Methylene C(sp3 )-H Arylation Enables the Stereoselective Synthesis and Structure Revision of Indidene Natural Products

The divergent synthesis of two indane polyketides of the indidene family, namely (±)-indidene A (11 steps, 1.7 %) and (+)-indidene C (13 steps, 1.3 %), is reported. The synthesis of the trans-configured common indane intermediate was enabled by palladium(0)-catalyzed methylene C(sp3 )-H arylation, which was performed in both racemic and enantioselective (e.r. 99 : 1) modes. Further elaboration of this common intermediate by nickel-catalyzed dehydrogenative coupling allowed the rapid installation of the aroyl moiety of (±)-indidene A. In parallel, the biphenyl system of (±)- and (+)-indidene C was constructed by Suzuki-Miyaura coupling. These investigations led us to revise the structures of indidenes B and C.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Rhodium-Catalyzed Diastereo- and Enantioselective Divergent Annulations between Cyclobutanones and 1,5-Enynes: Rapid Construction of Complex C(sp3)-Rich Scaffolds

Given the emerging demand to "escape from flatland" for drug discovery, synthetic methods that can efficiently construct complex three-dimensional structures with multi-stereocenters become increasingly valuable. Here, we describe the development of Rh(I)-catalyzed intramolecular annulations between cyclobutanones and 1,5-enyne groups to construct complex C(sp3)-rich scaffolds. Divergent reactivities are realized with different catalysts, and excellent diastereo- and enantioselectivity have been achieved. The use of (R)-H8-binap as the ligand favors forming the bis-bicyclic scaffolds with multiple quaternary stereocenters, while the (R)-segphos ligand prefers to generate the tetrahydro-azapinone products. Owing to the versatile reactivity of ketone moieties, these C(sp3)-rich scaffolds can be further functionalized. Experimental and computational mechanistic studies support a reaction pathway involving enyne-cyclometallation, 1,2-carbonyl addition, and then β-carbon elimination; the divergent reactivities are dictated by a product-determining Rh-alkyl migratory insertion step.

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.

2-(Pyridin-2-yl)isopropyl (PIP) Amine: An Enabling Directing Group for Divergent and Asymmetric Functionalization of Unactivated Methylene C(sp3)-H Bonds

Directing group (DG) assistance provides a good solution to the problems of reactivity and selectivity, two of the fundamental challenges in C(sp³)-H activation. However, the activation of unbiased methylene C(sp³)-H bonds remains challenging due to the high heterolytic bond dissociation energy and substantial steric hindrance. Two main strategies have been developed thus far, that is, use of a strongly coordinating bidentate DG pioneered by Daugulis and use of a weakly coordinating monodentate DG accelerated by pyridine-type ligands, as disclosed by Yu. The seminal work by Daugulis sparked significant interest in the application of the monoanionic bidentate auxiliary in aliphatic C-H activation reactions. Our research has focused on enabling the divergent functionalization and enantiotopic differentiation of unactivated methylene C-H bonds. Inspired by the structure of bidentate 8-aminoquinoline and the accelerating effect of the gem-dimethyl moiety in cyclometalations, we developed a strongly coordinating bidentate 2-(pyridine-yl)isopropyl (PIP) amine DG consisting of a pyridyl group, a gem-dimethyl moiety, and an amino group, which enabled the divergent functionalization of unactivated β-methylene C(sp³)-H bonds to forge C-O, C-N, C-C, and C-F bonds with palladium catalysts. The exclusive β-selectivity was ascribed to the preferential formation of kinetically favored [5,5]-bicyclic palladacycle intermediates. DFT calculations revealed that the well-designed gem-dimethyl group was responsible for the lowered energy and compressed bite angle of the key transition state related to C-H cleavage.More recently, the combination of PIP amine with axially chiral ligands was found to promote asymmetric functionalization of unbiased methylene C(sp³)-H bonds, a challenging research topic in the area of C-H activation that remains to be addressed. Two different types of axially chiral ligands, namely, non-C₂-symmetric chiral phosphoric acids (CPAs) and 3,3'-disubstituted BINOLs, have been developed. The former enabled Pd(II)-catalyzed inter- and intramolecular arylation of unbiased methylene C(sp³)-H bonds with high enantioselectivity, whereas the latter promoted a series of asymmetric functionalization reactions, such as alkynylation, arylation, alkenylation/aza-Wacker cyclization, and intramolecular amidation. The unexpectedly high stereocontrol compared with other bidentate DGs might be attributable to steric communication between the ligand and gem-dimethyl moiety of PIP amine. Thus far, the combination of PIP amine DG with 3,3'-disubstituted BINOL ligands is arguably the most general strategy for asymmetric functionalization of unbiased methylene C(sp³)-H bonds. Finally, the ease of installation and removal of PIP under mild conditions and synthetic applications are described.

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.

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.

Multiple Catalytic C-H Bond Functionalization for Natural Product Synthesis

In the past decade, multiple catalytic C-H bond functionalization has been successfully applied in natural product synthesis as a strategy to reduce the number of steps, increase overall yield and employ more easily available starting materials. This minireview presents selected examples making use of multiple C-H bond functionalization in conceptually different ways. First, linear syntheses are discussed, wherein multiple C-H functionalization is employed either from simple (hetero)cyclic cores, at a late stage, or to build polycyclic systems. Second, the use of multiple C-H functionalization as a strategic tool in convergent synthesis to access and couple complex fragments is discussed. Information on the scalability of the employed methods is provided when available. The presented cases indicate that multiple C-H functionalization strategies should play a great role to shape the future synthesis of functional complex molecules with improved sustainability.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

Efficient and Divergent Total Synthesis of (-)-Epicoccin G and (-)-Rostratin A Enabled by Double C(sp3)-H Activation

Dithiodiketopiperazines are complex polycyclic natural products possessing a variety of interesting biological activities. Despite their interest, relatively few total syntheses have been completed. We herein report the enantioselective, scalable, and divergent total synthesis of two symmetrical pentacyclic dithiodiketopiperazines, (-)-epicoccin G and (-)-rostratin A. A common intermediate was synthesized on a multigram scale from inexpensive, commercially available starting materials using an enantioselective organocatalytic epoxidation and a double C(sp³)-H activation as key steps, with the latter allowing the efficient simultaneous construction of the two five-membered rings. In addition to the cis,cis-fused target (-)-epiccocin G, the more challenging (-)-rostratin A, possessing two trans ring junctions, was obtained for the first time on a 500 mg scale through the optimization of each step and validation on multigram quantities. Both natural products were synthesized with high overall yields (13-20%). This study should facilitate access to this fascinating and yet understudied family of biologically active natural products.

Ring Construction by Palladium(0)-Catalyzed C(sp3)-H Activation

The catalytic activation and functionalization of unactivated C(sp³)-H bonds of alkyl groups has undergone intense development in recent years. In particular, a variety of directing groups as well as native functional groups have been employed in combination with palladium(II) catalysis in order to perform a variety of intermolecular, and to some extent intramolecular reactions. In parallel, inspired by precedents in C(sp²)-H arylation, our group and others have developed a different approach, which is the focus of this Account. This strategy relies on the use of oxidative addition of a carbon-leaving group bond to palladium(0) to induce intramolecular C(sp³)-H activation and the subsequent formation of a C(sp²)-C(sp³) or C(sp³)-C(sp³) bond. Since our first publication in 2003, the construction of olefins and, more interestingly, of an array of valuable monocyclic and polycyclic systems has been reported according to this principle. (Hetero)aryl bromides were initially employed as reactants, but the scope was later expanded to include (hetero)aryl chlorides and triflates, alkenyl bromides, carbamoyl chlorides and α-chloroamides. Mechanistic studies enabled a better understanding of the C-H activation step, which was proposed to occur through ambiphilic metal-ligand activation-6 (AMLA-6), also known as concerted metalation deprotonation (CMD), and a better rationalization of the observed selectivity patterns. Moreover, the wealth of accumulated experimental data indicate that the number of atoms separating the C-H bond from Pd and the type of C-H bond are the main factors controlling the site-selectivity of the C-H bond cleavage. Recent efforts have been devoted to the development of enantioselective reactions. To this purpose, two different strategies have been employed: a chiral ancillary ligand in combination with an achiral base, and a chiral base in combination with an achiral ligand, and allowed for the achievement of high enantioselectivities in the construction of both tri- and tetrasubstituted stereocenters. On the other hand, the current C-H activation-based ring-forming method was applied to the synthesis of pharmacologically active substances and agrochemicals, as well as complex natural products such as the aeruginosins, thereby demonstrating its great potential for step-economical organic synthesis.

Palladium-Catalyzed Pyrazole-Directed sp3 C-H Bond Arylation for the Synthesis of β-Phenethylamines

We have developed a method for palladium-catalyzed, pyrazole-directed sp³ C-H bond arylation by aryl iodides. The reaction employs a Pd(OAc)₂ catalyst at 5-10 mol % loading and silver(I) oxide as a halide-removal agent, and it proceeds in acetic acid or acetic acid/hexafluoroisopropanol solvent. Ozonolysis of the pyrazole moiety affords pharmaceutically important β-phenethylamines.

Metal-free one-pot α-carboxylation of primary alcohols

An efficient metal-free procedure for the formal α-carboxylation of primary alcohols has been developed. The method involves a one-pot oxidation/Passerini/hydrolysis sequence and provides access to α-hydroxy acids bearing a broad range of functional groups. A minor modification to the reaction conditions extends the range of accessible products to α-hydroxy esters.

Stereoselective alkoxycarbonylation of unactivated C(sp3)-H bonds with alkyl chloroformates via Pd(II)/Pd(IV) catalysis

Several examples on Pd-catalysed carbonylation of methyl C(sp³)-H bonds with gaseous CO via Pd(II)/Pd(0) catalysis have been reported. However, methylene C(sp³)-H carbonylation remains a great challenge, largely due to the lack of reactivity of C-H bonds and the difficulty in CO migratory insertion. Herein, we report the stereoselective alkoxycarbonylation of both methyl and methylene C(sp³)-H bonds with alkyl chloroformates through a Pd(II)/Pd(IV) catalytic cycle. A broad range of aliphatic carboxamides and alkyl chloroformates are compatible with this protocol. In addition, this process is scalable and the directing group could be easily removed under mild conditions with complete retention of configuration.

Synthesis and Application of Tetrahydro-2H-fluorenes by a Pd(0)-Catalyzed Benzylic C(sp(3) )-H Functionalization

A new method has been developed for the synthesis of tetrahydro-2H-fluorenes based on a Pd(0)-catalyzed benzylic C(sp(3) )-H functionalization. Importantly, the success of the cyclization step was dependent on there being substituents at the two positions ortho to the benzylic group to avoid an undesired C(sp(2) )-H functionalization. This method was subsequently used to prepare the right-hand fragment of the hexacyclic triterpenoid benzohopanes, and therefore represents a powerful tool for the construction of the related compounds.

Synthesis of Strained γ-Lactams by Palladium(0)-Catalyzed C(sp(3) )-H Alkenylation and Application to Alkaloid Synthesis

A variety of strained α-alkylidene-γ-lactams were synthesized by palladium(0)-catalyzed intramolecular C(sp(3) )-H alkenylation from easily accessible acyclic and monocyclic bromoalkene precursors. These lactams are valuable intermediates for accessing various classes of mono- and bicylic alkaloids containing a pyrrolidine ring, as illustrated with the synthesis of an advanced model of the marine natural product plakoridine A and of the indolizidine alkaloid δ-coniceine.