A Biomimetic Synthesis Elucidates the Origin of Preuisolactone A

Synthesis and biological evaluation of cleistocaltone A, an inhibitor of respiratory syncytial virus (RSV)

The first chemical synthesis of the phloroglucinol meroterpenoid cleistocaltone A (1) is presented. This compound, previously isolated from Cleistocalyx operculatus was reported to show promising antiviral properties. Based on a modified biosynthesis proposal, a synthetic strategy was devised featuring an intramolecular Diels-Alder reaction and an epoxidation/elimination sequence to generate the allyl alcohol handle in the side chain. The strategy was successfully executed and synthetic cleistcaltone A was evaluated against a contemporary RSV-A strain.

Regio- and Diastereoselective Radical Dimerization Reactions for the Construction of Benzo[f]isoindole Dimers

This study presents a novel approach for synthesizing benzo[f]isoindole dimers, which involves cascade cyclization and oxidative radical dimerization. Our method allows for the formation of up to five carbon-carbon bonds in a single reaction, exhibiting remarkable diastereoselectivity and regioselectivity. The mechanism and regioselectivity were investigated through a combination of experiments and calculations.

Multicomponent Cyclizative 1,2-Rearrangement Enabled Enantioselective Construction of 2,2-Disubstituted Pyrrolinones

The 1,2-rearrangement reaction is one of the most important approaches to construct carbon-carbon bonds in organic synthesis. However, the development of catalytic asymmetric 1,2-rearrangements is still far from mature and often suffers from problems such as complex substrates, single product structure, and lack of synthetic application. Multicomponent reaction has been recognized as a robust tool for the synthesis of diverse and tunable products from readily available starting material. Conceptionally and practically, the development of multicomponent asymmetric 1,2-rearrangements is highly desirable. In this regard, we report herein a three-component benzilic acid-type rearrangement of 2,3-diketoesters, aromatic amines and aldehydes for the asymmetric construction of synthetically challenging pyrrolinones bearing aza-quaternary stereocenters. To the best of our knowledge, this reaction represents the first example of organocatalyzed multicomponent asymmetric 1,2-rearrangements.

Recent Developments and Applications of Biocatalytic and Chemoenzymatic Synthesis for the Generation of Diverse Classes of Drugs

Biocatalytic and chemoenzymatic biosynthesis are powerful methods of organic chemistry that use enzymes to execute selective reactions and allow the efficient production of organic compounds. The advantages of these approaches include high selectivity, mild reaction conditions, and the ability to work with complex substrates. The utilization of chemoenzymatic techniques for the synthesis of complicated compounds has lately increased dramatically in the area of organic chemistry. Biocatalytic technologies and modern synthetic methods are utilized synergistically in a multi-step approach to a target molecule under this paradigm. Chemoenzymatic techniques are promising for simplifying access to essential bioactive compounds because of the remarkable regio- and stereoselectivity of enzymatic transformations and the reaction diversity of modern organic chemistry. Enzyme kits may include ready-to-use, reproducible biocatalysts. Its use opens up new avenues for the synthesis of active therapeutic compounds and aids in drug development by synthesizing active components to construct scaffolds in a targeted and preparative manner. This study summarizes current breakthroughs as well as notable instances of biocatalytic and chemoenzymatic synthesis. To assist organic chemists in the use of enzymes for synthetic applications, it also provides some basic guidelines for selecting the most appropriate enzyme for a targeted reaction while keeping aspects like cofactor requirement, solvent tolerance, use of whole cell or isolated enzymes, and commercial availability in mind.

Natural Product Synthesis: The Endless Quest for Unreachable Perfection

Total synthesis is a field in constant progress. Its practitioners aim to develop ideal synthetic strategies to build complex molecules. As such, they are both a driving force and a showcase of the progress of organic synthesis. In this Perspective, we discuss recent notable total syntheses. The syntheses selected herein are classified according to the key strategic considerations for each approach.

Synthesis of Suffranidine B

Efficiently generating intricate molecular complexity is a coveted goal in organic synthesis. This can be realized through the implementation of inventive and audacious strategies coupled with the exploration and advancement of novel molecular reactivity pathways. Herein, we present a concise two-step synthesis of a high-oxidation state heterotrimeric securinega alkaloid, suffranidine B, from 2,3-dehydroallosecurinine and the vinylogous ketoaldehyde compound derived from kojic acid. Key to the success was the astute selection of appropriate acids during both the heterotrimerization and the desymmetrizing cyclization steps. This study underscores the value of biomimicry in the synthesis of complex natural products.

Total Synthesis of Yuzurine-type Alkaloid Daphgraciline

The first total synthesis of daphgraciline has been achieved, which also represents the first example of the synthesis of Daphniphyllum yuzurine-type alkaloids (∼50 members). The unique bridged azabicyclo[4.3.1] ring system in the yuzurine-type subfamily was efficiently and diastereoselectively assembled via a mild type II [5+2] cycloaddition for the first time. The compact tetracyclic [6-7-5-5] skeleton was installed efficiently via an intramolecular Diels-Alder reaction, followed by a benzilic acid-type rearrangement. The synthetically challenging spiro tetrahydropyran moiety in the final product was installed diastereoselectively via a Ti^III-mediated reductive epoxide coupling reaction. Potential access to enantioenriched daphgraciline is presented.

Ring contraction in synthesis of functionalized carbocycles

Carbocycles are a key and widely present structural motif in organic compounds. The construction of structurally intriguing carbocycles, such as highly-strained fused rings, spirocycles or highly-functionalized carbocycles with congested stereocenters, remains challenging in organic chemistry. Cyclopropanes, cyclobutanes and cyclopentanes within such carbocycles can be synthesized through ring contraction. These ring contractions involve re-arrangement of and/or small molecule extrusion from a parental ring, which is either a carbocycle or a heterocycle of larger size. This review provides an overview of synthetic methods for ring contractions to form cyclopropanes, cyclobutanes and cyclopentanes en route to structurally intriguing carbocycles.

Epicoccanes A-D, Four Oxidative Dimers of Pyrogallol Analogues from Epicoccum nigrum

Epicoccanes A-D (1-4) are four novel metabolites of an endophytic fungus Epicoccum nigrum. Their distinct unprecedented structures are hypothesized as oxidative dimers of pyrogallol analogues. Compounds 1 and 2 possess a novel spirobicyclo[3.2.1]octane-6,1'-cyclopentane or -cyclohexane core skeleton. Compound 3 is of a unique cage-like pentacyclic system, which unusually contained three continuous spiro-carbons. Compound 4 is a highly rearranged dimer with five contiguous chiral centers. The absolute structures of 1 and 2 were deduced by electronic circular dichroism (ECD) calculations, and those of 3 and 4 were determined by X-ray crystallography. Compounds 1 and 4 showed potential antiliver fibrosis activity.

Cloning and Functional Characterization of the Polyketide Synthases Based on Genome Mining of Preussia isomera XL-1326

Fungal polyketides (PKs) are one of the largest families of structurally diverse bioactive natural products biosynthesized by multidomain megasynthases, in which thioesterase (TE) domains act as nonequivalent decision gates determining both the shape and the yield of the polyketide intermediate. The endophytic fungus Preussia isomera XL-1326 was discovered to have an excellent capacity for secreting diverse bioactive PKs, i.e., the hot enantiomers (±)-preuisolactone A with antibacterial activity, the single-spiro minimoidione B with α-glucosidase inhibition activity, and the uncommon heptaketide setosol with antifungal activity, which drive us to illustrate how the unique PKs are biosynthesized. In this study, we first reported the genome sequence information of P. isomera. Based on genome mining, we discovered nine transcriptionally active genes encoding polyketide synthases (PKSs), Preu1–Preu9, of which those of Preu3, Preu4, and Preu6 were cloned and functionally characterized due to possessing complete sets of synthetic and release domains. Through heterologous expression in Saccharomyces cerevisiae, Preu3 and Preu6 could release high yields of orsellinic acid (OA) derivatives [3-methylorsellinic acid (3-MOA) and lecanoric acid, respectively]. Correspondingly, we found that Preu3 and Preu6 were clustered into OA derivative synthase groups by phylogenetic analysis. Next, with TE domain swapping, we constructed a novel “non-native” PKS, Preu6-TE_Preu3, which shared a very low identity with OA synthase, OrsA, from Aspergillus nidulans but could produce a large amount of OA. In addition, with the use of Preu6-TE_Preu3, we synthesized methyl 3-methylorsellinate (synthetic oak moss of great economic value) from 3-MOA as the substrate, and interestingly, 3-MOA exhibited remarkable antibacterial activities, while methyl 3-methylorsellinate displayed broad-spectrum antifungal activity. Taken together, we identified two novel PKSs to biosynthesize 3-MOA and lecanoric acid, respectively, with information on such kinds of PKSs rarely reported, and constructed one novel “non-native” PKS to largely biosynthesize OA. This work is our first step to explore the biosynthesis of the PKs in P. isomera, and it also provides a new platform for high-level environment-friendly production of OA derivatives and the development of new antimicrobial agents.

Recent Advances in Biocatalysis for Drug Synthesis

Biocatalysis is constantly providing novel options for the synthesis of active pharmaceutical ingredients (APIs). In addition to drug development and manufacturing, biocatalysis also plays a role in drug discovery and can support many active ingredient syntheses at an early stage to build up entire scaffolds in a targeted and preparative manner. Recent progress in recruiting new enzymes by genome mining and screening or adapting their substrate, as well as product scope, by protein engineering has made biocatalysts a competitive tool applied in academic and industrial spheres. This is especially true for the advances in the field of nonribosomal peptide synthesis and enzyme cascades that are expanding the capabilities for the discovery and synthesis of new bioactive compounds via biotransformation. Here we highlight some of the most recent developments to add to the portfolio of biocatalysis with special relevance for the synthesis and late-stage functionalization of APIs, in order to bypass pure chemical processes.

Natural product anticipation through synthesis

Natural product synthesis remains one of the most vibrant and intellectually rewarding areas of chemistry, although the justifications for pursuing it have evolved over time. In the early years, the emphasis lay on structure elucidation and confirmation through synthesis, as exemplified by celebrated studies on cocaine, morphine, strychnine and chlorophyll. This was followed by a phase where the sheer demonstration that highly complex molecules could be recreated in the laboratory in a rational manner was enough to justify the economic expense and intellectual agonies of a synthesis. Since then, syntheses of natural products have served as platforms for the demonstration of elegant strategies, for inventing new methodology 'on the fly' or to demonstrate the usefulness and scope of methods established with simpler molecules. We now add another aspect that we find fascinating, viz. 'natural product anticipation'. In this Review, we survey cases where the synthesis of a compound in the laboratory has preceded its isolation from nature. The focus of our Review lies on examples where this anticipation of a natural product has triggered a successful search or where synthesis and isolation have occurred independently. Finally, we highlight cases where a potential natural product structure has been suggested as a result of synthetic endeavours but not yet confirmed by isolation, inviting further collaborations between synthetic and natural product chemists.

Complex Natural Products Derived from Pyrogallols

Pyrogallols (1,2,3-trihydroxybenzenes) are abundant in Nature, easily oxidized, and are central precursors to important natural products. The rich chemistry of their oxidized derivatives, the hydroxy-o-quinones, has been studied for over a century and still attracts the interest of the scientific community. Only in the last ten years have critical insights of pyrogallol chemistry from the mid-twentieth century been applied to modern natural product synthesis. Historical studies of pyrogallol chemistry, including [5+2], [4+2], and formal [5+5] cycloadditions are discussed here and reactivity guidelines established. The application and remarkable selectivity of these cycloadditions is then showcased in the recent syntheses of several fungal natural products, including dibefurin, epicolactone, the merocytochalasans, and preuisolactone A. The authors hope that this contribution will spark further interest in the fascinating chemistry of pyrogallols and natural products derived from them.

Stereoselective Synthesis of Cyclobutanes by Contraction of Pyrrolidines

Here we report a contractive synthesis of multisubstituted cyclobutanes containing multiple stereocenters from readily accessible pyrrolidines using iodonitrene chemistry. Mediated by a nitrogen extrusion process, the stereospecific synthesis of cyclobutanes involves a radical pathway. Unprecedented unsymmetrical spirocyclobutanes were prepared successfully, and a concise, formal synthesis of the cytotoxic natural product piperarborenine B is reported.

Reflections on the intriguing occurrence of some recently isolated natural products as racemates and scalemic mixtures

This review continues our interest in the intriguing reports of a variety of new racemic natural products (at least 11 in the past 4 years). These include the polyphenolic racemate galewone, the polycyclic prenylated acylphloroglucinol garcinielliptone; variecolortide, a combination of an anthraquinone and a isochinulin-type alkaloid; the isoindoline alkaloid irpexine, the new hybrid phenylproanoid asarone; colletopyandione an indolydenepyradione; the enantiomerically enriched (scalemic) neolignans, gardenifolins; and meroterpenoid pabmaragramin in addition to some marine lipids. We also present a recent biomimetic synthesis of the polyketide preuisolactone A; synthesis of the polyketide spiromamakone A, which also corrected the proposed structure of another metabolite as identical to spiromamakone A; and the melicolones A and B. The continuing reports of natural racemates provoke speculation as to their role in the producing organism.

Iron-catalyzed arene C-H hydroxylation

[Figure: see text].

Stereoelectronic power of oxygen in control of chemical reactivity: the anomeric effect is not alone

Although carbon is the central element of organic chemistry, oxygen is the central element of stereoelectronic control in organic chemistry. Generally, a molecule with a C-O bond has both a strong donor (a lone pair) and a strong acceptor (e.g., a σ*_C-O orbital), a combination that provides opportunities to influence chemical transformations at both ends of the electron demand spectrum. Oxygen is a stereoelectronic chameleon that adapts to the varying situations in radical, cationic, anionic, and metal-mediated transformations. Arguably, the most historically important stereoelectronic effect is the anomeric effect (AE), i.e., the axial preference of acceptor groups at the anomeric position of sugars. Although AE is generally attributed to hyperconjugative interactions of σ-acceptors with a lone pair at oxygen (negative hyperconjugation), recent literature reports suggested alternative explanations. In this context, it is timely to evaluate the fundamental connections between the AE and a broad variety of O-functional groups. Such connections illustrate the general role of hyperconjugation with oxygen lone pairs in reactivity. Lessons from the AE can be used as the conceptual framework for organizing disjointed observations into a logical body of knowledge. In contrast, neglect of hyperconjugation can be deeply misleading as it removes the stereoelectronic cornerstone on which, as we show in this review, the chemistry of organic oxygen functionalities is largely based. As negative hyperconjugation releases the "underutilized" stereoelectronic power of unshared electrons (the lone pairs) for the stabilization of a developing positive charge, the role of orbital interactions increases when the electronic demand is high and molecules distort from their equilibrium geometries. From this perspective, hyperconjugative anomeric interactions play a unique role in guiding reaction design. In this manuscript, we discuss the reactivity of organic O-functionalities, outline variations in the possible hyperconjugative patterns, and showcase the vast implications of AE for the structure and reactivity. On our journey through a variety of O-containing organic functional groups, from textbook to exotic, we will illustrate how this knowledge can predict chemical reactivity and unlock new useful synthetic transformations.

Bioinspired Network Analysis Enabled Divergent Syntheses and Structure Revision of Pentacyclic Cytochalasans

We accomplished the divergent total syntheses of ten pentacyclic cytochalasans (aspergillin PZ, trichodermone, trichoderones, flavipesines, and flavichalasines) from a common precursor aspochalasin D and revised the structures of trichoderone B, spicochalasin A, flavichalasine C, aspergilluchalasin based on structure network analysis of the cytochalasans biosynthetic pathways and DFT calculations. The key steps of the syntheses include transannular alkene/epoxyalkene and carbonyl-ene cyclizations to establish the C/D ring of pentacyclic aspochalasans. Our bioinspired approach to these pentacyclic cytochalasans validate the proposed biosynthetic speculation from a chemical view and provide a platform for the synthesis of more than 400 valuable cytochalasans bearing different macrocycles and amino-acid residues.

Early Oxidative Transformations During the Biosynthesis of Terrein and Related Natural Products

The mycotoxin terrein is derived from the C₁₀‐precursor 6‐hydroxymellein (6‐HM) via an oxidative ring contraction. Although the corresponding biosynthetic gene cluster (BGC) has been identified, details of the enzymatic oxidative transformations are lacking. Combining heterologous expression and in vitro studies we show that the flavin‐dependent monooxygenase (FMO) TerC catalyzes the initial oxidative decarboxylation of 6‐HM. The reactive intermediate is further hydroxylated by the second FMO TerD to yield a highly oxygenated aromatic species, but further reconstitution of the pathway was hampered. A related BGC was identified in the marine‐derived Roussoella sp. DLM33 and confirmed by heterologous expression. These studies demonstrate that the biosynthetic pathways of terrein and related (polychlorinated) congeners diverge after oxidative decarboxylation of the lactone precursor that is catalyzed by a conserved FMO and further indicate that early dehydration of the side chain is an essential step.

Catalytic Enantioselective Synthesis of Morpholinones Enabled by Aza-Benzilic Ester Rearrangement

Chiral morpholinone is an important building block in organic synthesis and a pharmacophore in medicinal chemistry. However, catalytic enantioselective methods for the construction of this N,O-heterocycle remain scarce. We report herein a chiral phosphoric acid-catalyzed enantioselective synthesis of C3-substituted morpholinones from aryl/alkylglyoxals and 2-(arylamino)ethan-1-ols. The reaction proceeds through a domino [4 + 2] heteroannulation followed by a 1,2-aryl/alkyl shift of the resulting cyclic α-iminium hemiacetals. It represents formally an unprecedented asymmetric aza-benzilic ester rearrangement reaction. A concise synthesis of L-742,694, a neurokinin-1 receptor antagonist, featuring this reaction is documented.

Computational Exploration of the Mechanism of Critical Steps in the Biomimetic Synthesis of Preuisolactone A, and Discovery of New Ambimodal (5 + 2)/(4 + 2) Cycloadditions

Computational studies with ωB97X-D density functional theory of the mechanisms of the steps in Trauner's biomimetic synthesis of preuisolactone A have elaborated and refined mechanisms of several unique processes. An ambimodal transition state has been identified for the cycloaddition between an o-quinone and a hydroxy-o-quinone; this leads to both (5 + 2) (with H shift) and (4 + 2) cycloaddition products, which can in principle interconvert via α-ketol rearrangements. The origins of periselectivity of this ambimodal cycloaddition have been investigated computationally with molecular dynamics simulations and tested further by an experimental study. In the presence of bicarbonate ions, the deprotonated hydroxy-o-quinone leads to only the (5 + 2) cycloaddition adduct. A new mechanism for a benzilic acid rearrangement resulting in ring contraction is proposed.

Natural Product Synthesis Enabled by Domino Processes Incorporating a 1,2-Rearrangement Step

The art of natural product total synthesis is closely associated with two major determinants: the development/application of novel chemical reactions and the innovation in strategic use of classic organic reactions. While purposely seeking/applying a new synthetic methodology allowing nonconventional bond disconnections could shorten the synthetic route, the development of domino processes composed of a series of well-established reactions could also lead to a concise, practical, and aesthetically appealing synthesis. As an important class of textbook reactions, the 1,2-anionotropic rearrangements discovered at the dawn of modern organic chemistry have important bearings not only on chemical synthesis but also on the conceptual breakthroughs in the field. In its basic form, the 1,2-shift affords nothing but a constitutional isomer of the starting material and is therefore not a complexity-generating transformation. However, such a simple 1,2-shift could in fact change the molecular topology if the precursor is cleverly designed. More dramatically, it can metamorphosize the structure of the substrate when it is combined with other transformations in a domino sequence. In this Outlook, we highlight recent examples of natural product synthesis featuring a key domino process incorporating a 1,2-anionotropic rearrangement. Specifically, domino reactions integrating Wagner-Meerwein, pinacol, α-ketol, α-aminoketone, α-iminol, or benzilic acid rearrangements will be discussed.

Recent advances in total syntheses of complex dimeric natural products

Dimeric natural products are a collection of molecules with diverse molecular architectures and significant bio-activities. In this tutorial review, total synthesis of complex dimeric natural products accomplished in recent years are summarized and various dimerization strategies are discussed. By highlighting the selected representative examples, this review aims to demonstrate the recent tactics of dimerization which is an important process integrated into the whole synthetic sequences of dimeric natural products, provide insights on structural and chemical properties of monomers and dimers of related natural products, and promote further technological advances in organic synthesis and biological studies of complex dimeric natural products.

Innovation of hypervalent(iii) iodine in the synthesis of natural products

The focus article discusses the innovation of hypervalent(iii) iodine regarding skeletal rearrangement, cycloaddition and cyclization, and sp³ C–H functionalization in natural product synthesis.

Total Synthesis of (-)-Mitrephorone A Enabled by Stereoselective Nitrile Oxide Cycloaddition and Tetrasubstituted Olefin Synthesis

A highly enantioselective and diastereoselective total synthesis of the diterpenoid (−)-mitrephorone A is presented. Key to the synthesis are stereocontrolled 1,4-semihydrogenation of a 1,3-diene to a tetrasubstituted double bond, enzyme-catalyzed malonate desymmetrization, and highly diastereoselective nitrile oxide cycloaddition. The streamlined strategy is a considerable improvement to those reported earlier in terms of diastereo- and enantioselectivity. For the first time, the combination of modern Pd-cross-coupling with Cr-catalyzed reduction allows for rapid access to tetrasubstituted olefins with full stereocontrol.

Catalytic Enantioselective Benzilic Ester Rearrangement

We report the first examples of catalytic enantioselective benzilic ester rearrangement reaction. In the presence of a catalytic amount of Cu(OTf)₂ and a chiral box ligand under mild conditions, reaction of 2,3-diketoesters with alcohols afforded structurally diverse α-aryl(alkyl) substituted-α-hydroxy malonates (tartronic esters) in good to excellent yields with high enantioselectivities. Preliminary mechanistic studies indicated that hemiketalization, rather than the dynamic kinetic resolution of hemiketal, was the enantiodetermining step under our reaction conditions.

Taxodisones A and B: bioactive C30-terpenes with new skeletons from Taxodium distichum and their biosynthetic origin

Herein, taxodisones A and B were isolated from the seeds of T. distichum, and the structures, configurations, bioactivities and origin of these compounds were determined.

Hot off the Press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as burlemarxione A from Clusia burle-marxii.