Total Synthesis of Natural Products Containing Enamine or Enol Ether Derivatives

Recent advances in site-selective transformations of β-enaminones via transition-metal-catalyzed C-H functionalization/annulation

β-Enaminone transformation strategies are widely employed in the synthesis of numerous biologically active drugs and natural products, highlighting their significance in medicinal chemistry. In recent years, various strategies have been developed for synthesizing several five- and six-membered heterocycles, as well as substituted polyaromatic scaffolds, which serve as crucial synthons in drug development, from β-enaminones. Among these approaches, site-selective transformations of β-enaminones via C-H activation and annulation have been particularly well explored. This review summarizes the most recent literature (over the past eight years) on β-enaminone transformations for developing bioactive scaffolds through site-selective C-H bond functionalization and annulation.

Amide-derived enols in enol-Ugi reactions: expanding horizons for peptidomimetic scaffold synthesis

A highly efficient enol-Ugi reaction of β,β-diketoamides has been developed using a novel non-heterocyclic amide-stabilised enol. This approach enables a broad reaction scope, affording β-enaminoamide peptidomimetics with constrained conformations due to CH-π interaction and C(sp3)H⋯O hydrogen bonding. Notably, the use of a five-membered cyclic enol is crucial for achieving stable products in excellent yields. This work highlights the potential of the enol-Ugi reaction for constructing diverse peptidomimetic scaffolds.

Unlocking nature's antioxidants: a novel method for synthesising plasmalogens

Plasmalogens are glycerophospholipids distinguished by their O-(Z)-vinyl ether at the sn-1 position. These lipids are implicated in several disease states requiring analytical, diagnostic and therapeutic interventions, which demand synthetic availability for a variety of structural types. By deploying the new O-protecting group 1,4-dimethoxynaphthyl-2-methyl ('DIMON') and a new stereospecific method for accessing Z-vinyl ethers, a reproducible, versatile synthetic route to plasmalogens [plasmenyl phosphocholines] has been developed. A key intermediate is (S,Z)-1-((1,4-dimethoxynaphthalen-2-yl)methoxy)-3-(hexadec-1-en-1-yloxy)propan-2-ol, which in principle, permits plasmalogen synthesis 'à la carte' at scale. The methodology compares favourably with all previous synthetic routes by virtue of the very high configurational (>99% Z) and optical purity (>99% ee), including the ability to incorporate polyunsaturated fatty acyl chains (e.g. all Z docosahexaenoic acid) reliably at the sn-2 position.

Base-mediated synthesis of aryl enol ethers from α-aryl allylic alcohols and arylsulfonium salts

A concise synthesis of aryl enol ethers from allylic alcohols and arylsulfonium salts by simply using an inorganic base as a mediator is described. The reaction enabled the facile conversion of various α-aryl allylic alcohols into the corresponding aryl enol ethers in good yields with excellent selectivity. The results demonstrated that both symmetric triarylsulfonium triflate and 10-methyl-5-aryl-5,10-dihydrophenothiazin-5-ium salts were effective arylation reagents for the base-initiated selective O-arylation and isomerization of α-aryl allylic alcohols. This reaction represents the first use of arylsulfonium salts as arylation reagents to access aryl enol ethers directly from allylic alcohols.

Nickel-Catalyzed Alkene Isomerization to Access Bench-Stable Enamines and Their [3 + 2] Annulation

Enamines are difficult to prepare on the bench due to their instability, which results in side reactions, decompositions, poor yields, etc. Herein, we developed a simple and effective method for making bench-stable enamines using a very low amount of nickel catalyst loading. The deuterium exchange, competitive reaction, and radical clock experiment have all been found to favor the ionic mechanism of this alkene isomerization. Scale-up and [3 + 2] annulation reaction of enamines with activated cyclopropane to deliver cyclopentane derivatives have shown the value of this method in organic synthesis.

Synthesis and utility of N-boryl and N-silyl enamines derived from the hydroboration and hydrosilylation of N-heteroarenes and N-conjugated compounds

Catalytic hydroboration and hydrosilylation have emerged as promising strategies for the reduction of unsaturated hydrocarbons and carbonyl compounds, as well as for the dearomatization of N-heteroarenes. Various catalysts have been employed in these processes to achieve the formation of reduced products via distinct reaction pathways and intermediates. Among these intermediates, N-silyl enamines and N-boryl enamines, which are derived from hydrosilylation and hydroboration, are commonly underestimated in this reduction process. Because these versatile intermediates have recently been utilized in situ as nucleophilic reagents or dipolarophiles for the synthesis of diverse molecules, an expeditious review of the synthesis and utilization of N-silyl and N-boryl enamines is crucial. In this review, we comprehensively discuss a wide range of hydrosilylation and hydroboration catalysts used for the synthesis of N-silyl and N-boryl enamines. These catalysts include main-group metals (e.g., Mg and Zn), transition metals (e.g., Rh, Ru, and Ir), earth-abundant metals (e.g., Fe, Co, and Ni), and non-metal catalysts (including P, B, and organocatalysts). Furthermore, we highlight recent research efforts that have leveraged these versatile intermediates for the synthesis of intriguing molecules, offering insights into future directions for these invaluable building blocks.

Harnessing the Peterson Reaction for the Stereospecific Synthesis of Z-Vinyl Ethers

Vinyl ethers are valuable synthetic intermediates which are also found as natural products, including aflatoxins, rifamycins and plasmalogens. The latter are ubiquitous phospholipids in human cells and contain a vinyl ether moiety with specifically Z configuration. Although numerous methods are available for synthesis of vinyl ethers, there is a lack of methods for obtaining Z isomers of molecules of the type RCH=CHOR' that are applicable to plasmalogens. A variant of the Peterson reaction is described that generates such molecules with very high stereoselectivity (Z/E ratio: 99 : 1). (R,R)/(S,S)-1-alkoxy-2-hydroxyalkylsilanes were synthesized from 1-trimethylsilylalkynes by a sequence of reduction with di-isobutylaluminium hydride to a (Z)-1-trimethylsilylalkene, epoxidation of the alkene to a 2-trimethylsilyl-3-substituted epoxide and regioselective, boron-trifluoride catalyzed ring-opening of the epoxide by reaction with an alcohol. Conversion of the (R,R)/(S,S)-1-alkoxy-2-hydroxyalkylsilanes to vinyl ethers (RCH=CHOR') was achieved under basic conditions as in a standard Peterson reaction. However, near exclusive formation of a Z vinyl ether was only achieved when the reaction was performed using potassium hydride in the non-polar solvent α,α,α-trifluorotoluene, more polar solvents giving increasing amounts of the E isomer. The sequence described embraces a variety of substituents and precursors, proceeds in overall high yield and is readily scalable.

Titanium-catalyzed highly stereoselective anti-Markovnikov intermolecular hydroalkoxylation of alkynes to prepare Z-enol ethers

Enol ethers are essential synthetic frameworks and widely applied in organic synthesis; however, high regio- and stereo-selective access to enol ethers remains challenging. Herein, we report a titanium-catalyzed stereospecific anti-Markovnikov hydroalkoxylation reaction of alkynes for the synthesis of Z-enol ethers with excellent functional group tolerance and yields. Mechanistic studies showed that the titanium coordinates with the alkyne and then an oxygen anion attacks the π-bond of the alkyne from the backside to provide a trans-oxygen metallation intermediate, which accounts for the high Z-stereoselectivity. Furthermore, Z-enol ethers could be applied as a kind of synthon for late-stage transformations and gram-scale synthesis, which demonstrates their potential value in organic synthesis.

Visible-Light-Irradiated Multicomponent Reactions of Aliphatic Amines, Propiolate Acid Esters, and CF3 SO2 Na for Accessing β-CF3 Enamines

A novel one-pot two-step multicomponent reaction has been achieved for the preparation of β-CF3 enamines by using different aliphatic amines, propiolates, and CF3 SO2 Na as starting material. In this protocol, various aliphatic amines including primary amines, cyclic or acyclic secondary amines were demonstrated to be good coupling partners, and different β-CF3 enamines were obtained in moderate to good yields. Among them, the primary aliphatic amines only gave pure (E)-β-CF3 enamines as products. The synthetic utility of the MCRs strategy was further demonstrated by mild conditions, gram-scale synthesis and natural sunlight-induced protocol. Preliminary mechanistic studies suggest that this trifluoromethylation of C(sp2 )-H involves radical process.

Catalyst-Free Trans-Selective Oxyiodination and Oxychlorination of Alkynes Employing N-X (Halogen) Reagents

β-halogenated enol esters and ethers are versatile building blocks in organic synthesis, which has attracted increasing attention. In this study, we report the facile trans-oxyiodination and oxychlorination of alkynes, leading to the direct construction of versatile halogenated enol esters and ethers. This transformation features an easy operation, optimal atomic economy, a strong functional group tolerance, broad substrate scope, and excellent trans-selectivity. Employing highly electrophilic bifunctional N-X (halogen) reagents was the key to achieving broad reaction generality. To our knowledge, this transformation represents the first oxyhalogenation system employing N-X (halogen) reagents as both oxylation and halogenation sources.

Metal-Free Stereoconvergent C-H Borylation of Enamides

Enamides, functional derivatives of enamines, play a significant role as synthetic targets. However, the stereoselective synthesis of these molecules has posed a longstanding challenge in organic chemistry, particularly for acyclic enamides that are less thermodynamically stable. In this study, we present a general strategy for constructing β-borylenamides by C-H borylation, which provides a versatile platform for generating the stereodefined enamides. Our approach involves the utilization of metalloid borenium cation, generated through the reaction of BBr3 and enamides in the presence of two different additives, avoiding any exogenous catalyst. Importantly, the stereoconvergent nature of this methodology allows for the use of starting materials with mixed E/Z configurations, thus highlighting the unique advantage of this chemistry. Mechanistic investigations have shed light on the pivotal roles played by the two additives, the reactive boron species, and the phenomenon of stereoconvergence.

Gold-Catalyzed Divergent N/O-Vinylations of trans-2-Butene-1,4-amino Alcohols with Alkynes and the Cascade Rearrangements/Cyclizations to Dihydropyrroles and Dihydrofurans

Although the transition-metal-catalyzed vinylations of amines and alcohols via the additions to alkynes have been well developed, the selective vinylations of amino alcohols have been merely investigated. Herein, we report the gold-catalyzed divergent additions of trans-2-butene-1,4-amino alcohols' N-H and O-H groups to alkynes. The allyl enamine and allyl vinyl ether adducts then underwent a cascade (Aza-) Claisen rearrangement/cyclization sequence, furnishing the functionalized dihydropyrrole and dihydrofuran products.

Microwave-Assisted Synthesis of E-Aldimines, N-Heterocycles, and H2 by Dehydrogenative Coupling of Benzyl Alcohol and Aniline Derivatives Using CoCl2 as a Catalyst

The acceptorless dehydrogenative coupling (ADC) between alcohols and amines to produce imines has been achieved mostly by employing precious-metal-based complexes or complexes of earth-abundant metal ions with sensitive and complicated ligand systems as catalysts mostly under harsh reaction conditions. Methodologies using readily available earth-abundant metal salts as catalysts without the requirement of ligand, oxidant, or any external additives are not explored. We report an unprecedented microwave-assisted CoCl₂-catalyzed acceptorless dehydrogenative coupling of benzyl alcohol and amine for the synthesis of E-aldimines, N-heterocycles, and H₂ under mild condition, without any complicated exogenous ligand template, oxidant, or other additives. This environmentally benign methodology exhibits broad substrate scope (43 including 7 new products) with fair functional-group tolerance on the aniline ring. Detection of metal-associated intermediate by gas chromatography (GC) and HRMS, H₂ detection by GC, and kinetic isotope effect reveal the mechanism of this CoCl_2-catalyzed reaction to be via ADC. Furthermore, kinetic experiments and Hammett analysis with variation in the nature of substituents over the aniline ring reveal the insight into the reaction mechanism with different substituents.

Development of a drug discovery approach from microbes with a special focus on isolation sources and taxonomy

After the successful discoveries of numerous antibiotics from microorganisms, frequent reisolation of known compounds becomes an obstacle in further development of new drugs from natural products. Exploration of biological sources that can provide novel scaffolds is thus an urgent matter in drug lead screening. As an alternative source to the conventionally used soil microorganisms, we selected endophytic actinomycetes, marine actinomycetes, and actinomycetes in tropical areas for investigation and found an array of new bioactive compounds. Furthermore, based on the analysis of the distribution pattern of biosynthetic gene clusters in bacteria together with available genomic data, we speculated that biosynthetic gene clusters for secondary metabolites are specific to each genus. Based on this assumption, we investigated actinomycetal and marine bacterial genera from which no compounds have been reported, which led to the discovery of a variety of skeletally novel bioactive compounds. These findings suggest that consideration of environmental factor and taxonomic position is critically effective in the selection of potential strains producing structurally unique compounds.

Anticancer Small-Molecule Agents Targeting Eukaryotic Elongation Factor 1A: State of the Art

Eukaryotic elongation factor 1A (eEF1A) canonically delivers amino acyl tRNA to the ribosomal A site during the elongation stage of protein biosynthesis. Yet paradoxically, the oncogenic nature of this instrumental protein has long been recognized. Consistently, eEF1A has proven to be targeted by a wide assortment of small molecules with excellent anticancer activity, among which plitidepsin has been granted approval for the treatment of multiple myeloma. Meanwhile, metarrestin is currently under clinical development for metastatic cancers. Bearing these exciting advances in mind, it would be desirable to present a systematic up-to-date account of the title topic, which, to the best of our knowledge, has thus far been unavailable in the literature. The present review summarizes recent advances in eEF1A-targeting anticancer agents, both naturally occurring and synthetically crafted, with regard to their discovery or design, target identification, structure–activity relationship, and mode of action. Their structural diversity and differential eEF1A-targeting mechanisms warrant continuing research in pursuit of curing eEF1A-driven malignancy.

Gold-Catalyzed Intermolecular Alkyne Insertion into the N-S Bond in Sulfenamides

Gold-catalyzed reactions between sulfenamides and terminal alkynes proceeded via cis-insertion of alkynes into the N-S bond in sulfenamides, affording the corresponding β-sulfenylenamines in yields up to 90%. Mechanistic studies revealed that the reactions proceeded via nucleophilic attack of the sulfenamide nitrogen atom on the π-activated alkyne, followed by tosylate-assisted intermolecular transfer of the sulfenyl group.

Visible-Light-Induced α-C(sp3)-H Phosphinylation of Unactivated Ethers under Photocatalyst- and Additive-Free Conditions

A photocatalyst- and additive-free visible-light-induced α-C(sp³)-H phosphinylation of unactivated ethers involving a C-O bond cleavage with molecular oxygen as the sole oxidant at room temperature has been achieved. This method provides a sustainable access to α-hydroxyphosphine oxides in up to 88% yield with good functional group compatibility under mild and neutral conditions (34 examples). Moreover, the subsequent two-step conversion of the resulting dihydroxy diarylphosphine oxides afforded α-phosphinylated cyclic ethers in good overall yields (10 examples).

Copper-Catalyzed Regioselective Olefination and Trifluoromethylation of Carboxylic Acids To Give (Z)-Trifluoromethyl Enol Esters

Herein, we describe a method to produce (Z)-trifluoromethyl enol esters via the olefination and trifluoromethylation of carboxylic acids with TMSCF₃. This synthetic method uses inexpensive and easy-to-handle TMSCF₃. It employs a commercially available CuCl catalyst to transform a broad range of carboxylic acids into versatile (Z)-trifluoromethyl enol esters with good regio- and stereoselectivity. This protocol allows the concise synthesis of highly functionalized (Z)-trifluoromethyl enol esters directly from carboxylic acids.

Semi-Synthesis of Chloroxaloterpin A and B and Their Antifungal Activity against Botrytis cinerea

Chloroxaloterpin A (1) and B (2) are two diterpenoids with potent inhibitory activities against spore germination of Botrytis cinerea, which were identified from Streptomyces sp. SN194 as minor products previously. In order to overcome the poor yields, 1 and 2 were synthesized using viguiepinol (3), the major metabolite of Streptomyces sp. SN194, as the precursor compound. Scanning electron microscope and transmission electron microscope observations revealed that after treating B. cinerea spores with 1 and 2, spores were obviously aberrant, the cytoplasm appeared withdrawn, and plasma membranes were blurred. Propidium iodide fluorescence assay indicated that 1 and 2 damaged plasma membranes of B. cinerea spores. In vivo assays showed that compounds 1 and 2 could effectively inhibit the sporulation of B. cinerea on tomato fruits, with sporulation inhibitory rates reaching 98.8% at 100 μg/mL. These findings provide new insights into chloroxaloterpin A and B and demonstrate their potential as lead candidates for novel fungicides.

Enantioselective Organocopper-Catalyzed Hetero Diels-Alder Reaction through in Situ Oxidation of Ethers into Enol Ethers

We disclose a catalytic method for the enantio- and diastereoselective union of alkyl ethers and heterodienes. We demonstrate that a chiral Cu-BOX complex catalyzes the efficient oxidation of ethers into enol ethers in the presence of trityl acetate. Then, the organocopper promotes stereoselective hetero Diels-Alder reaction between the in situ generated enol ethers and β,γ-unsaturated ketoesters, allowing for rapid access to an array of dihydropyran derivatives possessing three vicinal stereogenic centers.

Divergent cyanoalkylation/cyanoalkylsulfonylation of enamides under organophotoredox catalytic conditions

An organophotoredox catalyzed divergent cyanoalkylation/cyanoalkylsulfonylation of enamides is described.

Stereoselective Cyclopropanation of Enamides via C―C Bond Cleavage of Cyclopropenes

This work describes a straightforward protocol for the stereoselective synthesis of vinylcyclopropylamides in high E/Z and syn/anti ratios by cyclopropanation of N-tosyl substituted enamides with cyclopropenes in the presence of...

Ru(II)-Catalyzed Difunctional Pyridyloxy-Directed Regio- and Stereospecific Addition of Carboxylic Acids to Internal Alkynes

A highly efficient Ru(II)-catalyzed regio- and stereospecific hydro-oxycarbonylation of unsymmetrical internal alkynes bearing a difunctional 2-pyridyloxy directing group with carboxylic acids has been developed, which provides allylic (Z)-enol esters in good to excellent yields with a broad substrate scope under mild conditions. The difunctional directing group can be diversely derivatized, particularly undergoing allylic substitution with various nucleophiles to afford β-functionalized (Z)-enol esters without directing groups.

Solvent-controlled two-step one-pot syntheses of α-X (X = Br or Cl) enamino ketones/esters and 3-(2,5-dioxopyrrolidin-1-yl)acrylate by using terminal carbonyl alkynes

A new two-step one-pot aminobromination/chlorination of carbonyl alkynes has been achieved via a Michael addition of aliphatic secondary amines and subsequent β-bromination/chlorination of the obtained enamines to afford various α-X (X = Br or Cl) enamino ketones/esters in moderate to good yields. A solvent-controllable protocol has been developed to produce versatile 3-(2,5-dioxopyrrolidin-1-yl)acrylates in moderate yields by using toluene as the solvent and chain alkyl propiolates as alkynyl substrates.

Total Synthesis and Structure Revision of Boholamide A

The 15-membered cyclic depsipeptide boholamide A and an epimer were prepared by total synthesis for the first time, thus leading to a revision of C6 stereochemistry in the originally proposed structure of natural boholamide A. This convergent route features achievement of a macro-lactamization step in a gram scale. The revised boholamide A was sythesized with 16 linear steps in 5.46% overall yield. This work facilitates the investigations of boholamide A as a potential hypoxia-selective anticancer agent.

Iron catalyzed β-C(sp2)–H alkylation of enamides

An attractive and cheap alternative approach was developed for the β-C(sp2)–H (fluoro)alkylation of a range of cyclic and acyclic non-aromatic enamides using either FeCl2 as a catalyst or a stoichiometric amount of nontoxic iron powder.