Highly enantioselective synthesis of both enantiomers of tetrahydroquinoxaline derivatives via Ir-catalyzed asymmetric hydrogenation

A novel Ir-catalyzed asymmetric hydrogenation protocol for the synthesis of chiral tetrahydroquinoxaline (THQ) derivatives has been developed. By simply adjusting the reaction solvent, both enantiomers of mono-substituted chiral THQs could be selectively obtained in high yields with excellent enantioselectivities (toluene/dioxane: up to 93% yield and 98% ee (R); EtOH: up to 83% yield and 93% ee (S)). For 2,3-disubstituted chiral THQs, the cis-hydrogenation products were obtained with up to 95% yield, 20 : 1 dr, and 94% ee. Remarkably, this methodology was also applicable under continuous flow conditions, yielding gram-scale products with comparable yields and enantioselectivities (dioxane: 91% yield and 93% ee (R); EtOH: 90% yield and 87% ee (S)). Unlike previously reported Ir-catalyzed asymmetric hydrogenation protocols, this system exhibited a significant improvement as it required no additional additives. Furthermore, comprehensive mechanistic studies including deuterium-labeling experiments, control experiments, kinetic studies, and density functional theory (DFT) calculations were conducted to reveal the underlying mechanism of enantioselectivities for both enantiomers.

Asymmetric Transfer Hydrogenation of 2-Substituted Quinoxalines with Regenerable Dihydrophenanthridine

The asymmetric hydrogenation of quinoxalines represents one of the most efficient approaches for the synthesis of optically active tetrahyroquinoxalines. In this paper, we demonstrate a metal-free asymmetric transfer hydrogenation of 2-substituted quinoxalines with regenerable dihydrophenanthridine under H2 using a combination of chiral phosphoric acid and achiral borane as catalysts. A wide range of optically active 2-substituted tetrahydroquinoxalines were produced in high yields with ≤98% ee.

Cobalt-Catalyzed Enantioselective Reductive Amination of Ketones with Hydrazides

An efficient cobalt-catalyzed asymmetric reductive amination of ketones with hydrazides has been realized, directly producing valuable chiral hydrazines in high yields and enantioselectivities (up to 98% enantiomeric excess).

Rhodium-Catalyzed Asymmetric Functionalization of Quinoxalinium Salts

We report a rhodium-catalyzed asymmetric addition of aryl and alkenyl boronic acids to quinoxalinium salts that generates dihydroquinoxalines with high enantioselectivity. Functionalization of the reaction products, dihydroquinoxaline, allows the preparation of tetrahydroquinoxalines with various substitution patterns.

Protocol for stereodivergent asymmetric hydrogenation of quinoxalines

Chiral 1,2,3,4-tetrahydroquinoxalines are ubiquitous in natural products and bioactive molecules. Herein, we disclose a protocol for stereodivergent asymmetric hydrogenation of disubstituted quinoxalines for the preparation of both cis- and trans-enantioenriched disubstituted tetrahydroquinoxalines (up to >20:1 d.r. and 99% ee). We describe steps for synthesis of ligands and substrate, setup of hydrogenation of disubstituted quinoxalines, and purification of products. Additionally, we provide detailed diagrams of the hydrogenation installation. For complete details on the use and execution of this protocol, please refer to Liu et al.1.

Highly enantioselective synthesis of both tetrahydroquinoxalines and dihydroquinoxalinones via Rh-thiourea catalyzed asymmetric hydrogenation

Chiral tetrahydroquinoxalines and dihydroquinoxalinones represent the core structure of many bioactive molecules. Herein, a simple and efficient Rh-thiourea-catalyzed asymmetric hydrogenation for enantiopure tetrahydroquinoxalines and dihydroquinoxalinones was developed under 1 MPa H2 pressure at room temperature. The reaction was magnified to the gram scale furnishing the desired products with undamaged yield and enantioselectivity. Application of this methodology was also conducted successfully under continuous flow conditions. In addition, 1H NMR experiments revealed that the introduction of a strong Brønsted acid, HCl, not only activated the substrate but also established anion binding between the substrate and the ligand. More importantly, the chloride ion facilitated heterolytic cleavage of dihydrogen to regenerate the active dihydride species and HCl, which was computed to be the rate-determining step. Further deuterium labeling experiments and density functional theory (DFT) calculations demonstrated that this reaction underwent a plausible outer-sphere mechanism in this new catalytic transformation.

FePO4 supported Rh subnano clusters with dual active sites for efficient hydrogenation of quinoline under mild conditions

Chemoselective hydrogenation of quinoline and its derivatives under mild reaction conditions still remains a challenging topic, which requires a suitable interaction between reactants and a catalyst to achieve high performance and stability. Herein, FePO₄-supported Rh single atoms, subnano clusters and nanoparticle catalysts were synthesized and evaluated in the chemoselective hydrogenation of quinoline. The results show that the Rh subnano cluster catalyst with a size of ∼1 nm gives a specific reaction rate of 353 mol_quinoline mol_Rh^-1 h^-1 and a selectivity of >99% for 1,2,3,4-tetrahydroquinoline under mild conditions of 50 °C and 5 bar H₂, presenting better performance compared with the Rh single atoms and nanoparticle counterparts. Moreover, the Rh subnano cluster catalyst exhibits good stability and substrate universality for the hydrogenation of various functionalized quinolines. A series of characterization studies demonstrate that the acidic properties of the FePO₄ support favors the adsorption of quinoline while the Rh subnano clusters promote the dissociation of H₂ molecules, and then contribute to the enhanced hydrogenation performance. This work provides an important implication to design efficient Rh-based catalysts for chemoselective hydrogenation under mild conditions.

Chiral Frustrated Lewis Pair@Metal-Organic Framework as a New Platform for Heterogeneous Asymmetric Hydrogenation

Asymmetric hydrogenation, a seminal strategy for the synthesis of chiral molecules, remains largely unmet in terms of activation by non-metal sites of heterogeneous catalysts. Herein, as demonstrated by combined computational and experimental studies, we present a general strategy for integrating rationally designed molecular chiral frustrated Lewis pair (CFLP) with porous metal-organic framework (MOF) to construct the catalyst CFLP@MOF that can efficiently promote the asymmetric hydrogenation in a heterogeneous manner, which for the first time extends the concept of chiral frustrated Lewis pair from homogeneous system to heterogeneous catalysis. Significantly, the developed CFLP@MOF, inherits the merits of both homogeneous and heterogeneous catalysts, with high activity/enantio-selectivity and excellent recyclability/regenerability. Our work not only advances CFLP@MOF as a new platform for heterogeneous asymmetric hydrogenation, but also opens a new avenue for the design and preparation of advanced catalysts for asymmetric catalysis.

Intramolecular Copper-Catalyzed Asymmetric Propargylic [4 + 2]- Cycloaddition toward Optically Active Tetrahydroisoindolo[2,1-a]quinoxalines

An intramolecular Cu-catalyzed asymmetric propargylic [4 + 2] cycloaddition of bis-N-nucleophile-functionalized propargylic esters has been realized in the support of a chiral tridentate N-ligand, (S,S)-Pybox-diOAc, leading to chiral tetrahydroisoindolo[2,1-a]quinoxalines in high yields and with good to excellent enantioselectivities. The reaction features high efficiency, simplicity, and broad substrate scope, thus providing a powerful and concise strategy for stereoselective access to optically active polycyclic heterocycle frameworks that are otherwise difficult to synthesize.

Recent Advances in the Enantioselective Synthesis of Chiral Amines via Transition Metal-Catalyzed Asymmetric Hydrogenation

Chiral amines are key structural motifs present in a wide variety of natural products, drugs, and other biologically active compounds. During the past decade, significant advances have been made with respect to the enantioselective synthesis of chiral amines, many of them based on catalytic asymmetric hydrogenation (AH). The present review covers the use of AH in the synthesis of chiral amines bearing a stereogenic center either in the α, β, or γ position with respect to the nitrogen atom, reported from 2010 to 2020. Therefore, we provide an overview of the recent advances in the AH of imines, enamides, enamines, allyl amines, and N-heteroaromatic compounds.

A formal [4 + 2] annulation of diamines and prop-2-ynyl sulfonium salts for the synthesis of tetrahydroquinoxalines

A formal [4 + 2] annulation of diamines and prop-2-ynyl sulfonium salts was developed.

Enantioselective Desymmetrization of 3-Substituted Oxetanes: An Efficient Access to Chiral 3,4-Dihydro-2H-1,4-benzoxazines

Herein, we describe a versatile transition metal/oxidant free synthesis of the chiral 2H-1,4-benzoxazines through chiral phosphoric acid (CPA) catalyzed enantioselective desymmetrization of prochiral oxetanes (30 examples) in up to 99% yield and 99% enantioselectivity under mild reaction conditions. The reported strategy not only complements the conventional 2H-1,4-benzoxazine synthetic strategies but also provides access to key intermediates of therapeutic candidates, i.e., prostaglandin D2 receptor antagonist and M1 positive allosteric modulator (PAM) compound VU0486846.

Chemically robust and readily available quinoline-based PNN iron complexes: application in C-H borylation of arenes

Iron catalysts have been used for over a century to produce ammonia industrially. However, the use of iron catalysts generally remained quite limited until relatively recently, when the abundance and low toxicity of iron spurred the development of a variety of iron catalysts. Despite the fact that iron catalysts are being developed as alternatives to precious metal catalysts, their reactivities and stabilities are quite different because of their unique electronic structures. In this context, our group previously developed a new family of quinoline-based PNN pincer-type ligands for low- to mid-valent iron catalysts. These chemically robust PNN ligands provide air- and moisture-tolerant iron complexes, which exhibit excellent catalytic performances in the C-H borylation of arenes. This feature article summarises our recent work on PNN iron complexes, including their conception and design, as well as related reports on iron pincer complexes and iron-catalysed C-H borylation reactions.

Asymmetric hydrogenation of O-/N-functional group substituted arenes

Asymmetric hydrogenation of aromatic compounds represents one of the most straightforward synthetic methods to construct important chiral cyclic skeletons that are often found in biologically active agents and natural products. So far, the most successful examples in this field are largely limited to aromatics containing alkyl and aryl substituted groups due to the poor functional-group tolerance of hydrogenation. Direct asymmetric hydrogenation of functionalized aromatics provides enormous potential for expanding the structural diversity of reductive products of planar aromatic compounds, which is highly desirable and has not been well studied. This feature article focuses on the progress in catalytic asymmetric hydrogenation and transfer hydrogenation of O/N substituted arenes.

Catalytic Enantioselective Access to Dihydroquinoxalinones via Formal α-Halo Acyl Halide Synthon in One Pot

An enantioselective one-pot catalytic strategy to dihydroquinoxalinones, featuring novel 1-phenylsulfonyl-1-cyano enantioenriched epoxides as masked α-halo acyl halide synthons, followed by a domino ring-opening cyclization (DROC), is documented. A popular quinine-derived urea served as the catalyst in two out of the three steps performed in the same solvent using commercially available aldehydes, (phenylsulfonyl)acetonitrile, cumyl hydroperoxide and 1,2-phenylendiamines. Medicinally relevant 3-aryl/alkyl-substituted heterocycles are isolated in generally good to high overall yield and high enantioselectivity (up to 99 % ee). A rare example of excellent reusability of an organocatalyst at higher scale, subjected to oxidative conditions, is demonstrated. Mechanistically, labile α-ketosulfone has been detected as the intermediate involved in the DROC process. Theoretical calculations on the key epoxidation step rationalize the observed stereocontrol, highlighting the important role played by the sulfone group.

(Cyclopentadienone)iron Complexes: Synthesis, Mechanism and Applications in Organic Synthesis

(Cyclopentadienone)iron tricarbonyl complexes are catalytically active, inexpensive, easily accessible and air-stable that are extensively studied as an active pre-catalyst in homogeneous catalysis. Its versatile catalytic activity arises exclusively due to the presence of a non-innocent ligand, which can trigger its unique redox properties effectively. These complexes have been employed widely in (transfer)hydrogenation (e. g., reduction of polar multiple bonds, Oppenauer-type oxidation of alcohols), C-C and C-N bond formation (e. g., reductive aminations, α-alkylation of ketones) and other synthetic transformations. In this review, we discuss the remarkable advancement of its various synthetic applications along with synthesis and mechanistic studies, until February 2021.

Asymmetric Pnictogen-Bonding Catalysis: Transfer Hydrogenation by a Chiral Antimony(V) Cation/Anion Pair

Pnictogen-bonding catalysis based on σ-hole interactions has recently attracted the attention of synthetic chemists. As a proof-of-concept for asymmetric pnictogen-bonding catalysis, we report herein an enantioselective transfer hydrogenation of benzoxazines catalyzed by a novel chiral antimony cation/anion pair. The chiral pnictogen catalyst library could be rapidly accessed from triarylstibine with readily available mandelic acid analogues, and the catalyst displays remarkable efficiency and enantiocontrol potency even at 0.05 mol % loading. Moreover, the properties of the catalyst and the mechanistic insights have been investigated by nonlinear effect studies, ¹H NMR, LC-MS, and control experiments.

Multigram-scale HPLC enantioseparation as a rescue pathway for circumventing racemization problem during enantioselective synthesis of ethyl 3,4-dihydro-2H-1,4-benzoxazine-2-carboxylate

Racemic ethyl 3,4-dihydro-2H-1,4-benzoxazine-2-carboxylate is a key synthon for the design of promising therapeutic drugs. It is mainly synthesized from racemic ethyl 2,3-dibromopropionate and 2-aminophenol in presence of K₂ CO₃ in refluxed acetone. Unfortunately, synthesis of (R)- and (S)-enantiomers using the enantioselective version of this reaction, which should normally be performed with a double S_N 2 mechanism, is unsuitable due to a racemization process involving the dehydrobromination of enantiopure ethyl 2,3-dibromopropionate into ethyl 2-bromoacrylate. For the first time, the enantioselective version is studied (ee ≈ 55-66%), and the percentage of racemization process has estimated to around 34-46% after determination of the optimal experimental conditions for analytical HPLC enantioseparation of racemate. The influence of the experimental and purification conditions on the racemization rate is also studied. The results indicate that racemization occurs faster at the beginning of the reaction but the initiation of the double S_N 2 process takes place more faster to limit the racemization rate. The study of the influence of experimental conditions (reaction times, temperature, solvent or type of base, etc.) on the degree of racemization of the (R)- enantiomer is performed and shows that despite changes in the experimental conditions, the synthesis of the (R)- enantiomer is always accompanied by a racemization rate which is difficult in reducing. In parallel, (R)- and (S)-enantiomers are obtained in high enantiopurity (ee > 99.5%) by preparative HPLC enantioseparation of racemate on multigram scale and characterized by optical rotation measurements, ECD and UV spectra.

Catalytic asymmetric synthesis of N-substituted tetrahydroquinoxalines via regioselective Heyns rearrangement and stereoselective transfer hydrogenation in one pot

N-Substituted tetrahydroquinoxalines (37 examples) were step-economically obtained in good yield (<97%) and ee (<99%) with readily available substrates. The reaction proceeds through an interesting regioselective Heyns rearrangement/enantioselective transfer hydrogenation in one pot. The substrate scope and the reaction mechanism were systematically investigated.

N-Substituted tetrahydroquinoxalines were step-economically obtained in good yield and ee with readily available substrates.

Asymmetric hydrogenation catalyzed by first-row transition metal complexes

This review focuses on asymmetric direct and transfer hydrogenation with first-row transition metal complexes. The reaction mechanisms and the models of enantiomeric induction were summarized and emphasized.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Metal-Free Twofold Electrochemical C-H Amination of Activated Arenes: Application to Medicinally Relevant Precursor Synthesis

The efficient production of many medicinally or synthetically important starting materials suffers from wasteful or toxic precursors for the synthesis. In particular, the aromatic non-protected primary amine function represents a versatile synthetic precursor, but its synthesis typically requires toxic oxidizing agents and transition metal catalysts. The twofold electrochemical amination of activated benzene derivatives via Zincke intermediates provides an alternative sustainable strategy for the formation of new C-N bonds of high synthetic value. As a proof of concept, we use our approach to generate a benzoxazinone scaffold that gained attention as a starting structure against castrate-resistant prostate cancer. Further improvement of the structure led to significantly increased cancer cell line toxicity. Thus, exploiting environmentally benign electrooxidation, we present a new versatile and powerful method based on direct C-H activation that is applicable for example the production of medicinally relevant compounds.

Recent Advances in Homogeneous Catalysts for the Asymmetric Hydrogenation of Heteroarenes

The asymmetric hydrogenation of heteroarenes has recently emerged as an effective strategy for the direct access to enantioenriched, saturated heterocycles. Although several homogeneous catalyst systems have been extensively developed for the hydrogenation of heteroarenes with high levels of chemo- and stereoselectivity, the development of mild conditions that allow for efficient and stereoselective hydrogenation of a broad range of substrates remains a challenge. This Perspective highlights recent advances in homogeneous catalysis of heteroarene hydrogenation as inspiration for the further development of asymmetric hydrogenation catalysts, and addresses underdeveloped areas and limitations of the current technology.

Transition-Metal-Catalyzed Reductive Amination Employing Hydrogen

The reductive amination, the reaction of an aldehyde or a ketone with ammonia or an amine in the presence of a reducing agent and often a catalyst, is an important amine synthesis and has been intensively investigated in academia and industry for a century. Besides aldehydes, ketones, or amines, starting materials have been used that can be converted into an aldehyde or ketone (for instance, carboxylic acids or organic carbonate or nitriles) or into an amine (for instance, a nitro compound) in the presence of the same reducing agent and catalyst. Mechanistically, the reaction starts with a condensation step during which the carbonyl compound reacts with ammonia or an amine, forming the corresponding imine followed by the reduction of the imine to the alkyl amine product. Many of these reduction steps require the presence of a catalyst to activate the reducing agent. The reductive amination is impressive with regard to the product scope since primary, secondary, and tertiary alkyl amines are accessible and hydrogen is the most attractive reducing agent, especially if large-scale product formation is an issue, since hydrogen is inexpensive and abundantly available. Alkyl amines are intensively produced and use fine and bulk chemicals. They are key functional groups in many pharmaceuticals, agro chemicals, or materials. In this review, we summarize the work published on reductive amination employing hydrogen as the reducing agent. No comprehensive review focusing on this subject has been published since 1948, albeit many interesting summaries dealing with one or the other aspect of reductive amination have appeared. Impressive progress in using catalysts based on earth-abundant metals, especially nanostructured heterogeneous catalysts, has been made during the early development of the field and in recent years.

An iron variant of the Noyori hydrogenation catalyst for the asymmetric transfer hydrogenation of ketones

We report the design of a new iron catalyst for the asymmetric transfer hydrogenation of ketones. This type of iron catalyst combines the structural characteristics of the Noyori hydrogenation catalyst (an axially chiral 2,2'-bis(phosphino)-1,1'-binaphthyl fragment and the metal-ligand bifunctional motif) and an ene(amido) group that can activate the iron center. After activation by 8 equivalents of potassium tert-butoxide, (S_A,R_P,SS)-7a and (S_A,R_P,SS)-7b are active but nonenantioselective catalysts for the transfer hydrogenation of acetophenone and α,β-unsaturated aldehydes at room temperature in isopropanol. A maximum turnover number of 14480 was observed for (S_A,R_P,SS)-7a in the reduction of acetophenone. The right combination of the stereochemistry of the axially chiral 2,2'-bis(phosphino)-1,1'-binaphthyl group and the carbon-centered chiral amine-imine moiety in (S_A,R_P,RR)-7b' afforded an enantioselective catalyst for the preparation of chiral alcohols with moderate to good yields and a broad functional group tolerance.

The Synthesis of Primary Amines through Reductive Amination Employing an Iron Catalyst

The reductive amination of ketones and aldehydes by ammonia is a highly attractive method for the synthesis of primary amines. The use of catalysts, especially reusable catalysts, based on earth-abundant metals is similarly appealing. Here, the iron-catalyzed synthesis of primary amines through reductive amination was realized. A broad scope and a very good tolerance of functional groups were observed. Ketones, including purely aliphatic ones, aryl-alkyl, dialkyl, and heterocyclic, as well as aldehydes could be converted smoothly into their corresponding primary amines. In addition, the amination of pharmaceuticals, bioactive compounds, and natural products was demonstrated. Many functional groups, such as hydroxy, methoxy, dioxol, sulfonyl, and boronate ester substituents, were tolerated. The catalyst is easy to handle, selective, and reusable and ammonia dissolved in water could be employed as the nitrogen source. The key is the use of a specific Fe complex for the catalyst synthesis and an N-doped SiC material as catalyst support.

Synthesis and Catalytic Application of Knölker-Type Iron Complexes with a Novel Asymmetric Cyclopentadienone Ligand Design

Asymmetric catalysis is an essential tool in modern chemistry, but increasing environmental concerns demand the development of new catalysts based on cheap, abundant, and less toxic iron. As a result, Knölker-type catalysts have emerged as a promising class of iron catalysts for various chemical transformations, notably the hydrogenation of carbonyls and imines, while asymmetric versions are still under exploration to achieve optimal enantio-selectivities. In this work, we report a novel asymmetric design of a Knölker-type catalyst, in which the C2-rotational symmetric cyclopentadienone ligand possesses chiral substituents on the 2- and 5-positions near the active site. Four examples of the highly modular catalyst design were synthesized via standard organic procedures, and their structures were confirmed with NMR, IR, MS, and polarimetry analysis. Density functional theory (DFT) calculations were conducted to elucidate the spatial conformation of the catalysts, and therewith to rationalize the influence of structural alterations. Transfer- and H2-mediated hydrogenations were successfully established, leading to appreciable enantiomeric excesses (ee) values up to 70%. Amongst all reported Knölker-type catalysts, our catalyst design achieves one of the highest ee values for hydrogenation of acetophenone and related compounds.

Recent Advances in First-Row Transition Metal/Chiral Phosphoric Acid Combined Catalysis

Since the pioneering independent reports of Akiyama and Terada, the use of chiral phosphoric acids (CPAs) and derivatives as a versatile tool for asymmetric synthesis with good reactivity, regioselectivity, diastereoselectivity and enantioselectivity has emerged, forming an important part of the implementation of asymmetric counteranion-directed catalysis reported to date. In these achievements, the combination of metals with CPAs has enabled various catalytic modes beyond the scope of typical acid catalysis, such as relay catalysis, ion-pairing catalysis, and binary acid catalysis. The first-row transition metals (Sc-Zn) are considered to be sustainable transition metals and have received a great deal of attention. These naturally abundant metals display excellent Lewis acidity and function as powerful redox catalysts in synthesis involving both one and two-electron transfers. Hence, in this chapter, we summarize recent advances in the development of asymmetric reactions using a combination of first-row transition metals and CPAs. Furthermore, we provide a detailed discussion of the mechanisms involved in order to understand the interaction of the metal/phosphate and the origins of the asymmetric control of the transformations.

Stereoconvergent, Redox-Neutral Access to Tetrahydroquinoxalines through Relay Epoxide Opening/Amination of Alcohols

We present an economical catalytic procedure to convert readily available 1,2-diaminobenzenes and terminal epoxides into valuable 1,2,3,4-tetrahydroquinoxalines in a highly enantioselective fashion. This procedure operates through relay zinc and iridium catalysis, and achieves redox-neutral and stereoconvergent production of valuable chiral heterocycles from racemic starting materials with water as the only side product. The use of commercially available reagents and catalysts and a convenient procedure also make this catalytic method attractive for practical application.

Selective Arene Hydrogenation for Direct Access to Saturated Carbo- and Heterocycles

Arene hydrogenation provides direct access to saturated carbo- and heterocycles and thus its strategic application may be used to shorten synthetic routes. This powerful transformation is widely applied in industry and is expected to facilitate major breakthroughs in the applied sciences. The ability to overcome aromaticity while controlling diastereo-, enantio-, and chemoselectivity is central to the use of hydrogenation in the preparation of complex molecules. In general, the hydrogenation of multisubstituted arenes yields predominantly the cis isomer. Enantiocontrol is imparted by chiral auxiliaries, Brønsted acids, or transition-metal catalysts. Recent studies have demonstrated that highly chemoselective transformations are possible. Such methods and the underlying strategies are reviewed herein, with an emphasis on synthetically useful examples that employ readily available catalysts.

Carbene-Catalyzed α-Carbon Amination of Chloroaldehydes for Enantioselective Access to Dihydroquinoxaline Derivatives

An NHC-catalyzed α-carbon amination of chloroaldehydes was developed. Cyclohexadiene-1,2-diimines are used as amination reagents and four-atom synthons. Our reaction affords optically enriched dihydroquinoxalines that are core structures in natural products and synthetic bioactive molecules.