Trends in applying C-H oxidation to the total synthesis of natural products

Pd-catalyzed regioselective activation of C(sp2)-H and C(sp3)-H bonds

Differentiating between two highly similar C-H bonds in a given molecule remains a fundamental challenge in synthetic organic chemistry. Directing group assisted strategies for the functionalisation of proximal C-H bonds has been known for the last few decades. However, distal C-H bond functionalisation is strenuous and requires distinctly specialised techniques. In this review, we summarise the advancement in Pd-catalysed distal C(sp2)-H and C(sp3)-H bond activation through various redox manifolds including Pd(0)/Pd(II), Pd(II)/Pd(IV) and Pd(II)/Pd(0). Distal C-H functionalisation, where a Pd-catalyst is directly involved in the C-H activation step, either through assistance of an external directing group or directed by an inherent functionality or functional group incorporated at the site of the Pd-C bond is covered. The purpose of this review is to portray the current state of art in Pd-catalysed distal C(sp2)-H and C(sp3)-H functionalisation reactions, their mechanism and application in the late-stage functionalisation of medicinal compounds along with highlighting its limitations, thus leaving the field open for further synthetic adjustment.

Naturally Occurring Norsteroids and Their Design and Pharmaceutical Application

The main focus of this review is to introduce readers to the fascinating class of lipid molecules known as norsteroids, exploring their distribution across various biotopes and their biological activities. The review provides an in-depth analysis of various modified steroids, including A, B, C, and D-norsteroids, each characterized by distinct structural alterations. These modifications, which range from the removal of specific methyl groups to changes in the steroid core, result in unique molecular architectures that significantly impact their biological activity and therapeutic potential. The discussion on A, B, C, and D-norsteroids sheds light on their unique configurations and how these structural modifications influence their pharmacological properties. The review also presents examples from natural sources that produce a diverse array of steroids with distinct structures, including the aforementioned A, B, C, and D-nor variants. These compounds are sourced from marine organisms like sponges, soft corals, and starfish, as well as terrestrial entities such as plants, fungi, and bacteria. The exploration of these steroids encompasses their biosynthesis, ecological significance, and potential medical applications, highlighting a crucial area of interest in pharmacology and natural product chemistry. The review emphasizes the importance of researching these steroids for drug development, particularly in addressing diseases where conventional medications are inadequate or for conditions lacking sufficient therapeutic options. Examples of norsteroid synthesis are provided to illustrate the practical applications of this research.

Copper-catalysed dehydrogenation or lactonization of C(sp3)-H bonds

Cytochrome P450 enzymes are known to catalyse bimodal oxidation of aliphatic acids via radical intermediates, which partition between pathways of hydroxylation and desaturation1,2. Developing analogous catalytic systems for remote C-H functionalization remains a significant challenge3-5. Here, we report the development of Cu(I)-catalysed bimodal dehydrogenation/lactonization reactions of synthetically common N-methoxyamides through radical abstractions of the γ-aliphatic C-H bonds. The feasibility of switching from dehydrogenation to lactonization is also demonstrated by altering reaction conditions. The use of a readily available amide as both radical precursor and internal oxidant allows for the development of redox-neutral C-H functionalization reactions with methanol as the sole side product. These C-H functionalization reactions using a Cu(I) catalyst with loading as low as 0.5 mol.% is applied to the diversification of a wide range of aliphatic acids including drug molecules and natural products. The exceptional compatibility of this catalytic system with a wide range of oxidatively sensitive functionality demonstrates the unique advantage of using a simple amide substrate as a mild internal oxidant.

Recent advances in Rh(I)-catalyzed enantioselective C-H functionalization

Chiral carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds are pervasive and very essential in natural products, bioactive molecules, and functional materials, and their catalytic construction has emerged as one of the hottest research fields in synthetic organic chemistry. The last decade has witnessed vigorous progress in Rh(I)-catalyzed asymmetric C-H functionalization as a complement to Rh(II) and Rh(III) catalysis. This review aims to provide the most comprehensive and up-to-date summary covering the recent advances in Rh(I)-catalyzed C-H activation for asymmetric functionalization. In addition to the development of diverse reactions, chiral ligand design and mechanistic investigation (inner-sphere mechanism, outer-sphere mechanism, and 1,4-Rh migration) will also be highlighted.

Rhodium-Catalyzed Asymmetric Macrocyclization towards Crown Ethers Using Hydroamination of Bis(allenes)

Enantiomerically enriched crown ethers (CE) exhibit strong asymmetric induction in phase transfer catalysis, supramolecular catalysis and molecular recognition processes. Traditional methods have often been used to access these valuable compounds, which limit their diversity and consequently their applicability. Herein, a practical catalytic method is described for the gram scale synthesis of a class of chiral CEs (aza-crown ethers; ACEs) using Rh-catalyzed hydroamination of bis(allenes) with diamines. Using this approach, a wide range of chiral vinyl functionalized CEs with ring sizes ranging from 12 to 36 have been successfully prepared in high yields of up to 92 %, dr of up to >20 : 1 and er of up to >99 : 1. These vinyl substituted CEs allow for further diversification giving facile access to various CE derivatives as well as to their three-dimensional analogues using ring-closing metathesis. Some of these chiral CEs themselves display high potential for use in asymmetric catalysis.

Remote C-H Amination and Alkylation of Camphor at C8 through Hydrogen-Atom Abstraction

Camphor continues to serve as a versatile chiral building block for chemical synthesis. We have developed a novel method to functionalize the camphor skeleton at C8 using an intramolecular hydrogen atom abstraction. The key advance involves the use of a camphor-derived aminonitrile, which is converted to the corresponding nitrogen-centered radical under photoredox conditions to effect the 1,5-hydrogen atom transfer at C8. The resulting carbon-centered radical at C8 was utilized in a C-H amination to access topologically complex proline derivatives. Furthermore, the total synthesis of several sesquiterpenoids was accomplished by engaging the radical generated at C8 in alkylation reactions.

Synthesis of 25-Hydroxy-provitamin D3 by Direct Hydroxylation of Protected 7-Dehydrocholesterol

A new synthetic route to 25-hydroxy-provitamin D3 was elaborated. The synthesis consists of direct hydroxylation at C-25 of 7-dehydrocholesterol hetero Diels-Alder adducts. The adducts were prepared by [4 + 2] cycloaddition of azadienophiles to the steroidal diene. The hydroxylation reactions of adducts were carried out with different dioxiranes or with chromyl trifluoroacetate. The byproducts of these reactions were isolated and identified. The strengths and weaknesses of hydroxylation methods with different oxidizing agents were discussed.

Strategic application of C-H oxidation in natural product total synthesis

The oxidation of unactivated C-H bonds has emerged as an effective tactic in natural product synthesis and has altered how chemists approach the synthesis of complex molecules. The use of C-H oxidation methods has simplified the process of synthesis planning by expanding the choice of starting materials, limiting functional group interconversion and protecting group manipulations, and enabling late-stage diversification. In this Review, we propose classifications for C-H oxidations on the basis of their strategic purpose: type 1, which installs functionality that is used to establish the carbon skeleton of the target; type 2, which is used to construct a heterocyclic ring; and type 3, which installs peripheral functional groups. The reactions are further divided based on whether they are directed or undirected. For each classification, examples from recent literature are analysed. Finally, we provide two case studies of syntheses from our laboratory that were streamlined by the judicious use of C-H oxidation reactions.

Transition-metal catalyzed C-H activation as a means of synthesizing complex natural products

Over the past few decades, the advent of C-H activation has led to a rethink among chemists about the synthetic strategies employed for multi-step transformations. Indeed, deploying innovative and masterful tricks against the numerous classical organic transformations has been the need of the hour. Despite this, the immense importance of C-H activation remains unfulfilled unless the methodology can be deployed for large-scale industrial processes and towards the concise, step-economic synthesis of prodigious natural products and pharmaceutical drugs. Lately, the growing potential of C-H activation methodology has indeed driven the pioneers of synthetic organic chemists into finding more efficient methods to accelerate the synthesis of such complex molecular scaffolds. This review aims to draw a general overview of the various C-H activation procedures that have been adopted for synthesizing these vast majority of structurally complicated natural products. Our objective lies in drawing a complete picture and taking the readers through the synthesis of a series of such complex organic compounds by simplified techniques, making it step-economic on a larger scale and thus instigating the readers to trigger the use of such methodology and uncover new, unique patterns for future synthesis of such natural products.

Transition-metal-catalyzed C-H bond activation as a sustainable strategy for the synthesis of fluorinated molecules: an overview

The last decade has witnessed the emergence of innovative synthetic tools for the synthesis of fluorinated molecules. Among these approaches, the transition-metal-catalyzed functionalization of various scaffolds with a panel of fluorinated groups (XRF, X = S, Se, O) offered straightforward access to high value-added compounds. This review will highlight the main advances made in the field with the transition-metal-catalyzed functionalization of C(sp2) and C(sp3) centers with SCF3, SeCF3, or OCH2CF3 groups among others, by C-H bond activation. The scope and limitations of these transformations are discussed in this review.

Palladium-catalysed site-selective arene ortho C-H fluoroalkoxylation of 4-aryl-pyrrolo[2,3-d]pyrimidines

A palladium-catalysed direct arene C-H fluoroalkoxylation of 4-aryl-pyrrolo[2,3-d]pyrimidine derivatives with fluorinated alcohols is described. Highly site-selective mono- or bis-fluoroalkoxylation can be achieved by tuning the reaction conditions, affording various fluoroalkoxylated pyrrolo[2,3-d]pyrimidine derivatives in moderate to good yields, which offer rational tailoring of their biological activity for their application in the field of pharmaceutical chemistry.

A Predictably Selective Palladium-Catalyzed Aliphatic C-H Oxygenation

Direct oxygenation of nonactivated aliphatic C(sp³)-H groups with peroxycarboxylic acids in the presence of palladium tris(pyridylmethyl)amine complex (0.6 mol %) is reported, providing the corresponding hydroxylated derivatives in up to 94% yields. The oxidation of 3° C-H groups occurs stereospecifically, with the catalyst system demonstrating extremely high sensitivity to electronic effects (adamantane oxidation: 3°:2° up to >300). This suggests potential applications for the 3°-regioselective oxidative functionalization of complex molecules of natural origin.

Unified Total Syntheses of Benzenoid Cephalotane-Type Norditerpenoids: Cephanolides and Ceforalides

Detailed herein are our synthetic studies toward the preparation of the C₁₈- and C₁₉-benzenoid cephalotane-type norditerpenoids. Guided by chemical network analysis, the core structure of this natural product family was constructed in a concise manner using an iterative cross-coupling, followed by a formal inverse-electron-demand [4 + 2] cycloaddition. Initial efforts to functionalize an alkene group in the [4 + 2] cycloadduct using a Mukaiyama hydration and a subsequent olefination led to the complete C₁₈-carbon framework. While effective, this approach proved lengthy and prompted the development of a direct alkene difunctionalization that relies on borocupration to advance the cycloadduct to the natural products. Late-stage peripheral C-H functionalization facilitated access to all of the known cephanolides in 6-10 steps as well as five recently isolated ceforalides in 8-13 steps.

Rapid and Accurate Estimation of Activation Free Energy in Hydrogen Atom Transfer-Based C-H Activation Reactions: From Empirical Model to Artificial Neural Networks

A well-performing machine learning (ML) model is obtained by using proper descriptors and artificial neural network (ANN) algorithms, which can quickly and accurately predict activation free energy in hydrogen atom transfer (HAT)-based sp³ C-H activation. Density functional theory calculations (UωB97X-D) are used to establish the reaction system data sets of methoxyl (CH₃O·), trifluoroethoxyl (CF₃CH₂O·), tert-butoxyl (tBuO·), and cumyloxyl (CumO·) radicals. The simplified Roberts' equation proposed in our recent study works here [R ² = 0.84, mean absolute error (MAE) = 0.85 kcal/mol]. Its performance is comparable with univariate Mulliken-type electronegativity (χ) with the ANN model. The ANN model with bond dissociation free energy, χ, α-unsaturation, and Nolan buried volume (%V _buried) successively improves R ² and MAE to 0.93 and 0.54 kcal/mol, respectively. It reproduces the test sets of trichloroethoxyl (CCl₃CH₂O·) with R ² = 0.87 and MAE = 0.89 kcal/mol and accurately predicts the relative experimental barrier of the HAT reactions with CumO· and the site selectivity of CH₃O·.

Strategy Evolution in a Skeletal Remodeling and C-H Functionalization-Based Synthesis of the Longiborneol Sesquiterpenoids

Detailed herein are our synthesis studies of longiborneol and related natural products. Our overarching goals of utilizing a "camphor first" strategy enabled by skeletal remodeling of carvone, and late-stage diversification using C-H functionalizations, led to divergent syntheses of the target natural products. Our initial approach proposed a lithiate addition to unite two fragments followed by a Conia-ene or Pd-mediated cycloalkylation reaction sequence to install the seven-membered ring emblematic of the longibornane core. This approach was unsuccessful and evolved into a revised plan that employed a Wittig coupling and a radical cyclization to establish the core. A reductive radical cyclization, which was explored first, led to a synthesis of copaborneol, a structural isomer of longiborneol. Alternatively, a metal-hydride hydrogen atom transfer-initiated cyclization was effective for a synthesis of longiborneol. Late-stage C-H functionalization of the longibornane core led to a number of hydroxylated longiborneol congeners. The need for significant optimization of the strategies that were employed as well as the methods for C-H functionalization to implement these strategies highlights the ongoing challenges in applying these powerful reactions. Nevertheless, the reported approach enables functionalization of every natural product-relevant C-H bond in the longibornane skeleton.

Selective Oxidations in the Synthesis of Complex Natural ent-Kauranes and ent-Beyeranes

Syntheses of two natural products derived from the ent-kaurene kaurenoic acid are described for the first time using regio- and diastereoselective oxidations. Palladium- and manganese-mediated oxidations were used to accomplish the syntheses of two ent-beyerane metabolites. The use of the White-Gormisky-Zhao catalyst Mn(CF₃-PDP) enabled the first application of a nondirected metal-catalyzed oxidation in an unactivated C-H bond in a total synthesis.

Toolbox for Distal C-H Bond Functionalizations in Organic Molecules

Transition metal catalyzed C-H activation has developed a contemporary approach to the omnipresent area of retrosynthetic disconnection. Scientific researchers have been tempted to take the help of this methodology to plan their synthetic discourses. This paradigm shift has helped in the development of industrial units as well, making the synthesis of natural products and pharmaceutical drugs step-economical. In the vast zone of C-H bond activation, the functionalization of proximal C-H bonds has gained utmost popularity. Unlike the activation of proximal C-H bonds, the distal C-H functionalization is more strenuous and requires distinctly specialized techniques. In this review, we have compiled various methods adopted to functionalize distal C-H bonds, mechanistic insights within each of these procedures, and the scope of the methodology. With this review, we give a complete overview of the expeditious progress the distal C-H activation has made in the field of synthetic organic chemistry while also highlighting its pitfalls, thus leaving the field open for further synthetic modifications.

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

Ruthenium Catalyzed Intramolecular C-X (X=C, N, O, S) Bond Formation via C-H Functionalization: An Overview

Ruthenium catalyzed C-H activation is well known for its high tolerance towards the functional group and broad applicability in organic synthesis and molecular sciences, with significant applications in pharmaceutical industries, material sciences, and polymer industry. In the last few decades, enormous progress has been observed with ruthenium-catalyzed C-H activation chemistry. Notably, the vast majority of the C-H functionalization known in the literature are intermolecular, although the intramolecular variant provides fascinating new structural facet starting from the simple molecular scaffolds. Intramolecular C-H functionalization is atom economical and step efficient, results in less formation of undesired products which is easy to purify. This has created a lot of interest in organic chemistry in developing new synthetic strategies for such functionalization. The focus of this review is to present the relatively unexplored intramolecular functionalization of C-H bonds into C-X (X=C, N, O, S) bonds utilizing versatile ruthenium catalysts, their scope, and brief mechanistic discussion.

Preparation of Antiproliferative Terpene-Alkaloid Hybrids by Free Radical-Mediated Modification of ent-Kauranic Derivatives

A convenient strategy for molecular editing of available ent-kauranic natural scaffolds has been developed based on radical mediated C-C bond formation. Iodine atom transfer radical addition (ATRA) followed by rapid ionic elimination and radical azidoalkylation were investigated. Both reactions involve radical addition to the exo-methylenic double bond of the parent substrate. Easy transformations of the obtained adducts lead to extended diterpenes of broad structural diversity and artificial diterpene-alkaloid hybrids possessing lactam and pyrrolidine pharmacophores. The cytotoxicity of selected diterpenic derivatives was examined by in vitro testing on several tumor cell lines. The terpene-alkaloid hybrids containing N-heterocycles with unprecedented spiro-junction have shown relevant cytotoxicity and promising selectivity indexes. These results represent a solid basis for following research on the synthesis of such derivatives based on available natural product templates.

Recent advances in the application of tetrabromomethane in organic synthesis

This review article covers the use of tetrabromomethane as mediator, catalyst and reagents for organic synthesis for the period from 2007 to 2020.

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.

Site-selective C-H hydroxylation of pentacyclic triterpenoids directed by transient chiral pyridine-imino groups

Pentacyclic triterpenoids (PTs) constitute one of the biggest families of natural products, many with higher oxidation state at the D/E rings possess a wide spectrum of biological activties but are poorly accessible. Here we report a site-selective C-H hydroxylation at the D/E rings of PTs paving a way toward these important natural products. We find that Schönecker and Baran's Cu-mediated aerobic oxidation can be applied and become site-selective on PT skeletons, as being effected unexpectedly by the chirality of the transient pyridine-imino directing groups. To prove the applicability, starting from the most abundant triterpenoid feedstock oleanane, three representative saponins bearing hydroxyl groups at C16 or C22 are expeditiously synthesized, and barringtogenol C which bears hydroxyl groups at C16, C21, and C22 is synthesized via a sequential hydroxylation as the key steps.

2-Ketoglutarate-Generated In Vitro Enzymatic Biosystem Facilitates Fe(II)/2-Ketoglutarate-Dependent Dioxygenase-Mediated C-H Bond Oxidation for (2s,3r,4s)-4-Hydroxyisoleucine Synthesis

Fe(II)/2-ketoglutarate-dependent dioxygenase (Fe(II)/2-KG DO)-mediated hydroxylation is a critical type of C-H bond functionalization for synthesizing hydroxy amino acids used as pharmaceutical raw materials and precursors. However, DO activity requires 2-ketoglutarate (2-KG), lack of which reduces the efficiency of Fe(II)/2-KG DO-mediated hydroxylation. Here, we conducted multi-enzymatic syntheses of hydroxy amino acids. Using (2s,3r,4s)-4-hydroxyisoleucine (4-HIL) as a model product, we coupled regio- and stereo-selective hydroxylation of l-Ile by the dioxygenase IDO with 2-KG generation from readily available l-Glu by l-glutamate oxidase (LGOX) and catalase (CAT). In the one-pot system, H₂O₂ significantly inhibited IDO activity and elevated Fe²⁺ concentrations of severely repressed LGOX. A sequential cascade reaction was preferable to a single-step process as CAT in the former system hydrolyzed H₂O₂. We obtained 465 mM 4-HIL at 93% yield in the two-step system. Moreover, this process facilitated C-H hydroxylation of several hydrophobic aliphatic amino acids to produce hydroxy amino acids, and C-H sulfoxidation of sulfur-containing l-amino acids to yield l-amino acid sulfoxides. Thus, we constructed an efficient cascade reaction to produce 4-HIL by providing prerequisite 2-KG from cheap and plentiful l-Glu and developed a strategy for creating enzymatic systems catalyzing 2-KG-dependent reactions in sustainable bioprocesses that synthesize other functional compounds.

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.

Iron- and cobalt-catalyzed C(sp3)–H bond functionalization reactions and their application in organic synthesis

This review highlights the developments in iron and cobalt catalyzed C(sp³)–H bond functionalization reactions with emphasis on their applications in organic synthesis, i.e. natural products and pharmaceuticals synthesis and/or modification.

Continued Progress towards Efficient Functionalization of Natural and Non-natural Targets under Mild Conditions: Oxygenation by C-H Bond Activation with Dioxirane

The successful isolation and characterization of a dioxirane species in 1988 opened up one of the most attractive methods for the efficient oxidation of simple and/or structurally complex molecules. Dioxirane today rank among the most powerful tools in organic chemistry, with numerous applications in commercially important processes. They were quickly recognized as efficient oxygen transfer agents, especially for epoxidations and for a wide range of O-insertion reactions into C-H bonds. Dioxirane possess catalytic activity and appear as highly (chemo-, regio-, and stereo-) selective oxidants, despite their reactivity under mild and strictly neutral conditions being controlled by a combination of steric and electronic factors. In this review, we discuss some of the most recent and significant developments in the selective homogeneous and heterogeneous oxyfunctionalization of non-activated C-H bonds in hydrocarbons of natural and non-natural targets by using isolated dioxirane or, more generally, by using the ketones (i.e., the dioxirane precursors) as organocatalysts.

Ruthenium(ii)-catalysed selective C(sp2)-H bond benzoxylation of biologically appealing N-arylisoindolinones

Site- and regio-selective aromatic C-H bond benzoxylations were found to take place using biologically appealing N-arylisoindolinones under ruthenium(ii) catalysis in the presence of (hetero)aromatic carboxylic acid derivatives as coupling partners. Besides the presence of two potential C(sp²)-H sites available for functionalization in the substrates, exclusive ortho selectivity was achieved in the phenyl ring attached to the nitrogen atom. Notably, the reactions occurred in a selective manner as only mono-functionalized products were formed and they tolerated a large number of functional chemical groups. The ability of the cyclic tertiary amide within the isoindolinone skeleton to act as a weak directing group in order to accommodate six-membered ring ruthenacycle intermediates appears to be the key to reach such high levels of selectivity. In contrast, the more sterically demanding cyclic imides were unreactive under identical reaction conditions.

Unified Enantioselective, Convergent Synthetic Approach toward the Furanobutenolide-Derived Polycyclic Norcembranoid Diterpenes: Synthesis of a Series of Ineleganoloids by Oxidation-State Manipulation of the Carbocyclic Core

Late-stage synthetic efforts to advance the enatio- and diastereoselectively constructed [6,7,5,5]-fused tetracyclic scaffold toward the polycyclic norditerpenoid ineleganolide are disclosed. The described investigations focus on oxidation-state manipulation around the central cycloheptane ring. Computational evaluation of ground-state energies of dihydroineleganolide is used to rationalize empirical observations and provide insight for further synthetic development, enhancing the understanding of the conformational constraints of these compact polycyclic structures. Advanced synthetic manipulations generated a series of natural product-like compounds termed the ineleganoloids.

P450 Monooxygenases Enable Rapid Late-Stage Diversification of Natural Products via C-H Bond Activation

The biological potency of natural products has been exploited for decades. Their inherent structural complexity and natural diversity might hold the key to efficiently address the urgent need for the development of novel pharmaceuticals. At the same time, it is that very complexity, which impedes necessary chemical modifications such as structural diversification, to improve the effectiveness of the drug. For this purpose, Cytochrome P450 enzymes, which possess unique abilities to activate inert sp3-hybridised C-H bonds in a late-stage fashion, offer an attractive synthetic tool. In this review the potential of cytochrome P450 enzymes in chemoenzymatic lead diversification is illustrated discussing studies reporting late-stage functionalisations of natural products and other high-value compounds. These enzymes were proven to extend the synthetic toolbox significantly by adding to the flexibility and efficacy of synthetic strategies of natural product chemists, and scientists of other related disciplines.

Development of a Terpene Feedstock-Based Oxidative Synthetic Approach to the Illicium Sesquiterpenes

The Illicium sesquiterpenes are a family of natural products containing over 100 highly oxidized and structurally complex members, many of which display interesting biological activities. This comprehensive account chronicles the evolution of a semisynthetic strategy toward these molecules from (+)-cedrol, seeking to emulate key aspects of their presumed biosynthesis. An initial route generated lower oxidation state analogs but failed in delivering a crucial hydroxy group in the final step. Insight gathered during these studies, however, ultimately led to a synthesis of the pseudoanisatinoids along with the allo-cedrane natural product 11- O-debenzoyltashironin. A second-generation strategy was then developed to access the more highly oxidized majucinoid compounds including jiadifenolide and majucin itself. Overall, one dozen natural products can be accessed from an abundant and inexpensive terpene feedstock. A multitude of general observations regarding site-selective C(sp³)-H bond functionalization reactions in complex polycyclic architectures are reported.