Concise Stereoselective Synthesis of (−)‐Podophyllotoxin by an Intermolecular Iron(III)‐Catalyzed Friedel–Crafts Alkylation

Improving the Enantioselectivity of CHMOBrevi1 for Asymmetric Synthesis of Podophyllotoxin Precursor

(R)-β-piperonyl-γ-butyrolactones are key building blocks for the synthesis of podophyllotoxin, which have demonstrated remarkable potential in cancer treatment. Baeyer-Villiger monooxygenases (BVMOs)-mediated asymmetric oxidation is a green approach to produce chiral lactones. While several BVMOs were able to oxidize the corresponding cyclobutanone, most BVMOs gave the (S) enantiomer while Cyclohexanone monooxygenase (CHMO) from Brevibacterium sp. HCU1 gave (R) enantiomer, but with a low enantioselectivity (75 % ee). In this study, we use a strategy called "focused rational iterative site-specific mutagenesis" (FRISM) at residues ranging from 6 Å from substrate. The mutations by using a restricted set of rationally chosen amino acids allow the formation of a small mutant library. By generating and screening less than 60 variants, we achieved a high ee of 96.8 %. Coupled with the cofactor regeneration system, 9.3 mM substrate was converted completely in a 100-mL scale reaction. Therefore, our work reveals a promising synthetic method for (R)-β-piperonyl-γ-butyrolactone with the highest enantioselectivity, and provides a new opportunity for the chem-enzymatic synthesis of podophyllotoxin.

Stereoselective Total Synthesis of Formosanol, Tsugacetal, and Methyl β-Conidendral

The first enantioselective total synthesis of aryltetralin lignan acetals, (-)-formosanol, (+)-tsugacetal, (+)-methyl β-conidendral, and their enantiomers have been accomplished on the basis of the Pd-catalyzed asymmetric allylic cycloaddition as a key step. Six stereoisomers of the lignan acetals have been synthesized via a 7-8 step sequence in up to 14% overall yield. The in vitro cytotoxicity against several cancer cells has preliminarily been examined for the obtained six stereoisomers of lignan acetals.

Biosynthesis, total synthesis, and pharmacological activities of aryltetralin-type lignan podophyllotoxin and its derivatives

Covering: up to 2022Podophyllotoxin (PTOX, 1), a kind of aryltetralin-type lignan, was first discovered in the plant Podophyllum peltatum and its structure was clarified by W. Borsche and J. Niemann in 1932. Due to its potent anti-cancer and anti-viral activities, it is considered one of the molecules most likely to be developed into modern drugs. With the increasing market demand and insufficient storage of natural resources, it is crucial to expand the sources of PTOXs. The original extraction method from plants has gradually failed to meet the requirements, and the biosynthesis and total synthesis have become the forward-looking alternatives. As key enzymes in the biosynthetic pathway of PTOXs and their catalytic mechanisms being constantly revealed, it is possible to realize the heterogeneous biosynthesis of PTOXs in the future. Chemical and chemoenzymatic synthesis also provide schemes for strictly controlling the asymmetric configuration of the tetracyclic core. Currently, the pharmacological activities of some PTOX derivatives have been extensively studied, laying the foundation for clinical candidate drugs. This review focuses primarily on the latest research progress in the biosynthesis, total synthesis, and pharmacological activities of PTOX and its derivatives, providing a more comprehensive understanding of these widely used compounds and supporting the future search for clinical applications.

Recent strategies and tactics for the enantioselective total syntheses of cyclolignan natural products

Covering: 2000 to 2021Lignan natural products are found in many different plant species and possess numerous useful biological properties, such as anti-inflammatory, antiviral, antioxidant, antibacterial, and antitumor activities. Their utility in both traditional and conventional medicine, coupled with their structural diversity has made them popular synthetic targets over many decades. This review specifically addresses the cyclolignan subclass of the family, which possess both a C8-C8' and a C2-C7' linkage between two different phenylpropene units. We present a comprehensive overview of the diverse strategies employed by chemists to achieve enantioselective total syntheses of cyclolignans covering: 2000 to 2021.

Photo-Induced Ruthenium-Catalyzed C-H Benzylations and Allylations at Room Temperature

The ruthenium‐catalyzed synthesis of diarylmethane compounds was realized under exceedingly mild photoredox conditions without the use of exogenous photocatalysts. The versatility and robustness of the ruthenium‐catalyzed C−H benzylation was reflected by an ample scope, including multifold C−H functionalizations, as well as transformable pyrazoles, imidates and sensitive nucleosides. Mechanistic studies were indicative of a photoactive cyclometalated ruthenium complex, which also enabled versatile C−H allylations.

Asymmetric Total Synthesis of Aglacins A, B, and E

An asymmetric photoenolization/Diels-Alder (PEDA) reaction between electron-rich 2-methylbenzaldehydes and unsaturated γ-lactones was developed to directly construct the basic tricyclic core of aryltetralin lactone lignans. This methodology enabled the first asymmetric total synthesis of aglacins A, B, and E and revision of the absolute configuration of these natural lignans. The strategy was also used to prepare the naturally occurring aryldihydronaphthalene-type lignans (-)-7,8-dihydroisojusticidin B and (+)-linoxepin in four and six steps, as well as 27 natural-product-like molecules containing a C8' quaternary center. We believe that the synthetic aglacins and small-molecule library provide new opportunities to carry out the SAR studies of the podophyllotoxin family of natural products.

Biocatalytic routes to anti-viral agents and their synthetic intermediates

An assessment of biocatalytic strategies for the synthesis of anti-viral agents, offering guidelines for the development of sustainable production methods for a future COVID-19 remedy.

Assembly of Plant Enzymes in E. coli for the Production of the Valuable (-)-Podophyllotoxin Precursor (-)-Pluviatolide

Lignans are plant secondary metabolites with a wide range of reported health-promoting bioactivities. Traditional routes toward these natural products involve, among others, the extraction from plant sources and chemical synthesis. However, the availability of the sources and the complex chemical structures of lignans often limit the feasibility of these approaches. In this work, we introduce a newly assembled biosynthetic route in E. coli for the efficient conversion of the common higher-lignan precursor (+)-pinoresinol to the noncommercially available (-)-pluviatolide via three intermediates. (-)-Pluviatolide is considered a crossroad compound in lignan biosynthesis, because the methylenedioxy bridge in its structure, resulting from the oxidation of (-)-matairesinol, channels the biosynthetic pathway toward the microtubule depolymerizer (-)-podophyllotoxin. This oxidation reaction is catalyzed with high regio- and enantioselectivity by a cytochrome P450 monooxygenase from Sinopodophyllum hexandrum (CYP719A23), which was expressed and optimized regarding redox partners in E. coli. Pinoresinol-lariciresinol reductase from Forsythia intermedia (FiPLR), secoisolariciresinol dehydrogenase from Podophyllum pleianthum (PpSDH), and CYP719A23 were coexpressed together with a suitable NADPH-dependent reductase to ensure P450 activity, allowing for four sequential biotransformations without intermediate isolation. By using an E. coli strain coexpressing the enzymes originating from four plants, (+)-pinoresinol was efficiently converted, allowing the isolation of enantiopure (-)-pluviatolide at a concentration of 137 mg/L (ee ≥99% with 76% isolated yield).

Asymmetric Chemoenzymatic Synthesis of (-)-Podophyllotoxin and Related Aryltetralin Lignans

(-)-Podophyllotoxin is one of the most potent microtubule depolymerizing agents and has served as an important lead compound in antineoplastic drug discovery. Reported here is a short chemoenzymatic total synthesis of (-)-podophyllotoxin and related aryltetralin lignans. Vital to this approach is the use of an enzymatic oxidative C-C coupling reaction to construct the tetracyclic core of the natural product in a diastereoselective fashion. This strategy allows gram-scale access to (-)-deoxypodophyllotoxin and is readily adaptable to the preparation of related aryltetralin lignans.

A sterically encumbered photoredox catalyst enables the unified synthesis of the classical lignan family of natural products

Herein, we detail a unified synthetic approach to the classical lignan family of natural products that hinges on divergence from a common intermediate that was strategically identified from nature's biosynthetic blueprints.

Herein, we detail a unified synthetic approach to the classical lignan family of natural products that hinges on divergence from a common intermediate that was strategically identified from nature's biosynthetic blueprints. Efforts toward accessing the common intermediate through a convergent and modular approach resulted in the discovery of a sterically encumbered photoredox catalyst that can selectively generate carbonyl ylides from electron-rich epoxides. These can undergo concerted [3 + 2] dipolar cycloadditions to afford tetrahydrofurans, which were advanced (2–4 steps) to at least one representative natural product or natural product scaffold within all six subtypes in classical lignans. The application of those synthetic blueprints to the synthesis of heterolignans bearing unnatural functionality was demonstrated, which establishes the potential of this strategy to accelerate structure–activity-relationship studies of these natural product frameworks and their rich biological activity.

Oxidative Transformations of Lignans

Numerous oxidative transformations of lignan structures have been reported in the literature. In this paper we present an overview on the current findings in the field. The focus is put on transformations targeting a specific structure, a specific reaction, or an interconversion of the lignan skeleton. Oxidative transformations related to biosynthesis, antioxidant measurements, and total syntheses are mostly excluded. Non-metal mediated as well as metal mediated oxidations are reported, and mechanisms based on hydrogen abstractions, epoxidations, hydroxylations, and radical reactions are discussed for the transformation and interconversion of lignan structures. Enzymatic oxidations, photooxidation, and electrochemical oxidations are also briefly reported.

Applications of Friedel-Crafts reactions in total synthesis of natural products

Over the years, Friedel-Crafts (FC) reactions have been acknowledged as the most useful and powerful synthetic tools for the construction of a special kind of carbon-carbon bond involving an aromatic moiety. Its stoichiometric and, more recently, its catalytic procedures have extensively been studied. This reaction in recent years has frequently been used as a key step (steps) in the total synthesis of natural products and targeted complex bioactive molecules. In this review, we try to underscore the applications of intermolecular and intramolecular FC reactions in the total syntheses of natural products and complex molecules, exhibiting diverse biological properties.

Asymmetric total synthesis of (+)-ovafolinins A and B

(+)-Ovafolinins A and B are two homologous lignans containing unique polycyclic skeletons. Benefiting from a highly diastereoselective alkylation of (S)-Taniguchi lactone, a double Friedel-Crafts reaction, a global debenzylation and a Cu(OAc)₂-enabled benzylic oxidative cyclization, we present herein an efficient synthetic approach to (+)-ovafolinins A and B.

A silver-catalyzed three-component reaction via stabilized cation: synthesis of polysubstituted tetrahydronaphthols and tetrahydronaphthylamines

An efficient method for the synthesis of polysubstituted tetrahydronaphthols and tetrahydronaphthylamines via a stabilized cation intermediate has been developed.

Catalytic Enantioselective Synthesis of (-)-Podophyllotoxin

The first catalytic enantioselective total synthesis of (-)-podophyllotoxin is accomplished by a challenging organocatalytic cross-aldol Heck cyclization and distal stereocontrolled transfer hydrogenation in five steps from three aldehydes. Reversal of selectivity in hydrogenation led to the syntheses of other stereoisomers from the common precursor.

Recent Advances in the Chemistry and Biology of Podophyllotoxins

Podophyllotoxin and its related aryltetralin cyclolignans belong to a family of important products that exhibit various biological properties (e.g., cytotoxic, insecticidal, antifungal, antiviral, anti-inflammatory, neurotoxic, immunosuppressive, antirheumatic, antioxidative, antispasmogenic, and hypolipidemic activities). This Review provides a survey of podophyllotoxin and its analogues isolated from plants. In particular, recent developments in the elegant total chemical synthesis, structural modifications, biosynthesis, and biotransformation of podophyllotoxin and its analogues are summarized. Moreover, a deoxypodophyllotoxin-based chemosensor for selective detection of mercury ion is described. In addition to the most active podophyllotoxin derivatives in each series against human cancer cell lines and insect pests listed in the tables, the structure-activity relationships of podophyllotoxin derivatives as cytotoxic and insecticidal agents are also outlined. Future prospects and further developments in this area are covered at the end of the Review. We believe that this Review will provide necessary information for synthetic, medicinal, and pesticidal chemistry researchers who are interested in the chemistry and biology of podophyllotoxins.

Deconjugative alkylation/Heck reaction as a simple platform for dihydronaphthalene synthesis

A simple platform for carbocycle synthesis by Knoevenagel adduct deconjugative alkylation/“pairing” reaction is described.

Novel Podophyllotoxin Derivatives as Partial PPARγ Agonists and their Effects on Insulin Resistance and Type 2 Diabetes

Peroxisome proliferator-activated receptor γ (PPARγ) is recognized as a key regulator of insulin resistance. In this study, we searched for novel PPARγ agonists in a library of structurally diverse organic compounds and determined that podophyllotoxin exhibits partial agonist activity toward PPARγ. Eight novel podophyllotoxin-like derivatives were synthesized and assayed for toxicity and functional activity toward PPARγ to reduce the possible systemic toxic effects of podophyllotoxin and to maintain partial agonist activity toward PPARγ. Cell-based transactivation assays showed that compounds (E)-3-(hydroxy(3,4,5-trimethoxyphenyl)methyl)-4-(4(trifluoromethyl)styryl)dihydrofuran-2(3H)-one (3a) and (E)-4-(3-acetylstyryl)-3-(hydroxyl (3,4,5-trimethoxyphenyl)methyl)dihydrofuran-2(3H)-one (3f) exhibited partial agonist activity. An experiment using human hepatocarcinoma cells (HepG2) that were induced to become an insulin-resistant model showed that compounds 3a and 3f improved insulin sensitivity and glucose consumption. In addition, compounds 3a and 3f significantly improved hyperglycemia and insulin resistance in high-fat diet-fed streptozotocin (HFD-STZ)-induced type 2 diabetic rats at a dose of 15 mg/kg/day administered orally for 45 days, without significant weight gain. Cell toxicity testing also showed that compounds 3a and 3f exhibited weaker toxicity than pioglitazone. These findings suggested that compounds 3a and 3f improved insulin resistance in vivo and in vitro and that the compounds exhibited potential for the treatment of type 2 diabetes mellitus.

A Ruthenium/Phosphoramidite-Catalyzed Asymmetric Interrupted Metallo-ene Reaction

Allylic chlorides prepared from commercially available trans-1,4-dichloro-2-butene were converted to trans-disubstituted 5- and 6-membered ring systems with perfect diastereoselectivity and high enantioselectivity under chiral ruthenium catalysis. These products contain stereodefined secondary and tertiary alcohols that originate from the trapping of an alkylruthenium intermediate with adventitious water. Key to the success of this transformation was the development of a new BINOL-based phosphoramidite ligand containing bulky substitution at its 3- and 3'-positions. As a demonstration of product utility, diastereoselective Friedel-Crafts reactions were performed on the chiral benzylic alcohols in high yield and stereoselectivity.

The chemical synthesis of aryltetralin glycosides

Led by etoposide and teniposide, the synthesis of aryltetralin glycosides has been experiencing flourishing development in the past five decades. Herein, a review focusing on the total synthesis of aryltetralin glycosides is provided. The main body of this review is composed of two parts, one is the enantioselective synthesis of aryltetralin derivatives and the other one is the construction of key glycosidic linkages. In each part the contents are organised based on the different strategies or protocols applied in the original documents. The total synthesis of aryltetralin glycosides represents the developing direction of this field, and sooner or later will replace the currently applied semi-total synthesis method, using the aglycon residue acquired directly from natural sources. This account provides a comprehensive and deep insight into the field of aryltetralin glycoside synthesis for chemists who have the intention of committing themselves to the development of aryltetralin glycoside medicine.

Insight into 2α-Chloro-2'(2',6')-(Di)Halogenopicropodophyllotoxins Reacting with Carboxylic Acids Mediated by BF3·Et2O

Stereospecific nucleophilic substitution at the C-4α position of 2α-chloro-2'(2',6')-(di)halogenopicropodophyllotoxin derivatives with carboxylic acids mediated by BF3·Et2O was described. Interestingly, this stereoselective products were completely controlled by the reaction time. That is, if the reaction time was prolonged to 24.5-31 h, the resulting compounds were all transformed into the unusual C-ring aromatization products. Additionally, it demonstrated that BF3·Et2O and reaction temperature were the important factors for C-ring aromatization, and AlCl3 could be substituted for BF3·Et2O as a lewis acid for C-ring aromatization. Halogenation of E-ring of 2β-chloropodophyllotoxins with NCS or NBS also led to the same C-ring aromatization compounds. Especially compounds 5c, 6g and 7b exhibited insecticidal activity equal to that of toosendanin.

Iron-promoted C-C bond formation in the total synthesis of natural products and drugs

Iron salts are inexpensive and almost innocuous; they are thus the promoters of choice, even in stoichiometric amounts, for the formation of carbon-carbon bonds in the backbone of complex molecules. This review encompasses the key role of iron complexes in the total synthesis of some natural products or pharmacologically important compounds.

An SN1-type Reaction To Form the 1,2-Dioxepane Ring: Synthesis of 10,12-Peroxycalamenene

The synthesis of the sesquiterpene endoperoxide natural product 10,12-peroxycalamenene has been achieved. Featured transformations include an intramolecular Heck reaction to build the fused bicyclic core and a cobalt-catalyzed peroxidation to install the peroxide functional group. The final step involved an SN1-type ring closure catalyzed by DDQ to construct the 1,2-dioxepane ring.

The β-Silyl Effect on the Memory of Chirality in Friedel-Crafts Alkylation Using Chiral α-Aryl Alcohols

Iron salt-catalyzed Friedel-Crafts alkylation of chiral α-aryl alcohols with a trimethylsilyl group was found to proceed with retention of the configuration of the hydroxyl group as a leaving group. The memory of chirality of this system stems from the β-silyl effect of the trimethylsilyl group on the carbocation intermediate.

Biarylmethane and Fused Heterocyclic Arene Synthesis via in Situ Generated o- and/or p-Naphthoquinone Methides

o- and/or p-naphthoquinone methides (NQMs) can be selectively prepared by the ring opening of 1-(siloxymethyl)-1,4-epoxy-1,4-dihydronaphthalene derivatives based on a substituent effect at the 4 position of the substrates. The 4-alkyl- or silyl-substituted 1-(siloxymethyl)-1,4-epoxy-1,4-dihydronaphthalene was transformed to o-NQM (1-naphthoquinone-2-methide), which underwent Friedel-Crafts 1,4-addition of the α,β-unsaturated carbonyl moiety to provide the 2-benzyl-1-naphthol as the biarylmethane and [4 + 2]-cycloaddition with a dienophile to give the fused heterocyclic arene. Meanwhile, the 4-unsubstituted 1-(siloxymethyl)-1,4-epoxy-1,4-dihydronaphthalene could be converted to the corresponding 4-benzyl-1-naphthol by the Friedel-Crafts 1,6-addition of p-NQM (1-naphthoquinone-4-methide) generated by the site-selective ring opening of the 1,4-epoxy moiety. Furthermore, the 4-(siloxymethyl)-(1,4-bis(siloxymethyl))-1,4-epoxy-1,4-dihydronaphthalene was transformed into a 2,4-bisbenzyl-1-naphthol or pentacyclic derivative via both the o- and p-NQM intermediates.

Syntheses of all the stereoisomers of butanol type 1,7-seco-2,7'-cyclolignane

All the stereoisomers of butanol type 1,7-seco-2,7'-cyclolignane were stereoselectively synthesized by employing (S)- and (R)-Evans' auxiliaries to construct the stereochemistry. (+)- and (-)-Kadangustin J and their diastereomers were also prepared. The optical purity of the synthesized butanol type 1,7-seco-2,7'-cyclolignane was more than 99%ee.

Stereoselective synthesis of 2α-Chloropicropodophyllotoxins and insecticidal activity of their esters against oriental armyworm, Mythimna separata walker

As part of ongoing efforts to discover new natural-product-based insecticidal agents, in the present study, an efficient method for the stereoselective α-chlorination at the C-2 position of 2'(2',6')-(di)halogenopodophyllotoxin derivatives was first developed. Subsequently, a series of novel esters of 2α-chloro-2'(2',6')-(di)halogenopicropodophyllotoxin with modified C, D, and E rings of podophyllotoxin were smoothly obtained. Finally, all of the title compounds were tested against the pre-third-instar larvae of oriental armyworm (Mythimna separata Walker) at 1 mg/mL. It was found that besides their 2'-halogen-substituted E ring, the stereoselective α-chlorination at the C-2 position of 2'(2',6')-(di)halogenopodophyllotoxins was also related to the chlorination reagents. Especially 2α-chloro-4α-(benzoyl)oxy-2'-chloropicropodophyllotoxin (6e) and 2α-chloro-4α-(2-chlorophenylacyl)oxy-2'-bromopicropodophyllotoxin (8f) showed the most potent insecticidal activities, with final mortality rates of >60%. For 4α-(alkylacyl)oxy derivatives of 2α-chloro-2'(2',6')-(di)halogenopicropodophyllotoxin, the effect of the length of their side chain at the C-4 position of podophyllotoxin skeleton on the insecticidal activity was not very obvious. For 4α-(arylacyl)oxy derivatives of 2α-chloro-2'-chloro/bromopicropodophyllotoxin, an electronic effect of the substituents on their phenyl ring at the C-4 position of podophyllotoxin skeleton on the insecticidal activity was observed.

Stereocontrolled syntheses of tetralone- and naphthyl-type lignans by a one-pot oxidative [3,3] rearrangement/Friedel-Crafts arylation

The development of a stereoselective one-pot oxidative [3,3] sigmatropic rearrangement/Friedel-Crafts arylation that provides enantioenriched benzhydryl compounds is reported. The utility of this new transformation is demonstrated by the concise synthesis of several tetralone- and naphthyl-type lignan natural products, many of which display anti-malarial activity.