Recent applications of C-H functionalization in complex natural product synthesis

Rhodium(III)-Catalyzed Synthesis of Skipped Enynes via C(sp3 )-H Alkynylation of Terminal Alkenes

The Rh^III -catalyzed allylic C-H alkynylation of non-activated terminal alkenes leads selectively to linear 1,4-enynes at room-temperature. The catalytic system tolerates a wide range of functional groups without competing functionalization at other positions. Similarly, the vinylic C-H alkynylation of α,β- and β,γ- unsaturated amides gives conjugated Z-1,3-enynes and E-enediynes.

Direct Arylation of Azoles Enabled by Pd/Cu Dual Catalysis

A practical approach toward the synthesis of 2-arylazoles via direct arylation is described. The transformation relies on a Pd/Cu cocatalyst system that operates with low catalyst loadings. The reaction conditions were found to be tolerant of a wide range of functional groups and nitrogen-containing heterocycles commonly employed in a drug discovery setting.

Rh(III)-Catalyzed Redox-Neutral C-H Activation/[3 + 2] Annulation of N-Phenoxy Amides with Propargylic Monofluoroalkynes

An efficient and redox-neutral Rh(III)-catalyzed C-H activation/[3 + 2] annulation of N-phenoxy amides with propargylic monofluoroalkynes has been realized to afford 3-alkylidene dihydrobenzofurans with an interesting α-quaternary carbon center. Combined experimental and computational mechanistic studies revealed that a Rh(III)-Rh(V)-Rh(III) catalytic pathway/uncatalyzed intramolecular [H···F] bonding-assisted S_N2'-type substitution cascade might be involved in the catalytic cycle, thereby enabling an excellent site-/regioselectivity with broad substrate/functional group compatibility, including the complete retention of the highly strained cyclobutyl structure in the 3-position.

Kinetic Resolution of Allyltriflamides through a Pd-Catalyzed C-H Functionalization with Allenes: Asymmetric Assembly of Tetrahydropyridines

Enantioenriched, six-membered azacycles are essential structural motifs in many products of pharmaceutical or agrochemical interest. Here we report a simple and practical method for enantioselective assembly of tetrahydropyridines, which is paired to a kinetic resolution of α-branched allyltriflamides. The reaction consists of a formal (4+2) cycloaddition between the allylamine derivatives and allenes and is initiated by a palladium(II)-catalyzed C-H activation process. Both the chiral allylamide precursors and the tetrahydropyridine adducts were successfully obtained in high yields, with excellent enantioselectivity (up to 99% ee) and selectivity values of up to 127.

Enantioselective Pd0 -Catalyzed C(sp2 )-H Arylation for the Synthesis of Chiral Warped Molecules

C-H activation-based ring-forming methods are a powerful approach for the construction of complex molecular architectures, especially those containing a congested stereocenter. Therefore, this strategy seems perfectly suited to address the synthesis of chiral polycyclic aromatic hydrocarbons (PAHs) and bowl-shaped molecules, which are important target molecules in the field of organic electronic materials. Herein, we describe an enantioselective Pd⁰ -catalyzed C(sp² )-H arylation protocol for the synthesis of chiral fluoradenes and other warped molecules, which could serve to the bottom-up construction of chiral PAHs. The current approach relies on the use of chiral bifunctional phosphine-carboxylate ligands and delivers diverse polycyclic compounds in high yield and with good to excellent enantioselectivity. The chiroptical properties of the obtained products were investigated, and some of them were found to have a strong ellipticity and an emission band located in the visible region.

Enzymatic Kinetic Resolution by Addition of Oxygen

Kinetic resolution using biocatalysis has proven to be an excellent complementary technique to traditional asymmetric catalysis for the production of enantioenriched compounds. Resolution using oxidative enzymes produces valuable oxygenated structures for use in synthetic route development. This Minireview focuses on enzymes which catalyse the insertion of an oxygen atom into the substrate and, in so doing, can achieve oxidative kinetic resolution. The Baeyer-Villiger rearrangement, epoxidation, and hydroxylation are included, and biological advancements in enzyme development, and applications of these key enantioenriched intermediates in natural product synthesis are discussed.

Catalyst-free and additive-free reactions enabling C-C bond formation: a journey towards a sustainable future

This review focuses on the catalyst- and additive-free C-C bond forming reactions reported mostly from the year 2005 to date. C-C bond forming reactions are highly important as large and complex organic molecules can be derived from simpler ones via these reactions. On the other hand, catalyst- and additive-free reactions are economical, environmentally friendly and less sensitive to air/moisture, allow easy separation of products and are operationally simple. Hence, a large number of research articles have been published in this area. Though a few reviews are available on the catalyst-free organic reactions, most of them were published a few years ago. The current review excludes catalysts as well as additives and is specific to only C-C bond formation. Besides many organic name reactions, catalyst/additive-free C-H functionalizations, coupling reactions and UV-visible-light-promoted reactions are also discussed. Undoubtedly, the contents of this review will motivate readers to do more novel work in this area which will accelerate the journey towards a sustainable future.

Catalytic C-H Functionalization of Unreactive Furan Cores in Bio-Derived Platform Chemicals

C-H functionalization is one of the most convenient and powerful tools in the arsenal of modern chemistry, deservedly nominated as the "Holy Grail" of organic synthesis. A frequent disadvantage of this method is the need for harsh reaction conditions to carry out transformations of inert C-H bonds, which limits the possibility of its use for modifying less stable substrates. Biomass-derived furan platform chemicals, which have a relatively unstable aromatic furan core and highly reactive side chain substituents, are extremely promising and valuable organic molecules that are currently widely used in a variety of research and industrial fields. The high sensitivity of furan derivatives to acids, strong oxidants, and high temperatures significantly limits the use of classical methods of C-H functionalization for their modification. New methods of catalytic functionalization of non-reactive furan cores are urgently required to obtain a new generation of materials with controlled properties and potentially bioactive substances.

C(sp2)-H functionalization in non-aromatic azomethine-based heterocycles

Direct C(sp2)-H functionalization of the endocyclic azomethine and aldonitrone moieties in non-aromatic azaheterocycles has established itself as a promising methodology over the last decade. Transition metal-catalyzed cross-coupling reactions, α-metalation-electrophile quenching protocols, and (metal-free) nucleophilic substitution of hydrogen reactions (SNH) are the major routes applied on cyclic imines and their derivatives. In this overview, we show the tangible progress made in this area during the period from 2008 to 2020.

Palladium-Catalyzed Regiospecific peri- and ortho-C-H Oxygenations of Polyaromatic Rings Mediated by Tunable Directing Groups

An efficient divergent approach of Pd-catalyzed C-H oxygenation of polyaromatic rings is described. Reversible directing groups enable regiospecific peri- and ortho-oxygenation to readily access a wide array of polyaromatic phenols without pre- and postmanipulation of directing groups. The systematic mechanistic investigation, including deuterium-labeling experiments, palladacycle trapping, and DFT calculations, reveals that the tunable ligand-assisted C-H bond cleavage played a crucial role during the reaction process.

Reframing primary alkyl amines as aliphatic building blocks

While primary aliphatic amines are ubiquitous in natural products, they are traditionally considered inert to substitution chemistry. This review highlights historical and recent advances in the field of aliphatic deamination chemistry which demonstrate these moieties can be harnessed as valuable C(sp3) synthons. Cross-coupling and photocatalyzed transformations proceeding through polar and radical mechanisms are compared with oxidative deamination and other transition metal catalyzed reactions.

Rapid Construction of Tetralin, Chromane, and Indane Motifs via Cyclative C-H/C-H Coupling: Four-Step Total Synthesis of (±)-Russujaponol F

The development of practical C-H/C-H coupling reactions remains a challenging yet appealing synthetic venture because it circumvents the need to prefunctionalize both coupling partners for the generation of C-C bonds. Herein we report a cyclative C(sp³)-H/C(sp²)-H coupling reaction of free aliphatic acids enabled by a cyclopentane-based mono-N-protected β-amino acid ligand. This reaction uses inexpensive sodium percarbonate (Na₂CO₃·1.5H₂O₂) as the sole oxidant and generates water as the only byproduct. A range of biologically important scaffolds, including tetralins, chromanes, and indanes, can be easily prepared by this protocol. Finally, the synthetic application of this methodology is demonstrated by the concise total synthesis of (±)-russujaponol F in a four-step sequence starting from readily available phenylacetic acid and pivalic acid through sequential functionalizations of four C-H bonds.

Rhodium-catalyzed enone carbonyl directed C–H activation for the synthesis of indanones containing all-carbon quaternary centers

Rhodium(iii)-catalyzed enone carbonyl directed C–H activation/annulation of α-aroyl ketene dithioacetals with diazo compounds has been realized for the synthesis of β-quaternary indanones.

Recent advances in transition metal-catalyzed olefinic C–H functionalization

Recent advances during 2015–2020 in the field of metal-catalyzed olefinic C–H functionalization are organized according to the metal center of the catalyst, with an emphasis on the similarities and differences among different catalysts.

Recent advances and perspectives in manganese-catalyzed C–H activation

Manganese-catalyzed C–H activation has become an emerging area in organic chemistry. These efficient and eco-friendly manganese catalysed reactions provides new opportunities in the field of synthetic organic chemistry.

Visible-light-promoted α-methoxymethylation and aminomethylation of ketones with methanol as the C1 source

Visible-light-promoted α-methoxymethylation and aminomethylation of ketones using methanol as a sustainable C1 source have been developed. With rose bengal as the photosensitizer and air as the green oxidant, the methoxymethylation reactions proceeded smoothly under visible light irradiation at ambient temperature. Additionally, a one-pot one-step α-aminomethylation of ketones was achieved by adding N-nucleophiles. Preliminary mechanism studies suggest that the reaction mainly proceeds via a radical pathway.

Asymmetric formal synthesis of (−)-tetrazomine

The asymmetric formal synthesis of (−)-tetrazomine, a structurally unique member of the tetrahydroisoquinoline alkaloids, is presented.

Recent advances in hydride transfer-involved C(sp3)–H activation reactions

This review summarizes the recent progresses (2016–2020) in the hydride transfer-enabled C(sp³)–H activation according to the reaction types, categorized into the intramolecular/intermolecular C(sp³)–H functionalization, and hydride reduction.

Remote C(sp3)–H functionalization via catalytic cyclometallation: beyond five-membered ring metallacycle intermediates

Despite impressive recent momentum gained in C(sp3)–H activation, achieving high regioselectivity in molecules containing different C–H bonds with similar high energy without abusing tailored substitution remains as one of the biggest challenges.

Iridium-catalyzed regioselective C–H sulfonamidation of 1,2,4-thiadiazoles with sulfonyl azides in water

We have developed a regioselective C–N cross-coupling of 1,2,4-thiadiazoles with sulfonyl azides through iridium catalysis in water. This method tactically linked the 1,2,4-thiadiazoles and sulfonamides together, and the novel molecules increased the diversity of 1,2,4-thiadiazoles which may have potential applications.

We have developed a regioselective C–N cross-coupling of 1,2,4-thiadiazoles with sulfonyl azides through iridium catalysis in water.

Divergent Synthesis of Bioactive Dithiodiketopiperazine Natural Products Based on a Double C(sp3 )-H Activation Strategy

This article provides a detailed report of our efforts to synthesize the dithiodiketopiperazine (DTP) natural products (-)-epicoccin G and (-)-rostratin A using a double C(sp³ )-H activation strategy. The strategy's viability was first established on a model system lacking the C8/C8' alcohols. Then, an efficient stereoselective route including an organocatalytic epoxidation was secured to access a key bis-triflate substrate. This bis-triflate served as the functional handles for the key transformation of the synthesis: a double C(sp³ )-H activation. The successful double activation opened access to a common intermediate for both natural products in high overall yield and on a multigram scale. After several unsuccessful attempts, this intermediate was efficiently converted to (-)-epicoccin G and to the more challenging (-)-rostratin A via suitable oxidation/reduction and protecting group sequences, and via a final sulfuration that occurred in good yield and high diastereoselectivity. These efforts culminated in the synthesis of (-)-epicoccin G and (-)-rostratin A in high overall yields (19.6 % over 14 steps and 12.7 % over 17 steps, respectively), with the latter being obtained on a 500 mg scale. Toxicity assessments of these natural products and several analogues (including the newly synthesized epicoccin K) in the leukemia cell line K562 confirmed the importance of the disulfide bridge for activity and identified dianhydrorostratin A as a 20x more potent analogue.

New Strategies in the Efficient Total Syntheses of Polycyclic Natural Products

Polycyclic natural products are an inexhaustible source of medicinal agents, and their complex molecular architecture renders challenging synthetic targets where innovative and effective approaches for their rapid construction are urgently required. The total synthesis of polycyclic natural products has witnessed exponential progression along with the emergence of new synthetic strategies and concepts, such as sequential C-H functionalizations, radical-based transformations, and functional group pairing strategies. Our group exerts continued interest in the construction of bioactive and structurally complex natural products as well as evaluation of the mode of action of these molecules. In this Account, we will showcase how these new synthetic strategies are employed and guide our total synthesis endeavors.During the last two decades, a series of remarkable advances in C-H functionalization have led to the emergence of many new approaches to directly functionalize C-H bonds into useful functional groups. These selective transformations have provided a great opportunity for the step- and atom-economical construction of key fragments in complex molecule synthesis. We recently furnished the total syntheses for polycyclic natural products: incarviatone A, chrysomycin A, polycarcin V, and gilvocarcin V by employing a multiple C-H bond functionalization strategy. The polysubstituted benzene or naphthalene skeleton was constructed through sequential and site-selective C-H functionalizations from readily available simple starting materials, which reduced the number of steps and streamlined synthesis.Recently, we have also completed the total syntheses for a number of skeletally diverse tetracyclic Isodon diterpenoids inspired by their biogenesis and radical-based retrosynthetic disconnections. Radical transformations are strategically and tactically utilized in our syntheses, and radical-based reactions, including organo-SOMO catalysis, Birch reduction, regioselective 1,6-dienyne reductive cyclization, visible-light-mediated Schenck ene reaction, and photoradical-mediated late-stage skeletal rearrangement, play significant roles in our synthetic endeavors. Protecting-group-free and scalable syntheses are also built into our work to achieve the "ideal" synthesis. Furthermore, our synthetic work reveals that late-stage skeletal rearrangement through a photo radical process is possible in a biological setting in complement with nature's carbocation chemistry in complex natural product biosynthesis.Lycopodium alkaloids are a large family of structurally unique polycyclic natural products with impressive biological activities. Owing to their fascinating polycyclic architectures and diverse biological activities, these alkaloids have continued to serve as targets as well as inspirations for the synthetic community for decades. To access these bioactive natural products or natural product-like molecules for biological exploration and drug discovery, we applied a novel functional group pairing strategy to furnish the total syntheses for several Lycopodium alkaloids and obtained numerous skeletally diverse compounds with structural complexity comparable to natural products.

Polarclean/Water as a Safe and Recoverable Medium for Selective C2-Arylation of Indoles Catalyzed by Pd/C

Herein, we report the use of nontoxic, water-miscible Polarclean as a safe dipolar aprotic solvent for the metal-catalyzed direct C2-H arylation of indoles using Pd/C as a catalyst. The developed method allows reaching excellent yields and regioselectivities, and it tolerates various substituents on both indole and diaryliodonium salt scaffolds. Polarclean is fully recoverable and reusable; it shows a very low leaching of the metal catalyst, allowing its complete recovery and reuse for at least six representative reaction runs.

Spatial Anion Control on Palladium for Mild C-H Arylation of Arenes

C-H arylation of arenes without the use of directing groups is a challenge, even for simple molecules, such as benzene. We describe spatial anion control as a concept for the design of catalytic sites for C-H bond activation, thereby enabling nondirected C-H arylation of arenes at ambient temperature. The mild conditions enable late-stage structural diversification of biologically relevant small molecules, and site-selectivity complementary to that obtained with other methods of arene functionalization can be achieved. These results reveal the potential of spatial anion control in transition-metal catalysis for the functionalization of C-H bonds under mild conditions.

Recent Advances in Construction of Polycyclic Natural Product Scaffolds via One-Pot Reactions Involving Alkyne Annulation

Polycyclic scaffolds are omnipresent in natural products and drugs, and the synthetic strategies and methods toward construction of these scaffolds are of particular importance. Compared to simple cyclic ring systems, polycyclic scaffolds have higher structure complexity and diversity, making them suitable for charting broader chemical space, yet bringing challenges for the syntheses. In this review, we surveyed progress in the past decade on synthetic methods for polycyclic natural product scaffolds, in which the key steps are one-pot reactions involving intermolecular or intramolecular alkyne annulation. Synthetic strategies of selected polycyclic carbocycles and heterocycles with at least three fused, bridged, or spiro rings are discussed with emphasis on the synthetic efficiency and product diversity. Recent examples containing newly developed synthetic concepts or toolkits such as collective and divergent total synthesis, gold catalysis, C–H functionalization, and dearomative cyclization are highlighted. Finally, several “privileged synthetic strategies” for “privileged polycyclic scaffolds” are summarized, with discussion of remained challenges and future perspectives.

Activity-Directed Synthesis: A Flexible Approach for Lead Generation

Activity-directed synthesis (ADS) is a structure-blind, functional-driven molecular discovery approach. In this Concept, four case studies highlight the general applicability of ADS and showcase its flexibility to support different medicinal chemistry strategies. ADS deliberately harnesses reactions with multiple possible outcomes, and allows many chemotypes to be evaluated in parallel. Resources are focused on bioactive molecules, which emerge in tandem with associated synthetic routes. Some of the future challenges for ADS are highlighted, including the realisation of an autonomous molecular discovery platform. The prospects for ADS to become a mainstream lead generation approach are discussed.

Palladium(II)-catalyzed asymmetric C-H carbonylation to diverse isoquinoline derivatives bearing all-carbon quaternary stereocenters

Enantioselective synthesis of tetrahydroisoquinolines bearing an all-carbon quaternary stereogenic center, was achieved via asymmetric C-H activation with high enantioselectivities (up to 93% ee). Fair substrate tolerance was indicated throughout the scope investigation and no evident loss of enantioselectivity was exhibited in late-stage derivatization. This study provides incentives for the construction of diverse chiral isoquinoline derivatives, which are prevalent among pharmaceuticals, natural products, etc.