A Guide to Directing Group Removal: 8-Aminoquinoline

Cobalt(II)-Catalyzed Selective C2-H Heck Reaction of Native (N-H) Indoles Enabled by Salicylaldehyde Ligand

Direct functionalization of native (N-H) indoles via C-H activation remains a challenge. Herein, we report a salicylaldehyde-promoted cobalt-catalyzed selective C2-H Heck reaction of native (N-H) indoles with both active and unactivated olefins in the presence of free N-H bonds. A series of structurally diverse C2-alkenylated native (N-H) indoles including natural product and drug derivatives were prepared directly and effectively without additional preprotection and deprotection procedures.

Salicylaldehyde-Enabled Co(II)-Catalyzed Oxidative C-H Alkenylation of Indoles with Olefins

A ligand-promoted oxidative dehydrogenation C-H alkenylation of indoles and olefins was achieved using commercial and low-cost Co(NO3)2·6H2O as a catalyst and Mn(OAc)2 as an oxidant. The design and selection of electrically unique methyl-substituted salicylaldehyde as a ligand is the key to achieve this transformation. This protocol can introduce an indole backbone into diverse bioactive molecules such as ibuprofen, naproxen, and Estrol for late-stage synthetic modification, which has potential applications in the discovery of drug molecules containing an indole motif.

Synthesis of biaryl-based carbazoles via C-H functionalization and exploration of their anticancer activities

The synthesis of a library of new biaryl-based carbazoles via bidentate directing group-assisted C-H functionalization and preliminary screening of the anticancer properties of biaryl-based carbazoles is reported. While various classes of modified carbazoles are known for their applications in materials and medicinal chemistry, to our knowledge, the biological activities of designed biaryl-based carbazoles have been rarely known. Given the prominence of carbazoles in research in medicinal chemistry, we envisioned the scope for new scaffolds of carbazole-based biaryl structures. We screened the synthesized biaryl-based carbazoles for their anticancer properties against various cancer cell lines such as HeLa (cervical cancer), HCT116 (colon cancer), MDA-MB-231 and MDA-MB-468 (breast cancer). In addition, the hits were also tested in the human embryonic kidney cell line HEK293T to assess their impact on the viability of normal human cells in the presence of these compounds. In this preliminary study, we identified some of the biaryl-based carbazoles as lead compounds with anticancer activities.

Transition Metal-Catalyzed C-H Activation/Functionalization of 8-Methylquinolines

8-Methylquinoline is regarded as an ideal substrate to participate in diversely C(sp3)-H functionalization reactions. The presence of the chelating nitrogen atom enables 8-methylquinoline to easily form cyclometallated complexes with various transition metals, leading to the selective synthesis of functionalized quinolines. Considering the great importance of quinoline cores in medicinal chemistry, in this review article, we have covered the publications related to the C-H activation and functionalization of 8-methylquinoline under transition metal catalysis during the last decade.

Anti-selective Cyclopropanation of Nonconjugated Alkenes with Diverse Pronucleophiles via Directed Nucleopalladation

A facile approach to obtaining densely functionalized cyclopropanes is described. The reaction proceeds under mild conditions via the directed nucleopalladation of nonconjugated alkenes with readily available pronucleophiles and gives excellent yields and good anti-selectivity using I2 and TBHP as oxidants. Pronucleophiles bearing a diverse collection of electron-withdrawing groups, including -CN, -CO2R, -COR, -SO2Ph, -CONHR, and -NO2, are well tolerated. Internal alkenes, which are generally challenging substrates in other cyclopropanation methods, provide excellent yields and good diastereoselectivity in this methodology, allowing for controlled access to cyclopropanes substituted at all three C atoms. DFT calculations and mechanistic experiments reveal that the major mechanistic pathway involves the initial α-iodination of the nucleophile, followed by anti-carbopalladation and intramolecular C(sp3)-I oxidative addition. Strain-release-promoted C(sp3)-C(sp3) reductive elimination then furnishes the cyclopropanated product.

Palladium-Catalyzed anti-Michael-Type (Hetero)arylation of Acrylamides

This paper reports a direct α-(hetero)arylation of acrylamides through an inverse electron-demand nucleophilic addition, specifically an anti-Michael-type addition. The introduction of a quinolyl directing group facilitates the nucleophilic addition of (hetero)arenes to the α-position of acrylamides. The quinolyl directing group effectively suppresses undesired β-hydrogen elimination and is removable for subsequent derivatization. The presented method provides an atom economical synthesis of α-(hetero)arylamide with a high degree of functional group tolerance.

Synthesis of 13C-methyl-labeled amino acids and their incorporation into proteins in mammalian cells

Isotopic labeling of methyl-substituted proteinogenic amino acids with 13C has transformed applications of solution-based NMR spectroscopy and allowed the study of much larger and more complex proteins than previously possible with 15N labeling. Procedures are well-established for producing methyl-labeled proteins expressed in bacteria, with efficient incorporation of 13C-methyl labeled metabolic precursors to enable the isotopic labeling of Ile, Val, and Leu methyl groups. Recently, similar methodology has been applied to enable 13C-methyl labeling of Ile, Val, and Leu in yeast, extending the approach to proteins that do not readily fold when produced in bacteria. Mammalian or insect cells are nonetheless preferable for production of many human proteins, yet 13C-methyl labeling using similar metabolic precursors is not feasible as these cells lack the requisite biosynthetic machinery. Herein, we report versatile and high-yielding synthetic routes to 13C methyl-labeled amino acids based on palladium-catalyzed C(sp3)-H functionalization. We demonstrate the efficient incorporation of two of the synthesized amino acids, 13C-γ2-Ile and 13C-γ1,γ2-Val, into human receptor extracellular domains with multiple disulfides using suspension-cultured HEK293 cells. Production costs are reasonable, even at moderate expression levels of 2-3 mg purified protein per liter of medium, and the method can be extended to label other methyl groups, such as 13C-δ1-Ile and 13C-δ1,δ2-Leu. In summary, we demonstrate the cost-effective production of methyl-labeled proteins in mammalian cells by incorporation of 13C methyl-labeled amino acids generated de novo by a versatile synthetic route.

Programmable Deuteration of Indoles via Reverse Deuterium Exchange

Methods for selective deuterium incorporation into drug-like molecules have become extremely valuable due to the commercial, mechanistic, and biological importance of deuterated compounds. Herein, we report a programmable labeling platform that allows access to C2, C3, or C2- and C3-deuterated indoles under mild, user-friendly conditions. The C2-deuterated indoles are accessed using a reverse hydrogen isotope exchange strategy which represents the first non-directed C2-deuteration of indoles.

Synthesis of HC-Toxin via Matteson Homologation and C-H Functionalization

A new synthetic route toward host-specific HC-toxin was developed. The HC-toxin belongs to a group of cyclic, tetrapeptide histone deacetylase inhibitors containing the unusual amino acid Aeo. Key steps in the synthesis of this building block include the Matteson homologation to generate the stereogenic centers in the side chain and a C-H functionalization to connect the side chain to a protected alanine.

Redox-Paired Alkene Difunctionalization Enables Skeletally Divergent Synthesis

Multistep organic synthesis enables conversion of simple chemical feedstocks into a more structurally complex product that serves a particular function. The target compound is forged over several steps, with concomitant generation of byproducts in each step to account for underlying mechanistic features of the reactions (e.g., redox processes). To map structure-function relationships, libraries of molecules are often needed, and these are typically prepared by iterating an established multistep synthetic sequence. An underdeveloped approach is designing organic reactions that generate multiple valuable products with different carbogenic skeletons in a single synthetic operation. Taking inspiration from paired electrosynthesis processes that are widely used in commodity chemical production (e.g., conversion of glucose to sorbitol and gluconic acid), we report a palladium-catalyzed reaction that converts a single alkene starting material into two skeletally distinct products in a single operation through a series of carbon-carbon and carbon-heteroatom bond-forming events enabled by mutual oxidation and reduction, a process that we term redox-paired alkene difunctionalization. We demonstrate the scope of the method in enabling simultaneous access to reductively 1,2-diarylated and oxidatively [3 + 2]-annulated products, and we explore the mechanistic details of this unique catalytic system using a combination of experimental techniques and density functional theory (DFT). The results described herein establish a distinct approach to small-molecule library synthesis that can increase the rate of compound production. Furthermore, these findings demonstrate how a single transition-metal catalyst can mediate a sophisticated redox-paired process through multiple pathway-selective events along the catalytic cycle.

Mechanistic Insights Into the Rhodium-Catalyzed C-H Alkenylation/Directing Group Migration and [3+2] Annulation: A DFT Study

The mechanism of the rhodium-catalyzed C-H alkenylation/directing group migration and [3+2] annulation of N-aminocarbonylindoles with 1,3-diynes has been investigated with DFT calculations. On the basis of mechanistic studies, we mainly focus on the regioselectivity of 1,3-diyne inserting into the Rh-C bond and the N-aminocarbonyl directing group migration involved in the reactions. Our theoretical study uncovers that the directing group migration undergoes a stepwise β-N elimination and isocyanate reinsertion process. As studied in this work, this finding is also applicable to other relevant reactions. Additionally, the role of Na⁺ versus Cs⁺ involved in the [3+2] cyclization reaction is also probed.

Selective Hydrolysis of Primary and Secondary Amides Enabled by Visible Light

Amide hydrolysis is a fundamentally important transformation in organic chemistry. Developing hydrolysis procedures under mild conditions with a broad substrate scope is desirable. Herein, by leveraging a photoresponsive auxiliary o-nitroanilide, we established a mild two-step protocol for the hydrolysis of primary and secondary amides. This protocol is driven by visible light irradiation at room temperature under neutral conditions, which tolerates numerous acid- and base-sensitive functional groups. Various drugs, natural product-, and amino acid-derived amides can be selectively hydrolyzed.

Copper-catalyzed selective C5-H bromination and difluoromethylation of 8-aminoquinoline amides using ethyl bromodifluoroacetate as the bifunctional reagent

A simple and effective method for the copper-catalyzed selective C5-H bromination and difluoromethylation of 8-aminoquinoline amides with ethyl bromodifluoroacetate as the bifunctional reagent was developed. The combination of cupric catalyst and alkaline additive results in a C5-bromination reaction, whereas cuprous catalyst combined with silver additive results in the C5-difluoromethylation reaction. This method has a broad substrate scope and allows for easy and convenient access to desired C5-functionalized quinolones with good to excellent yields.

Redox-neutral rhodium(III)-catalyzed divergent synthesis of tetrasubstituted 1,3-enynes and alkynylated benzofurans

With the assistance of the acetamido directing group (DG), a rhodium-catalyzed C-H alkenylation/DG migration cascade for the synthesis of tetrasubstituted 1,3-enynes from N-phenoxyacetamides and 1,3-diynes has been achieved in this work. Alternatively, a rhodium-catalyzed [3 + 2] annulation for the synthesis of alkynylated benzofurans from the same set of substrates has also been achieved by simply changing the reaction conditions. This work highlights the tunable divergent synthesis of valuable compounds triggered by C-H activation.

Site-Selective Palladium(II)-Catalyzed Methylene C(sp3)-H Diarylation of a Tropane Scaffold

A series of 2,4-di-arylated tropane derivatives was synthesized through a site-selective palladium-catalyzed β-C(sp³)-H di-arylation process. This type of structure has been scarcely reported in literature. They nevertheless represent an interesting class of biologically relevant molecules as illustrated by the observed activity at the micromolecular level of eight derivatives toward human colorectal cancer cell line HCT116.

Atroposelective Total Synthesis of Darobactin A

A concise, modular synthesis of the novel antibiotic darobactin A is disclosed. The synthesis successfully forges the hallmark strained macrocyclic ring systems in a sequential fashion. Key transformations include two atroposelective Larock-based macrocyclizations, one of which proceeds with exquisite regioselectivity despite bearing an unprotected alkyne. The synthesis is designed with medicinal chemistry considerations in mind, appending key portions of the molecule at a late stage. Requisite unnatural amino acid building blocks are easily prepared in an enantiopure form using C-H activation and decarboxylative cross-coupling tactics.

4-Aminobenzotriazole (ABTA) as a Removable Directing Group for Palladium-Catalyzed Aerobic Oxidative C-H Olefination

4-Aminobenzotriazole (ABTA) was applied as an effective removable directing group (DG) in Pd-catalyzed C-H activation for the first time. Compared with the widely applied pyridine and quinoline analogs, ABTA showed significantly improved reactivity, achieving aerobic oxidative C-H olefination in excellent yields (up to 95% vs <50% with other reported DGs under identical conditions). Using this new strategy, macrocyclization was achieved to give cyclic peptides in good yields with easy ABTA removal under mild conditions, highlighting the promising potential of this new DG.

Rhodium-catalyzed cascade C–H activation/annulation/1,6-acyl migration: direct construction of free N–H indoles under mild conditions

Rh-Catalyzed regioselective cascade C–H activation/annulation/1,6-acyl migration of N-acetanilides with alkynes via C–C/C–N/C–O bond formation is developed.

Transamidation and Decarbonylation of N-Phthaloyl-Amino Acid Amides Enabled by Palladium-Catalyzed Selective C-N Bond Cleavage

Amides are important functional synthons that have been widely used in the construction of peptides, natural products, and drugs. The C-N bond cleavage provides the direct method for amide conversion. However, amides, especially secondary amides, tend to be chemically inert due to the resonance of the amide bond. Here, we describe an efficient Pd-catalyzed transamidation and decarbonylation of multiamide structure molecules through C-N bond cleavage with excellent chemoselectivity. The transamidation of secondary amides and the decarbonylation of phthalimide provide meaningful tools for the modification of amino acid derivatives. Moreover, further transformations of azidation and C(sp³)-H monoarylation emphasized the potential utility of this selective C-N bond cleavage method.

Transition-Metal-Catalyzed, Coordination-Assisted Functionalization of Nonactivated C(sp3)-H Bonds

Transition-metal-catalyzed, coordination-assisted C(sp³)-H functionalization has revolutionized synthetic planning over the past few decades as the use of these directing groups has allowed for increased access to many strategic positions in organic molecules. Nonetheless, several challenges remain preeminent, such as the requirement for high temperatures, the difficulty in removing or converting directing groups, and, although many metals provide some reactivity, the difficulty in employing metals outside of palladium. This review aims to give a comprehensive overview of coordination-assisted, transition-metal-catalyzed, direct functionalization of nonactivated C(sp³)-H bonds by covering the literature since 2004 in order to demonstrate the current state-of-the-art methods as well as the current limitations. For clarity, this review has been divided into nine sections by the transition metal catalyst with subdivisions by the type of bond formation. Synthetic applications and reaction mechanism are discussed where appropriate.

Facile Synthesis of Alkylidene Phthalides by Rhodium-Catalyzed Domino C-H Acylation/Annulation of Benzamides with Aliphatic Carboxylic Acids

The Rh-catalyzed ortho-C(sp² )-H functionalization of 8-aminoquinoline-derived benzamides with aliphatic acyl fluorides generated in situ from the corresponding acids has been developed. This reaction initiated with 8-aminoquinoline-directed ortho-C(sp² )-H acylation, which was accompanied by subsequent intramolecular nucleophilic acyl substitution of amide group to produce alkylidene phthalides This approach exhibits high stereo-selectivity for Z-isomer products, and tolerates a variety of functional groups as well as aliphatic carboxylic acids with diverse structural scaffolds.

Salicylaldehyde-Promoted Cobalt-Catalyzed C-H/N-H Annulation of Indolyl Amides with Alkynes: Direct Synthesis of a 5-HT3 Receptor Antagonist Analogue

A cobalt-catalyzed annulation of the C(sp²)-H/N-H bond of indoloamides with alkynes assisted by 8-aminoquinoline is reported for the synthesis of six-membered indololactams. The use of salicylaldehyde as the ligand is crucial for this transformation. The protocol has a broad scope for both alkynes and indoles. Preparing an active Co complex illustrates that salicylaldehyde plays a key role in the C-H activation step. The synthetic applications are proven by the gram-scale reaction and one-step construction of the multicyclic 5-HT3 receptor antagonist.

Pd/Cu-Catalyzed amide-enabled selectivity-reversed borocarbonylation of unactivated alkenes

The addition reaction between CuBpin and alkenes to give a terminal boron substituted intermediate is usually fast and facile. In this communication, a selectivity-reversed procedure has been designed and established. This selectivity-reversed borocarbonylation reaction is enabled by a cooperative action between palladium and copper catalysts and proceeds with complete regioselectivity. The key to the success of this transformation is the coordination of the amide group and slower CuBpin formation by using KHCO₃ as the base. A wide range of β-boryl ketones were produced from terminal unactivated aliphatic alkenes and aryl iodides. Further synthetic transformations of the obtained β-boryl ketones have been developed as well.

A selectivity-reversed borocarbonylation reaction has been developed with complete regioselectivity.

Progress and perspectives on directing group-assisted palladium-catalysed C-H functionalisation of amino acids and peptides

Peptide modifications can unlock a variety of compounds with structural diversity and abundant biological activity. In nature, peptide modifications, such as functionalisation at the side-chain position of amino acids, are performed using post-translational modification enzymes or incorporation of unnatural amino acids. However, accessing these modifications remains a challenge for organic chemists. During the past decades, selective C-H activation/functionalisation has attracted considerable attention in synthetic organic chemistry as a pathway to peptide modification. Various directing group strategies have been discovered that assist selective C-H activation. In particular, bidentate directing groups that enable tuneable and reversible coordination are now recognised as one of the most efficient methods for the site-selective C-H activation and functionalisation of numerous families of organic compounds. Synthetic peptide chemists have harnessed bidentate directing group strategies for selective functionalisation of the β- and γ-positions of amino acids. This method has been expanded and recognised as an effective device for the late stage macrocyclisation and total synthesis of complex peptide natural products. In this review, we discuss various β-, γ-, and δ-C(sp³)-H bond functionalisation reactions of amino acids for the formation of C-X bonds with the aid of directing groups and their application in late-stage macrocyclisation and the total synthesis of complex peptide natural products.

Copper-mediated oxidative C-H/N-H activations with alkynes by removable hydrazides

The efficient copper-mediated oxidative C-H alkynylation of benzhydrazides was accomplished with terminal alkynes. Thus, a heteroaromatic removable N-2-pyridylhydrazide allowed for domino C-H/N-H functionalization. The approach featured remarkable functional group compatibility and ample substrate scope. Thereby, highly functionalized aromatic and heteroaromatic isoindolin-1-ones were accessed with high efficacy with rate-limiting C-H cleavage.