Synthesis Of Natural Products Research Articles

Asymmetric organocatalytic synthesis of chiral homoallylic amines.

In recent decades, the chiral allylation of imines emerged as a key methodology in the synthesis of alkaloids and natural products with 4-, 5- and 6-membered cyclic amine motifs. Initially reliant on stoichiometric reagents, synthetic chemists predominantly used N-substituted chiral imines, organometallic chiral reagents and achiral reagents with an equimolar chiral controller. However, recent years have witnessed the rise of asymmetric transition-metal catalysts and, importantly, organocatalytic allylation, reshaping the landscape of modern synthetic chemistry. This review explores the latest developments in the asymmetric allylation of imines, encompassing cutting-edge advances in hydrogen-bond catalysis and non-classical approaches. Furthermore, practical examples showcasing the application of these innovative methodologies in total synthesis are presented.

Chemoenzymatic Oxidation of Labdane and Formal Synthesis of Nimbolide.

In nature, basic terpene skeletons are produced and subsequently undergo enzymatic or nonenzymatic oxidative transformations, leading to diverse structural variations. To date, thousands of natural products featuring a variety of oxidation patterns have been isolated solely from the labdane family. This work describes a strategy for the comprehensive introduction of oxidation states into the labdane core by employing a combination of enzyme library screening, directed evolution, and sequential chemical oxidation processes. Furthermore, we showcase the functional viability of our chemoenzymatic approach by accomplishing a formal synthesis of nimbolide, highlighting its potential for streamlining the synthesis of complex natural products.

Copper and Photocatalytic Synergistic Strategies for Radical Cyclization Reactions

AbstractThe rapid development of photo‐synergistic transition metal catalytic systems has provided a green paradigm to complement thermal transition metal catalytic methods. However, the most commonly used iridium or ruthenium complexes involve expensive in nature, in contrast to the abundant copper elements in the earth's crust, which are highly valued for their unique electronic structure and light‐absorbing properties. Recently, the application of copper and photocatalytic synergistic strategies in radical cyclization reactions has progressed considerably, leading to a renaissance in the synthesis of functional natural products, drugs and their analogues, but summary work addressing this aspect has not been reported. In this review, we briefly analyze the effect of ligand choice on copper complexes and some inorganic copper salts and even on light sources. We then summarize the copper and photocatalytic synergistic strategies in radical cyclization reactions and classify them into three categories, C, N and O radicals, according to the class of the central atom of the radical in each work, and in each category will be elaborated in turn from coordination cyclization via Cu catalysts, direct radical cyclization and other cyclization mode. For individual more complex reactions, the mechanisms are explored and briefly discussed.

Collective Synthesis of Furanocoumarin Natural Products Through a Radical‐mediated Construction of Furanocoumarin Skeletons

AbstractFuranocoumarins are attractive targets in the synthetic organic and medicinal chemistry field because of their structural diversity and interesting biological properties. Herein, we describe the synthesis of furanocoumarin natural products from common furanocoumarin skeletons. A key step was a novel intramolecular radical cyclization onto nitrile to synthesize angular and linear franocoumarins, which was optimized by reacting iodocoumarin derivatives in the presence of (Me3Si)3SiH and oxygen. These compounds served as intermediates in the syntheses of vaginidiol, vaginidin, oroselol, multivittan D, gaudichaudine, oreoselone, peucedanin, and smyrindiol, successfully demonstrating the versatility of the intermediates and utility of the radical cyclization method for accessing furanocoumarin natural products.

TMSCN-Promoted Difunctionalization of Alkenes for the Synthesis of Alcohol Derivatives.

A TMSCN-promoted difunctionalization of styrenes with CHCl3 and TBHP is reported via the radical addition/cross coupling process. A wide range of dichloromethyl-substituted alcohol derivatives were synthesized under transition-metal-free conditions. Besides, this method is also applicable to unactive alkenes. The key to this success lies in the role of TMSCN, which prevents the reaction toward dichloromethylperoxylation of olefins. This represents an alternative approach for synthesizing diverse alcohol derivatives using readily available substrates, holding significant promise in the fields of pharmaceutical chemistry and natural product synthesis.

Palladium-catalyzed intramolecular Heck-type carbonylative cyclization cascade reaction and its application in total synthesis of complex natural products

Palladium-catalyzed intramolecular Heck-type carbonylative cyclization cascade reaction and its application in total synthesis of complex natural products

Metathesis Mastery: Crafting Linear Triquinane Frameworks for Natural Product Synthesis

AbstractOrganic and medicinal chemists have shown great interest in developing simple approaches for creating linear triquinane structures, as they are crucial for synthesizing both natural and synthetic products. Linear triquinane shows various biological characteristics like being commonly used for antitumor, antibacterial, and antimalarial purposes, making it a popular subject of study among synthetic chemists. These come from different sources like marine organisms, plants, and fungi and make up a significant group of sesquiterpenoids. These can be classified as cis‐syn‐cis or cis‐anti‐cis based on the stereochemistry at the ring junction, with cis‐anti‐cis being the most common type and exhibiting significant biological activity. In this review, we summarized different methods using a combination of photothermal and olefin metathesis strategy to construct a triquinane framework by utilizing both traditional heat and microwave radiation. Some commonly used synthetic methods, including metathesis protocol and key reactions like Grignard reaction and Diels–Alder reaction, were utilized together with ring‐closing, ring‐opining, and ring‐rearrangement metathesis to create the linear triquinane core for use in total synthesis of natural products. Additionally, the use of photothermal olefin metathesis along with Hosomi‐Sakurai reaction conditions allows for the creation of intricate polyquinanes, which can be used to produce complex natural compounds and for designing unique molecules theoretically.

Copper-catalyzed three-component reaction to construct thietane ring using elemental sulfur

Thietanes are vital aliphatic four-membered rings associated with various pharmaceuticals, building blocks and synthons of intermediates used in the total synthesis of natural products and biomolecules. Also, 4-thiazolidinone is a core pharmacophore of many drug molecules. Based on their various applications in medicinal chemistry and organic synthesis, an effort has been made to achieve a thietane ring on 4-thiazolidinone nucleus via quasi-benzylic acid-type intramolecular rearrangement using elemental sulfur, benzyl bromide and Cu(I). The designed methodology is environmentally benign, cheap, and easily accessible, with a broad range of substrate scopes and carried out by the employment of earth-abundant metal catalysts like copper.

Generation and Coupling of Radical Species from α-Alkoxy Bridgehead Carboxylic Acid, Selenide, Telluride, Acyl Selenide, and Acyl Telluride.

α-Alkoxy bridgehead radicals enable intermolecular construction of sterically congested C-C bonds due to their sterically accessible nature. We implemented these radical species into total syntheses of various densely oxygenated natural products and demonstrated their exceptional versatility. Herein, we employed different precursors to generate the same α-alkoxy bridgehead radical and compared the efficacy of the precursors for coupling reactions. Specifically, the bridgehead radical of the trioxaadamantane structure was formed from α-alkoxy carboxylic acid, selenide/telluride, and acyl selenide/acyl telluride, and reacted with 4-((tert-butyldimethylsilyl)oxy)cyclopent-2-en-1-one and 5-oxo-1-cyclopentene-1-carbonitrile. The efficiency of the bridgehead radical formation and subsequent coupling reaction significantly depended on the structures of the precursors and acceptors as well as the reaction conditions. Our findings provide new insights for selecting the appropriate substrates of key coupling reactions in the total synthesis of complex natural products.

Enantioselective Total Synthesis of (-)-Cyathin B2: A Desymmetric Double-Allylboration Approach.

A powerful Pt-catalyzed asymmetric diboration/desymmetric double-allylboration cascade reaction has been developed for the construction of synthetically useful, densely functionalized hydrindanes with five stereocenters, including three quaternary ones, in good yields and excellent enantiomeric excess (ee) values within a single synthetic operation. A unified strategy utilizing this key tandem methodology enabled the concise asymmetric total synthesis of cyathane diterpene (-)-Cyathin B2 in 14 steps from commercially available starting materials, thereby demonstrating its remarkable potential in the synthesis of hydrindane-containing natural products and pharmaceuticals.

Palladium-Catalyzed γ-C–H Arylation of Unsaturated Carbonyl Compounds: An Emerging Remote Buchwald–Hartwig–Miura Arylation

AbstractAs an important complement to the Buchwald–Hartwig–Miura arylation, Pd-catalyzed γ-C–H arylations, including γ-C(sp3)–H and γ-C(sp2)–H arylations, provide a more direct route to install an aryl group on the less reactive γ-site of unsaturated carbonyl compounds, and have attracted considerable interest from the chemistry community in recent years. This review summarizes the applications of this method with both cyclic and linear unsaturated carbonyl compounds (aldehydes, ketones, esters, amide, and nitriles), as well as in the total synthesis of natural products (NPs), natural product skeletons, and bioactive analogues.1 Introduction2 γ-C–H Arylation of Cyclic Unsaturated Carbonyl Substrates2.1 Exocyclic γ-Arylation2.1.1 Unsaturated Ketones and the Corresponding Silyl-Dienol Ethers2.1.2 Unsaturated Lactones2.2 Endocyclic γ-C–H Arylation2.2.1 Unsaturated Ketones and the Corresponding Silyl-Dienol Ethers2.2.2 Unsaturated Lactones2.2.3 Unsaturated Nitriles3 γ-C–H Arylation of Linear Unsaturated Carbonyl Substrates3.1 Unsaturated Aldehydes3.2 Unsaturated Ketones3.3 Unsaturated Amides3.4 Unsaturated Nitriles3.5 Silyl Ketene Acetals of α,β-Unsaturated Esters4 Conclusion

Chemoenzymatic Synthesis of Plant‐Derived Kavalactone Natural Products by Dynamic Resolution Using a Biosynthetic O‐Methyltransferase Tailoring Enzyme

Biosynthetic enzymes have enormous potential for the chemoenzymatic synthesis of natural products and other bioactive compounds. Methyltransferases are promising tools for the selective enzymatic modification of complex structures. This paper describes the production, purification and biochemical characterization of the O‐methyltransferase JerF, which catalyzes unique 4‐methoxy‐5,6‐dihydropyranone formation in jerangolid A biosynthesis. Isolation problems had hitherto prevented detailed studies on JerF and were solved by the production of the maltose‐binding protein fusion protein. The differentiation of JerF between styryl‐substituted dihydropyrandion enantiomers was investigated. In combination with a spontaneous racemization occurring with this type of substrates, a new enzymatic dynamic kinetic resolution was observed, which was used for the enantioselective chemoenzymatic synthesis of kavalactone natural products and new derivatives. In combination with an HMT‐based SAM regeneration system, (+)‐kavain, (+)‐11,12‐dimethoxykavain and (+)‐12‐fluorokavain were prepared in 3‐4 steps on the 100 µmol scale with overall yields of 37‐57% and ees of 70‐86%. A mutational study based on an AlphaFold 2 model provided indications for active site residues with an influence on the performance of the enzyme that could be targets for engineering. This example illustrates how the exceptional enzymatic activities and specificities of biosynthetic enzymes can be exploited for the development of new synthesis approaches.

Stereodivergent syntheses of diospongins A and B

Syntheses of the tetrahydropyranol natural products (±)-diospongin A and (±)-diospongin B, in four steps (37 % overall) and seven (14 % overall) or nine steps (25 % overall) respectively, are reported. A common homoallylic alcohol serves as a key intermediate for stereodivergent, one-pot olefin metathesis/deprotection/oxa-Michael cyclization steps that establish the requisite 2,6-stereochemical relationships of the diastereomeric targets. Cross metathesis results in formation of an acyclic, unsaturated ketone that undergoes conjugate addition to give diospongin A directly. Ring-closing metathesis/conjugate addition provides a diastereomerically pure bicyclic lactone, possessing the contra-thermodynamic 2,6-trans stereochemistry, that can be converted to diospongin B in either two or four steps.

Zirconium Hydride Catalysis Initiated by Tetrabutylammonium Fluoride.

In our drug discovery campaigns to target the oncogenic drivers of cancers, the demand for a chemoselective, stereoselective and economical synthesis of chiral benzylamines drove the development of a catalytic zirconium hydride reduction. This methodology uses the inexpensive, bench stable zirconocene dichloride, and a novel tetrabutylammonium fluoride activation tactic to catalytically generate a metal hydride under ambient conditions. The diastereo- and chemoselectivity of this reaction was tested with the preparation of key intermediates from our discovery programs and in the scope of sulfinyl ketimines and carbonyls relevant to medicinal chemistry and natural product synthesis. A preliminary mechanistic investigation conducted into the role of tetrabutylammonium fluoride indicates that formation of a zirconocene fluoride occurs to initiate catalysis. The implications of this convenient activation approach may provide expanded roles for zirconium hydrides in catalytic transformations.

Unified, Biosynthesis-Inspired, Completely Stereocontrolled Total Synthesis of All Highest-Order [n + 1] Oligocyclotryptamine Alkaloids.

We describe the unified enantioselective total synthesis of the polycyclotryptamine natural products (+)-quadrigemine H, (+)-isopsychotridine C, (+)-oleoidine, and (+)-caledonine. Inspired by our hypothesis for the biogenesis of these alkaloids via an iterative concatenative addition of homochiral cyclotryptamines to a meso-chimonanthine headcap, we leverage the modular, diazene-directed assembly of stereodefined cyclotryptamines to introduce successive C3a-C7' quaternary stereocenters on a heterodimeric meso-chimonanthine surrogate with full stereochemical control at each quaternary linkage. We developed a new strategy for iterative aryl-alkyl diazene synthesis using increasingly complex oligomeric hydrazide nucleophiles and a bifunctional cyclotryptamine bearing a C3a leaving group and a pendant C7 pronucleophile. The utility of this strategy is demonstrated by the first total synthesis of heptamer (+)-caledonine and hexamer (+)-oleoidine. Enabled by our completely stereoselective total syntheses and expanded characterization data sets, we provide the first complete stereochemical assignment of pentamer (+)-isopsychotridine C, provide evidence that it is identical to the alkaloid known as (+)-isopsychotridine B, and report that tetramer (+)-quadrigemine H is identical to the alkaloid called (+)-quadrigemine I, resolving longstanding questions about the structures of the highest-order [n + 1] oligocyclotryptamine alkaloids.

Biosynthetic Strategies of Berberine Bridge Enzyme-like Flavoprotein Oxidases toward Structural Diversification in Natural Product Biosynthesis.

Berberine bridge enzyme-like oxidases are often involved in natural product biosynthesis and are seen as essential enzymes for the generation of intricate pharmacophores. These oxidases have the ability to transfer a hydride atom to the FAD cofactor, which enables complex substrate modifications and rearrangements including (intramolecular) cyclizations, carbon-carbon bond formations, and nucleophilic additions. Despite the diverse range of activities, the mechanistic details of these reactions often remain incompletely understood. In this Review, we delve into the complexity that BBE-like oxidases from bacteria, fungal, and plant origins exhibit by providing an overview of the shared catalytic features and emphasizing the different reactivities. We propose four generalized modes of action by which BBE-like oxidases enable the synthesis of natural products, ranging from the classic alcohol oxidation reactions to less common amine and amide oxidation reactions. Exploring the mechanisms utilized by nature to produce its vast array of natural products is a subject of considerable interest and can lead to the discovery of unique biochemical activities.

Cobalt-Catalyzed Chemo- and Stereoselective Transfer Semihydrogenation of 1,3-Dienes with Water as a Hydrogen Source.

(Z)-1,2-Disubstituted, trisubstituted, and tetrasubstituted alkenes are not only important units in medicinal chemistry, natural product synthesis, and material science but also useful intermediates in organic synthesis. Development of catalytic stereoselective transformations to access multisubstituted alkenes with various substitution patterns from easily accessible modular starting materials and readily available catalysts is a crucial goal in the field of catalysis. Water is an ideal hydrogen source for catalytic transfer hydrogenation despite of the high difficulty to activate water. Here, we report a cobalt-catalyzed protocol for regio- and stereoselective transfer semihydrogenation of 1,3-dienes to construct a broad scope of (Z)-1,2-disubstituted, (Z)-, (E)-trisubstituted, and tetrasubstituted alkenes in high stereoselectivity with H2O as the hydrogen source. Mechanistic studies revealed that the reactions proceeded through a unique Co(I)/Co(III) cycle and involved a 1,4-cobalt shift process, which is an unprecedented reaction pathway, providing a new platform for modular synthesis of multisubstituted alkenes as well as opportunities for designing novel reaction modes and pushing forward the advancement in organocobalt chemistry.

Facile Construction of Quaternary Carbon Centers via Dinuclear Titanium(III)‐Catalyzed Reductive Coupling of Tertiary Chlorides and Activated Olefins

AbstractThe establishment of all‐carbon quaternary carbon centers represents a pivotal aspect in the synthesis of numerous natural products and pharmaceutical compounds, underscoring its paramount significance in synthetic organic chemistry. Herein, we present a novel synthetic strategy for the generation of quaternary carbon centers through the reductive coupling of tertiary halides, catalyzed by dinuclear (salen)titanium catalysts. This catalytic framework demonstrates notable reactivity towards tertiary chlorides while maintaining inertness towards secondary and primary chlorides, thereby facilitating exceptional chemoselectivity for C−C bond formation. The reaction proceeds with remarkable efficiency across a diverse range of substrates under mild reaction conditions.

Development of Organocatalytic Reactions for the Synthesis of Natural Products and Pharmaceuticals

This review describes novel organocatalytic methods for the enantioselective construction of spiroindans and spirochromans and the application of the methods to the total synthesis of natural products. We developed an intramolecular Friedel-Craftstype 1,4-addition in which the substrates were a resorcinol derivative and 2-cyclohexenone linked by an alkyl chain. The reaction proceeded smoothly in the presence of a cinchonidine-based primary amine (30 mol%) with water and p-bromophenol as additives. A variety of spiroindanes were obtained with high enantioselectivity under these conditions. The reaction was applied in the first total synthesis of the unusual proaporphine alkaloid (-)-misramine, which included the key steps of enantioselective spirocyclization and double reductive amination of the keto-aldehyde to form a piperidine ring toward the end of the synthesis. The total synthesis of misrametine was achieved by selective demethylation of the methoxy group from the precursor to misramine. Next, a method for highly enantioselective organocatalytic construction of spirochromans containing a tetrasubstituted stereocenter was developed. An intramolecular oxy-Michael addition was catalyzed by a bifunctional cinchona alkaloid thiourea catalyst. A variety of spirochroman compounds containing a tetrasubstituted stereocenter were obtained with excellent enantioselectivity of up to 99% enantiomeric excess. The reaction was applied to the asymmetric formal synthesis of (-)-(R)-cordiachromene.

Total synthesis of Comfreyn A and structural analogues via two photochemical key steps.

In this work the influence of o-fluorine substituents on the photo-dehydro-Diels-Alder (PDDA) reaction was investigated and the findings of this study were applied to the total synthesis of natural products. The reactant molecules consisted of two alkyl arylpropiolates, connected by a suberic acid tether and bearing fluorine substituents in each of the o-positions. While quantum chemical calculations suggested that a fluorine substituent prevents an attack of the adjacent carbon atom in the second C-C bond forming step of the PDDA reaction, this attack took place nevertheless. The subsequent fluoride elimination, assisted by protic solvents or trialkylsilanes, resulted in an "Umpolung" of the 4-position of the cycloallene intermediate enabling the introduction of nucleophiles at this position. The nucleophilic replacement of the second fluorine substituent could also be triggered photochemically. After removal of the tether, the two arene moieties stand nearly perpendicular to each other and a selective excitation of the naphthalene moiety was possible. This led to an intramolecular photoinduced electron transfer (PET) followed by a nucleophilic replacement of the fluoride according to a SR+N1Ar* mechanism. The formed phenolic hydroxyl group underwent spontaneous lactonization with the adjacent ester group. Based on these results, the first total synthesis of the lignan Comfreyn A and some structural analogues were developed.