Exploring the Boundaries of “Practical”: De Novo Syntheses of Complex Natural Product-Based Drug Candidates

Asymmetric Synthesis of Trisubstituted Vicinal Diols through Copper(I)-Catalyzed Diastereoselective and Enantioselective Allylation of Ketones with Siloxypropadienes

Chiral trisubstituted vicinal diols are a type of important organic compounds, serving as both common structure units in bioactive natural products and chiral auxiliaries in asymmetric synthesis. Herein, by using siloxypropadienes as the precursors of allyl copper(I) species, a copper(I)-catalyzed diastereoselective and enantioselective reductive allylation of ketones was achieved, providing both syn-diols and anti-diols in good to excellent enantioselectivity. DFT calculations show that cis-γ-siloxy-allyl copper species are generated favorably with either 1-TBSO-propadiene or 1-TIPSO-propadiene. Moreover, the steric difference of TBS group and TIPS group distinguishes the face selectivity of acetophenone, leading to syn-selectivity for 1-TBSO-propadiene and anti-selectivity for 1-TIPSO-propadiene. Easy transformations of the products were performed, demonstrating the synthetic utility of the present method. Moreover, one chiral diol prepared in the above transformations was used as a suitable organocatalyst for the catalytic asymmetric reductive self-coupling of aldimines generated in situ with B2(neo)2.

An Efficient and Scalable "Second Generation" Total Synthesis of the Marine Polyketide Limaol Endowed with Antiparasitic Activity

The cluster of four skipped exo-methylene substituents on the "northern" wing of limaol renders this dinoflagellate-derived marine natural product unique in structural terms. This arguably non-thermodynamic array gains kinetic stability by virtue of populating local conformations which impede isomerization to a partly or fully conjugated polyene. This analysis suggested that the difficulties encountered during the late stages of our first total synthesis of this polyketide had not been caused by an overly fragile character of this unusual substructure; rather, an unfavorable steric microenvironment about the spirotricyclic core was identified as the likely cause. To remedy the issue, the protecting groups on this central fragment were changed; in effect, this amendment allowed all strategic and practical problems to be addressed. As a result, the overall yield over the longest linear sequence was multiplied by a factor of almost five and the material throughput increased more than eighty-fold per run. Key-to-success was a gold-catalyzed spirocyclization reaction; the reasons why a Brønsted acid cocatalyst is needed and the origin of the excellent levels of selectivity were delineated. The change of the protecting groups also allowed for much improved fragment coupling processes; most notably, the sequence of a substrate-controlled carbonyl addition reaction followed by Mitsunobu inversion that had originally been necessary to affix the southern tail to the core could be replaced by a reagent controlled asymmetric allylation. Finally, a much-improved route to the "northern" sector was established by leveraging the power of asymmetric hydrogenation of a 2-pyrone derivative. Limaol was found to combine appreciable antiparasitic activity with very modest cytotoxicity.

Ligand-Enabled Palladium(II)-Catalyzed γ-C(sp3)-H Arylation of Primary Aliphatic Amines

The Pd(II)/sulfoxide-2-hydroxypyridine catalytic system shows promising activity in C-H activation chemistry. In this study, we showcase how this catalytic system solves the problem of native primary amine-directed γ-C(sp3)-H arylation. Primary amines with different complexities are compatible with the established methodology, and the range of applicable substrates can be expanded to include pyridine, oxime ether, and pyridine N-oxide.

Synthesis, structure-activity relationship, and biological evaluation of quinolines for development of anticancer agents

Tetrahydro-β-carbolines (THβCs) are a kind of natural alkaloids with multiple pharmaceutical activities. Herein, a focused compound library derived from THβCs was synthesized and their anticancer activities were studied in several cancer cell lines. Among them, three compounds showed considerable anticancer activities with low micromolar to submicromolar IC₅₀ values. The abilities to induce apoptosis and alter mitochondrial membrane potential levels, which are comparable to those of the commercial anticancer drug adriamycin, were confirmed by one representative compound (21) on the B16/F10 cell line. Our preliminary structure-activity relationship studies indicated that alkylamines with suitable lengths are very important for potency improvement.

Native Amino Group Directed Site-Selective ε-C(sp2)-H Iodination of Primary Amines

Selective remote C-H activating amines using unmodified NH₂ as a native directing group demonstrate compelling synthetic utilities. The 3-arylpropan-1-amine moiety is present in many drugs and candidates in clinical trials. Selective iodination of 3-arylpropan-1-amines on remote aryl rings gives valuable intermediates for modifying bioactive molecules and synthesizing quinolones. Here we report the first palladium-catalyzed selective ε-C(sp²)-H iodination of free 3-arylpropan-1-amines via a seven-membered palladacycle.

Chiral Co3Y Propeller-Shaped Chemosensory Platforms Based on 19F-NMR

Two propeller-shaped chiral Co^III₃Y^III complexes built from fluorinated ligands are synthesized and characterized by single-crystal X-ray diffraction (SXRD), IR, UV-vis, circular dichroism (CD), elemental analysis, thermogravimetric analysis (TGA), electron spray ionization mass spectroscopy (ESI-MS), and NMR (¹H, ¹³C, and ¹⁹F). This work explores the sensing and discrimination abilities of these complexes, thus providing an innovative sensing method using a ¹⁹F NMR chemosensory system and opening new directions in 3d/4f chemistry. Control experiments and theoretical studies shed light on the sensing mechanism, while the scope and limitations of this method are discussed and presented.

Antifungal and Cytotoxic Activity of Diterpenes and Bisnorsesquiterpenoides from the Latex of Euphorbia resinifera Berg

Euphorbia resinifera latex has been extensively utilized in traditional medicine due to its range of bioactivities. Chromatographic separations on silica gel of ethanol extract of E. resinifera latex led to the development of a new procedure for isolating resiniferatoxin (4) via dried E. resinifera latex and the identification of nine compounds. Among these, catechol (7), protocatechuic acid (8) and 3,4-dihydroxyphenylacetic acid (9), known phenolic compounds, were identified for the first time in E. resinifera latex. Herein we investigated the effects of major compounds of the latex of E. resinifera on the yeast Saccharomyces cerevisiae, on the growth of Aspergillus carbonarius, a widespread fungal contaminant, and on the breast cancer cell line MCF7 as well as on MCF10A normal breast cells. 12-deoxyphorbol-13-isobutyrate-20-acetate (2) had an inhibiting effect on the growth of A. carbonarius, and 7-p-metoxyphenylacetate-3,8,12-triacetate ingol (3) showed a negative effect on yeast cell growth and also a cytotoxic effect on breast cancer cell line MCF7, but not on MCF10A cells. Deglucosyl euphorbioside A (5) and euphorbioside A (6) showed a discoloration effect that was possibly related to mitochondrial functionality in yeast, and also cytotoxicity only on the cancer cell line that was tested. Interestingly, treatment of MCF7 cells with 7-p-metoxyphenylacetate-3,8,12-triacetate ingol (3) and deglucosyl euphorbioside A (5) not only led to a specific cytotoxic effect but also to the increase in the level of intracellular ROS.

Indispensable role of microbes in anticancer drugs and discovery trends

Recent years have seen an increased focus on the advancement of naturally derived products for the treatment of cancer. Since the beginning of recorded history, nature has provided a variety of medicinal agents, and an overwhelming number of drugs that we have today are derived from natural sources. Such natural agents are prominently used to treat several diseases such as diabetes, malaria, Alzheimer's, pulmonary disorders, etc. with cancer being the highlight of this review. Due to the rapid development of resistance to chemotherapeutic drugs, the hunt for effective novel drugs is still a paramount concern in cancer treatment. Moreover, many chemotherapy drugs typically have high toxicity and adverse side effects, which necessitates the need to develop anti-tumor drugs that can be employed to treat deadly tumors with fewer negative effects on health and better efficacy. Isolation of several chemotherapeutic drugs has been conducted from a wide range of natural sources which include plants, microbes, fungi, and marine microorganisms. Considering the trends of previous decades, microbial diversity has grown to play a significant role in the formulation of pharmaceuticals and drugs, especially antibiotics and anti-cancer medications. Microbe-derived antitumor antibiotics such as anthracycline, epothilones, bleomycin, actinomycin, and staurosporine are amongst the widely used cancer chemotherapeutic agents. This review deals majorly with microbe-derived anticancer drugs taking into account their derivatives, mechanism of action, isolation procedures, limitations, and tumors targeted by them. This article also reports the phase of clinical study these drugs are undergoing. Moreover, it intends to portray the indispensable part that these microbes have been playing since time immemorial in the odyssey of chemotherapeutic agents. KEY POINTS: • Microbial diversity contributes heavily towards the formulation of anticancer drugs. • Polypeptides, carbohydrates, and alkaloids are prevalent microbe-based drug classes. • Microbe-derived anticancer agents target various sarcomas, carcinomas, and lymphomas.

[Total Synthesis and Functional Analysis of Complex Peptidic Natural Products and Their Artificial Analogues]

This review focuses on a new solid-phase synthetic strategy for an anticancer natural product yaku'amide B (1) and its target identification and structure-function relationship study using synthetic analogues and probes. To realize the Fmoc-based solid-phase synthesis of 1, we developed new synthetic methods for enamide formation. Namely, modified traceless Staudinger ligation using alkenyl azides and newly designed phosphinophenol esters enabled stereoselective construction of the (E)- and (Z)-ΔIle moieties. Furthermore, resin-cleavage and C-terminus modification were simultaneously achieved with an ester-amide exchange reaction using C-terminal amine and AlMe₃, which successfully afforded 1 via a full solid-phase route. The developed strategy was applied to the construction of seven E/Z isomers of 1. In the target identification of 1, fluorescent imaging study and affinity pull-down assay using the synthetic probes revealed that 1 exerts potent cytostatic activity by binding to subunits α and β of mitochondrial F_oF₁-ATP synthase. On the basis of the mode of action of 1, we conducted biological evaluation of the seven E/Z-isomers of 1. Assessment of growth inhibition activity and the effect on F_oF₁-ATP synthase indicates that the E/Z-stereochemistry of the three ΔIle residues controls the magnitude of biological functions of 1.

Samarium(ii) iodide-mediated reactions applied to natural product total synthesis

Natural product synthesis remains a field in which new synthetic methods and reagents are continually being evaluated. Due to the demanding structures and complex functionality of many natural products, only powerful and selective methods and reagents will be highlighted in this proceeding. Since its introduction by Henri Kagan, samarium(ii) iodide (SmI₂, Kagan's reagent) has found increasing use in chemical synthesis. Over the years, many reviews have been published on the application of SmI₂ in numerous reductive coupling procedures as well as in natural product total synthesis. This review highlights recent advances in SmI₂-mediated synthetic strategies, as applied in the total synthesis of natural products since 2004.

Natural product synthesis remains a field in which new synthetic methods and reagents are continually being evaluated.

Itol A May Affect the Growth and Development of Spodoptera frugiperda through Hijacking JHBP and Impeding JH Transport

Isoryanodane and ryanodane diterpenes have a carbon skeleton correlation in structures, and their natural product-oxidized diterpenes show antifeedant and insecticidal activities against Hemiptera and Lepidoptera. While ryanodine mainly acts on the ryanodine receptor (RyR), isoryanodane does not. In this study, we demonstrated that itol A, an isoryanodane diterpenoid, could significantly downregulate the expression level of juvenile hormone-binding protein (JHBP), which plays a vital role in JH transport. RNAi bioassay indicated that silencing the Spodoptera frugipreda JHBP (SfJHBP) gene decreased itol A activity, which confirmed the developmental phenotypic observation. Parallel reaction monitoring (PRM) further confirmed that itol A affected JHBP's expression abundance. Although JHBP is not proven as the direct or only target of itol A, we confirmed that itol A's action effect depends largely on JHBP and that JHBP is a potential target of itol A. We present foundational evidence that itol A inhibits the growth and development of Spodoptera frugiperda mainly through hijacking JHBP.

Pd(ii)-catalyzed meta-C–H bromination and chlorination of aniline and benzoic acid derivatives

The classic electrophilic bromination leads to ortho- and para-bromination of anilines due to their electron-rich properties. Herein we report the development of an unprecedented Pd-catalyzed meta-C–H bromination of aniline derivatives using commercially available N-bromophthalimide (NBP), which overcomes the competing ortho/para-selectivity of electrophilic bromination of anilines. The addition of acid additives is crucial for the success of this reaction. A broad range of substrates with various substitution patterns can be tolerated in this reaction. Moreover, benzoic acid derivatives bearing complex substitution patterns are also viable with this mild bromination reaction, and meta-C–H chlorination is also feasible under similar reaction conditions. The ease of the directing group removal and subsequent diverse transformations of the brominated products demonstrate the application potential of this method and promise new opportunities for drug discovery.

An unprecedented Pd-catalyzed meta-C–H bromination and chlorination of highly substituted aniline and benzoic acid derivatives using N-bromophthalimide is reported.

A brief overview of classical natural product drug synthesis and bioactivity

This manuscript briefly overviewed the total synthesis and structure–activity relationship studies of eight classical natural products, which emphasizes the important role of total synthesis in natural product-based drug development.

Unified Total Syntheses of Rhamnofolane, Tigliane, and Daphnane Diterpenoids

Rhamnofolane, tigliane, and daphnane diterpenoids are structurally complex natural products with multiple oxygen functionalities, making them synthetically challenging. While these diterpenoids share a 5/7/6-trans-fused ring system (ABC-ring), the three-carbon substitutions at the C13- and C14-positions on the C-ring and appending oxygen functional groups differ among them, accounting for the disparate biological activities of these natural products. Here, we developed a new, unified strategy for expeditious total syntheses of five representative members of these three families, crotophorbolone (1), langduin A (2), prostratin (3), resiniferatoxin (4), and tinyatoxin (5). Retrosynthetically, 1-5 were simplified into their common ABC-ring 6 by detaching the three-carbon units and the oxygen-appended groups. Intermediate 6 with six stereocenters was assembled from four achiral fragments in 12 steps by integrating three powerful transformations, as follows: (i) asymmetric Diels-Alder reaction to induce formation of the C-ring; (ii) π-allyl Stille coupling reaction to set the trisubstituted E-olefin of the B-ring; and (iii) Eu(fod)₃-promoted 7-endo cyclization of the B-ring via the generation of a bridgehead radical. Then 6 was diversified into 1-5 by selective installation of the different functional groups. Attachment of the C14-β-isopropenyl and isopropyl groups led to 1 and 2, respectively, while oxidative acetoxylation and C13,14-β-dimethylcyclopropane formation gave rise to 3. Finally, formation of an α-oriented caged orthoester by C13-stereochemical inversion and esterification with two different homovanillic acids delivered 4 and 5 with a C13-β-isopropenyl group. This unified synthetic route to 1-5 required only 16-20 total steps, demonstrating the exceptional efficiency of the present strategy.

Tetracycline antibiotics

Tetracyclines belong to the first broad-spectrum, well-tolerated, and easy-to-administer antibiotics, which are effective against plague, cholera, typhoid, syphilis, Legionnaire’s disease, and anthrax. Some can also be used to treat malaria, Lyme disease, tuberculosis, Rocky Mountain spotted fever, and leprosy. Humans first encountered these chemical species involuntarily in ancient times, as evidenced from the analysis of bone samples dating back more than 1500 years. Shortly after World War II, they were “rediscovered” at Lederle Laboratories and Pfizer as a result of an intense search for new antibiotics. Their bacteriostatic action is based on the inhibition of protein biosynthesis. Since the structure elucidation by Robert Woodward, Lloyd Hillyard Conover, and others in the 1950s, tetracyclines have become preferred targets for natural product synthesis. However, on industrial scale, they became readily available by fermentation and partial synthesis. Their casual and thoughtless use in the initial decades after launch not only in humans but for veterinary purposes and as growth-enhancement agents in meat production rapidly led to the emergence of resistance. In an arms race for new antibiotics, more and more new drugs have been developed to deal with the threat. In this ongoing endeavor, a remarkable milestone was set by Andrew Myers in 2005 with the convergent total synthesis of (−)-doxycycline, as well as numerous azatetracyclines and pentacyclines, which has inspired chemists in the pharmaceutical industry to discover novel and highly active tetracyclines in recent years.

Graphic abstract

Convergent Assembly of Highly Oxygenated Natural Products Enabled by Intermolecular Radical Reactions

Natural products with a high ratio of sp³-hybridized atoms and oxygen-substituted stereogenic centers represent privileged structures for the development of pharmaceuticals and chemical probes. The multiple oxygen functionalities of these natural products endow their potent and selective biological activities, although they significantly heighten the challenge of their chemical assemblies. We focused on developing efficient strategies for the total syntheses of this biologically and chemically important class of molecules. A convergent strategy is more advantageous than a linear strategy for designing a shorter synthetic route because a convergent strategy enables direct coupling of functionalized fragments whereas a linear strategy involves stepwise construction of a molecule from its terminus. Radical reactions are preferred over polar reactions for the coupling of heavily functionalized and sp³-rich fragments, as they allow for C(sp³)-C(sp³) coupling without damaging diverse polar functional groups. With these considerations in mind, we designed radical-based convergent strategies for assembling highly oxygenated natural products. Here we summarize the concise total syntheses of asimicin (1, antibiotic activity), 1-hydroxytaxinine (2, cytotoxicity), polyoxins (3, antifungal activity), and hikizimycin (4, anthelmintic activity) as representative examples. Retrosynthetic disconnection at the central part of these molecules produces highly substituted α-alkoxy radicals as synthons. In the synthetic direction, the α-alkoxy radicals were generated from the corresponding α-alkoxyacyl tellurides in a unified fashion, and then utilized for four distinct coupling reactions. Formation of the Et radical from Et₃B and O₂ homolytically cleaves the C-Te bond of α-alkoxyacyl telluride, and the facile expulsion of carbon monoxide from the acyl radical leads to the α-alkoxy radical. Dimerization of the stabilized α-alkoxy radical resulted in the core structure of 1 with 10 contiguous stereocenters. The coupling adduct was derivatized to 1 through the attachment of two different carbon chains (17 steps as the longest linear sequence). Alternatively, intermolecular addition reactions of the α-alkoxy radicals to electron-deficient C═C, C═N, and C═O bonds, followed by Et₃B-mediated radical termination, led to the core structures of 2, 3, and 4, respectively. Intermolecular coupling between the α-alkoxy radical and the cyclohexenone derivative and intramolecular pinacol coupling gave rise to the 6/8/6-fused ring system of 2, which was transformed to 2 (26 steps). The two amino acid moieties of 3 were prepared by combining the α-alkoxy radical and the oxime and were then condensed to complete the synthesis of 3 (11 steps). Furthermore, a combination of α-alkoxyacyl telluride and Et₃B/O₂ realized a novel addition reaction of α-alkoxy radicals to aldehydes. This method was incorporated in the construction of the core 4-amino-5-deoxyundecose with 10 contiguous stereocenters, which was fabricated with two appendage structures to deliver 4. The four total syntheses described here demonstrate the versatility and robustness of intermolecular radical reactions. These syntheses will also provide new insights for retrosynthetic analyses in the field of organic chemistry and streamline synthetic routes to various bioactive natural products with multiple oxygen functionalities.

Synthesis of α-Amidoketones through the Cascade Reaction of Carboxylic Acids with Vinyl Azides under Catalyst-Free Conditions

An efficient synthesis of α-amidoketone derivatives through the cascade reactions of carboxylic acids with vinyl azides is presented. Compared with literature protocols, notable features of this new method include catalyst-free conditions, broad substrate scope, good tolerance of a wide range of functional groups, and high efficiency. In addition, the synthetic potential of this method as a tool for late-stage modification was convincingly manifested by its application in the structural elaborations of a number of carboxylic acid drug molecules.

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.

Total Synthesis of Diospyrodin and Its Three Diastereomers

Antibacterial diospyrodin (1) was synthesized in 13 steps. Et₃B and O₂ promoted the formation of an α-alkoxy carbon radical from l-ribose-derived α-alkoxyacyl telluride 5, which reacted with d-glucose-derived aldehyde 4. The radical addition realized the convergent assembly of the contiguously hydroxylated carbon-chain of 3-α and greatly contributed to streamlining the synthetic route. Compound 3-α was transformed not only to 1 but also to its three diastereomers by functional group manipulations.

Convergent Total Synthesis of Hikizimycin Enabled by Intermolecular Radical Addition to Aldehyde

Hikizimycin (1), which exhibits powerful anthelmintic activity, has the most densely functionalized structure among nucleoside antibiotics. A central 4-amino-4-deoxyundecose of 1 possesses 10 contiguous stereocenters on a C1-C11 linear chain and is decorated with a cytosine base at C1 and a 3-amino-3-deoxyglucose at C6-OH. These distinctive structural features of 1 make it an extremely challenging target for de novo construction. Herein, we report a convergent total synthesis of 1 from four known components: 3-azide-3-deoxyglucose derivative 4, bis-TMS-cytosine 5, d-mannose 9, and d-galactose derivative 10. We first designed and devised a novel radical coupling reaction between multiply hydroxylated aldehydes and α-alkoxyacyl tellurides. The generality and efficiency of this process was demonstrated by the coupling of 7c and 8, which were readily accessible from two hexoses, 9 and 10, respectively. Et₃B and O₂ rapidly induced decarbonylative radical formation from α-alkoxyacyl telluride 8, and intermolecular addition of the generated α-alkoxy radical to aldehyde 7c yielded 4-amino-4-deoxyundecose 6-α with installation of the desired C5,6-stereocenters. Subsequent attachments of the cytosine with 5 and of the 3-azide-3-deoxyglucose with 4 were realized through selective activation of the C1-acetal and selective deprotection of the C6-hydroxy group. Finally, the 3 amino and 10 hydroxy groups were liberated in a single step to deliver the target 1. Thus, the combination of the newly developed radical-coupling and protective-group strategies minimized the functional group manipulations and thereby enabled the synthesis of 1 from 10 in only 17 steps. The present total synthesis demonstrates the versatility of intermolecular radical addition to aldehyde for the first time and offers a new strategic design for multistep target-oriented syntheses of various nucleoside antibiotics and other bioactive natural products.

Solid-Phase Total Synthesis of Yaku'amide B Enabled by Traceless Staudinger Ligation

We report a solid-phase strategy for total synthesis of the peptidic natural product yaku'amide B (1), which exhibits antiproliferative activity against various cancer cells. Its linear tridecapeptide sequence bears four β,β-dialkylated α,β-dehydroamino acid residues and is capped with an N-terminal acyl group (NTA) and a C-terminal amine (CTA). To realize the Fmoc-based solid-phase synthesis of this complex structure, we developed new methods for enamide formation, enamide deprotection, and C-terminal modification. First, traceless Staudinger ligation enabled enamide formation between sterically encumbered alkenyl azides and newly designed phosphinophenol esters. Second, application of Eu(OTf)₃ led to chemoselective removal of the enamide Boc groups without detaching the resin linker. Finally, resin-cleavage and C-terminus modification were simultaneously achieved with an ester-amide exchange reaction using CTA and AlMe₃ to deliver 1 in 9.1 % overall yield (24 steps from the resin).

Synthesis of polysubstituted arenes through organocatalytic benzannulation

Polysubstituted arenes serve as ubiquitous structural cores of aromatic compounds with significant applications in chemistry, biological science, and materials science. Among all the synthetic approaches toward these highly functionalized arenes, organocatalytic benzannulation represents one of the most efficient and versatile transformations in the assembly of structurally diverse arene architectures under mild conditions with exceptional chemo-, regio- or stereoselectivities. Thus, the development of new benzannulation reactions through organocatalysis has attracted much attention in the past ten years. This review systemically presents recent advances in the organocatalytic benzannulation strategies, categorized as follows: (1) Brønsted acid-catalysis, (2) secondary amine catalysis, (3) primary amine catalysis, (4) tertiary amine catalysis, (5) tertiary phosphine catalysis, and (6) N-heterocyclic carbene catalysis. Each part is further classified into several types according to the number of carbon atoms contributed by different synthons participating in the cyclization reaction. The reaction mechanisms involved in different benzannulation strategies were highlighted.

Organocatalytic benzannulation represents one of the most efficient transformations for assembling polysubstituted arenes, this review presents recent advances in organocatalytic benzannulation strategies to construct functionalized benzenes.

Chiral Discrimination by a Binuclear Pd Complex Sensor Using 31P{1H} NMR

An axially chiral binuclear μ-hydroxo Pd complex (BPHP) first served as an excellent chiral sensor for discriminating a variety of analytes including amino alcohol, amino amide, amino acid, mandelic acid, diol, diamine, and monoamine by ³¹P{¹H} NMR. A detailed recognition mechanism was proposed based on the single crystal and mass spectrum of Pd-complexes. In general, BPHP sensor, through extracting the acidic hydrogen of an analyte by its Pd-OH group, forms stable diastereomeric complexes with two enantiomers of the analyte giving well distinguishable split ³¹P{¹H} NMR signals for chiral discrimination.

Fixing the Unfixable: The Art of Optimizing Natural Products for Human Medicine

Molecules isolated from natural sources including bacteria, fungi, and plants are a long-standing source of therapeutics that continue to add to our medicinal arsenal today. Despite their potency and prominence in the clinic, complex natural products often exhibit a number of liabilities that hinder their development as therapeutics, which may be partially responsible for the current trend away from natural product discovery, research, and development. However, advances in synthetic biology and organic synthesis have inspired a new generation of natural product chemists to tackle powerful undeveloped scaffolds. In this Perspective, we will present case studies demonstrating the historical and current focus on making targeted, but significant, changes to natural product scaffolds via biosynthetic gene cluster manipulation, total synthesis, semisynthesis, or a combination of these methods, with a focus on increasing activity, decreasing toxicity, or improving chemical and pharmacological properties.

Natural products as modulators of the cyclic-AMP pathway: evaluation and synthesis of lead compounds

It is now well recognized that the normal cellular response in mammalian cells is critically regulated by the cyclic-AMP (cAMP) pathway through the appropriate balance of adenylyl cyclase (AC) and phosphodiesterase-4 (PDE4) activities. Dysfunctions in the cAMP pathway have major implications in various diseases like CNS disorders, inflammation and cardiac syndromes and, hence, the modulation of cAMP signalling through appropriate intervention of AC/PDE4 activities has emerged as a promising new drug discovery strategy of current interest. In this context, synthetic small molecules have had limited success so far and therefore parallel efforts on natural product leads have been actively pursued. The early promise of using the diterpene forskolin and its semi-synthetic analogs as AC activators has given way to new leads in the last decade from novel natural products like the marine sesterterpenoids alotaketals and ansellones and the 9,9'-diarylfluorenone cored selaginpulvilins, etc. and their synthesis has drawn much attention. This review captures these contemporary developments, particularly total synthesis campaigns and structure-guided analog design in the context of AC and PDE-4 modulating attributes and the scope for future possibilities.

Microfluidic light-driven synthesis of tetracyclic molecular architectures

Herein we report an effective synthetic method for the direct assembly of highly functionalized tetracyclic pharmacophoric cores. Coumarins and chromones undergo diastereoselective [4 + 2] cycloaddition reactions with light-generated photoenol intermediates. The reactions occur by aid of a microfluidic photoreactor (MFP) in high yield (up to >98%) and virtually complete diastereocontrol (>20:1 dr). The method is easily scaled-up to a parallel setup, furnishing 948 mg of product over a 14 h reaction time. Finally, a series of manipulations of the tetracyclic scaffold obtained gave access to valuable precursors of biologically active molecules.

Bioinspired Synthesis of (+)-Cinchonidine Using Cascade Reactions

The development of efficient syntheses of complex natural products has long been a major challenge in synthetic chemistry. Designing cascade reactions and employing bioinspired transformations are an important and reliable means of achieving this goal. Presented here is a combination of these two strategies, which allow efficient asymmetric synthesis of the cinchona alkaloid (+)-cinchonidine. The key steps of this synthesis are a controllable, visible-light-induced photoredox radical cascade reaction to efficiently access the tetracyclic monoterpenoid indole alkaloid core, as well as a practical biomimetic cascade rearrangement for the indole to quinoline transformation. The use of stereoselective chemical transformations in this work makes it an efficient synthesis of (+)-cinchonidine.

Multicomponent Mechanochemical Synthesis of Cyclopentadienyl Titanium tert-Butoxy Halides, Cp x TiX y (O t Bu)4-(x+y) (x, y = 1, 2; X = Cl, Br)

Titanium tert-butoxy halides of the formula Cp _x TiX _y (O ^t Bu)_4-(x+y) (x, y = 1, 2; X = Cl, Br) have been prepared thorough milling the reagents without solvent. In the case of the chloride derivatives, Cp₂TiCl₂ is used as a starting material; in the case of the bromides, a mixture of LiCp, TiBr₄, and Li[O ^t Bu] is used. The stoichiometric ratios of the starting materials are reflected in the major products of the reactions. Single-crystal X-ray structures are reported for Cp₂TiCl(O ^t Bu), Cp₂TiBr(O ^t Bu), and CpTiBr₂(O ^t Bu), as well as for Cp₂TiCl(O ⁱ Pr) and a redetermination of Cp₂TiCl(OMe). The tert-butoxy derivatives are notable for their nearly linear Ti-O-C angles (>170°) that reflect Ti-O π-bonding, an interpretation supported with density functional theory calculations.

Regioselective Formal [4 + 2] Cycloadditions of Enaminones with Diazocarbonyls through RhIII-Catalyzed C-H Bond Functionalization

A regioselective formal [4 + 2] cycloaddition for the assembly of highly functionalized benzene rings was successfully developed. In this reaction, olefinic C-H bond functionalization/cyclization cascade reaction followed by rearomatization led to the desired molecules in one step under mild reaction conditions. This protocol also displays a broad substrate scope and good tolerance to a wide range of functional groups. Additionally, the potential utility for the synthesis of highly conjugated polybenzenes and diversification of natural products was also demonstrated.

Carbon Dioxide-Mediated C(sp3)-H Arylation of Amine Substrates

Elaborating amines via C-H functionalization has been an important area of research over the past decade but has generally relied on an added directing group or sterically hindered amine approach. Since free-amine-directed C(sp³)-H activation is still primarily limited to cyclization reactions and to improve the sustainability and reaction scope of amine-based C-H activation, we present a strategy using CO₂ in the form of dry ice that facilitates intermolecular C-H arylation. This methodology has been used to enable an operationally simple procedure whereby 1° and 2° aliphatic amines can be arylated selectively at their γ-C-H positions. In addition to potentially serving as a directing group, CO₂ has also been demonstrated to curtail the oxidation of sensitive amine substrates.

Chiral sensors for determining the absolute configurations of α-amino acid derivatives

Two simple ¹H NMR tests give the absolute configurations of α-amino acids.

Natural Products as Platforms To Overcome Antibiotic Resistance

Natural products have served as powerful therapeutics against pathogenic bacteria since the golden age of antibiotics of the mid-20th century. However, the increasing frequency of antibiotic-resistant infections clearly demonstrates that new antibiotics are critical for modern medicine. Because combinatorial approaches have not yielded effective drugs, we propose that the development of new antibiotics around proven natural scaffolds is the best short-term solution to the rising crisis of antibiotic resistance. We analyze herein synthetic approaches aiming to reengineer natural products into potent antibiotics. Furthermore, we discuss approaches in modulating quorum sensing and biofilm formation as a nonlethal method, as well as narrow-spectrum pathogen-specific antibiotics, which are of interest given new insights into the implications of disrupting the microbiome.