolides are readily obtained via chiral N,N′-dioxide-scandium(iii) complex-promoted asymmetric tandem Friedel–Crafts alkylation/intermolecular macrolactonization of ortho-quinone methides with C3-substituted indoles. This protocol provides an array of enantioenriched macrodiolides with 16, 18 or 20-membered rings in moderate to good yields with high diastereoselectivities and excellent enantioselectivities through adjusting the length of the tether at the C3 position of indoles. Density functional theory calculations indicate that the formation of macrocycles is more favorable than that of 9-membered-ring lactones in terms of kinetics and thermodynamics. The potential utility of these intriguing chiral macrodiolide molecules is demonstrated in the enantiomeric recognition of aminols and chemical recognition of metal ions.

An asymmetric tandem Friedel–Crafts alkylation/intermolecular macrolactonization of ortho-quinone methides with C3-substituted indoles was achieved by using a chiral N,N′-dioxide-scandium(iii) complex.

Asymmetric Transformation Driven by Confinement and Self-Release in Single-Layered Porous Nanosheets

Reported here is the use of single-layered, chiral porous sheets with induced pore chirality for repeatable asymmetric transformations and self-separation without the need for chiral catalysts or chiral auxiliaries. The asymmetric induction is driven by chiral fixation of absorbed achiral substrates inside the chiral pores for transformation into enantiopure products with enantioselectivities of greater than 99 % ee. When the conversion is completed, the products are spontaneously separated out of the pores, enabling the porous sheets to perform repeated cycles of converting achiral substrates into chiral products for release without compromising pore performance. Confinement of achiral substrates into two-dimensional chiral porous materials provides access to a highly efficient alternative to current asymmetric synthesis methodologies.

Unconventional Macrocyclizations in Natural Product Synthesis

Over the past several decades, macrocyclic compounds have emerged as increasingly significant therapeutic candidates in drug discovery. Their pharmacological activity hinges on their rotationally restricted three-dimensional orientation, resulting in a unique conformational preorganization and a high enthalpic gain as a consequence of high-affinity macrocycle-protein binding interactions. Synthetic access to macrocyclic drug candidates is therefore crucial. From a synthetic point of view, the efficiency of macrocyclization events commonly suffers from entropic penalties as well as undesired intermolecular couplings (oligomerization). Although over the past several decades ring-closing metathesis, macrolactonization, or macrolactamization have become strategies of choice, the toolbox of organic synthesis provides a great number of versatile transformations beyond the aforementioned. This Outlook focuses on a selection of examples employing what we term unconventional macrocyclizations toward the synthesis of natural products or analogues.

Landscape of Lankacidin Biomimetic Synthesis: Structural Revisions and Biogenetic Implications

In this report, a unified biomimetic approach to all known macrocyclic lankacidins is presented. By taking advantage of the thermolysis of N,O-acetal to generate the requisite N-acyl-1-azahexatriene species, we eventually realized the biomimetic Mannich macrocyclization, from which all of the macrocyclic lankacidins can be conquered by orchestrated desilylation. The reassignments of the reported structures of isolankacidinol (7 to 10) and the discovery of a recently isolated "lankacyclinol" found to be in fact 2,18-bis-epi-lankacyclinol (72) unraveled the previously underappreciated chemical diversity exhibited by the enzymatic macrocyclization. In addition, the facile elimination/decarboxylation/protonation process for the depletion of C1 under basic conditions resembling a physiological environment may implicate more undiscovered natural products with variable C2/C18 stereochemistries (i.e., 62, 73, and 75). The notable aspect provided by a biomimetic strategy is significantly reducing the step count compared with the two previous entries to macrocyclic lankacidins.

Cationic NHC-Phosphine Iridium Complexes: Highly Active Catalysts for Base-Free Hydrogenation of Ketones

Novel bidentate N-heterocyclic carbene-phosphine iridium complexes have been synthesized and evaluated in the hydrogenation of ketones. Reported catalytic systems require base additives and, if excluded, need elevated temperature or high pressure of hydrogen gas to achieve satisfactory reactivity. The developed catalysts showed extremely high reactivity and good enantioselectivity under base-free and mild conditions. In the presence of 1 mol % catalyst under 1 bar hydrogen pressure at room temperature, hydrogenation was complete in 30 minutes giving up to 96 % ee. Again, this high reactivity was achieved in additive-free conditions. Mechanistic experiments demonstrated that balloon pressure of hydrogen was sufficient to form the activate species by reducing and eliminating the 1,5-cyclooctadiene ligand. The pre-activated catalyst was able to hydrogenate acetophenone with 89 % conversion in 5 min.

Regio- and stereoselective synthesis of spiropyrrolidine-oxindole and bis-spiropyrrolizidine-oxindole grafted macrocycles through [3 + 2] cycloaddition of azomethine ylides

A convenient and efficient method for the regioselective macrocyclization of triazole bridged spiropyrrolidine-oxindole, and bis-spiropyrrolizidine-oxindole derivatives was accomplished through intra and self-intermolecular [3 + 2] cycloaddition of azomethine ylides. The chalcone isatin precursors 9a-i required for the click reaction were obtained from the reaction of N-alkylazidoisatin 4 and propargyloxy chalcone 8a-i which in turn were obtained by the aldol condensation of propargyloxy salicylaldehyde 6 and substituted methyl ketones 7a-i. The regio- and stereochemical outcome of the cycloadducts were assigned based on 2D NMR and confirmed by single crystal XRD analysis. High efficiency, mild reaction conditions, high regio- and stereoselectivity, atom economy and operational simplicity are the exemplary advantages of the employed macrocyclization procedure.