Synthesis of enamides related to the salicylate antitumor macrolides using copper-mediated vinylic substitution

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.

Total Synthesis of Lobatamides A and C

The total synthesis of lobatamides A (1 a) and C (1 c) via a common bislactone intermediate is reported. The allylic aryl moiety including a trisubstituted Z-olefin was constructed by hydroboration of a 1,1-disubstituted allene and subsequent Migita-Kosugi-Stille coupling. Although the seco acid proved to be highly unstable even in the presence of weak bases, Zhao macrolactonization under acidic conditions via the α-acyloxyenamide successfully provided the common bislactone intermediate. Hydrozirconation-iodination of the terminal alkyne and subsequent copper-mediated coupling with primary amides proceeded successfully in the presence of the sensitive bislactone framework. The developed synthetic route enables the late-stage installation of enamide side chains, which are crucial structures for V-ATPase inhibition.

Unlock the C-N Bond Amidation of Enaminones: Metal-Free Synthesis of Enamides by Water-Assisted Transamidation

The C-N bond transamidation of primary amides with N,N-dimethyl enaminones has been efficiently realized by heating in the presence of trifluoromethanesulfonic acid (TfOH). The method enables the practical synthesis of valuable enamides without the use of any metal reagent. In addition, this transamidation protocol can also be expanded to the reactions of sulfonamides, and the late-stage functionalization on sulfonamide drugs such as Celecoxib and Valdecoxib has been verified. Moreover, the participation of water in assisting the transamidation process has been identified by the isotope labeling experiments using D2O, disclosing a new possibility in designing catalytic tactic to other transamidation reactions.

Decarboxylative Enamide Synthesis from Carboxylic Acid and Alkenyl Isocyanate

Herein, we reported a protocol to access the enamide via employing carboxylic acid and alkenyl isocyanate as the precursors promoted by DMAP without involving any metal catalysts and dehydration reagents. This protocol is simple and practical and tolerates numerous functional groups. Considering the simplicity, the ready availability of both starting materials, and the significance of the enamides, we expect that this reaction will find broad application.

Different Lewis Acid Promotor-Steered Highly Regioselective Phosphorylation of Tertiary Enamides

Different Lewis acid promotor-steered highly regioselective phosphorylation of tertiary enamides with diverse H-phosphonates or H-phosphine oxides was developed. Under the catalysis of iron salt, the phosphonyl group was introduced into the α-position of tertiary enamides, affording various α-phosphorylated amides in high efficiency. On the other hand, the β-phosphorylated tertiary enamides were efficiently obtained as the products in the presence of manganese(III) acetylacetonate.

Synthesis of vinyl iodide chain-end polymers via organocatalyzed chain-end transformation

In the presence of alkynes (CH[triple bond, length as m-dash]C-R2), iodide chain-end polymers (Polymer-I) were successfully transformed to vinyl iodide chain-end polymers (polymer-CH[double bond, length as m-dash]CR2-I) in a single step via organocatalysis. This reaction is completely metal-free and easy to carry out without using special reagents or special conditions. The polymers encompassing polyacrylates and polymethacrylate, and additional functionalities (e.g., OH and CF3) were also incorporated into the R2 moiety. The obtained Polymer-CH[double bond, length as m-dash]CR2-I further served as a useful precursor for copper-catalyzed cross-coupling reactions with various thiols (R3-SH) to yield vinyl sulfide chain-end polymers (polymer-CH[double bond, length as m-dash]CR2-SR3) with various R3 moieties. Interestingly, under selected conditions, this organocatalysis also offered block-like copolymers containing a conjugated oligo-alkyne segment and a non-conjugated polyacrylate segment. Exploiting the unique structure, the block-like copolymer was used as an efficient dispersant of carbon nanotubes.

Convergent and Stereoselective Method for the Synthesis of Acyclic α-Chloroenamides

A direct, stereocontrolled synthesis of acyclic α-chloroenamides is presented. Our methodology showed good yields and substrate scope. Mechanistic insights are provided that account for the high levels of stereoselectivity reported. Subsequent synthetic manipulation of the α-chloroenamides provides direct entry to polyfunctionalized acyclic enamides, compounds of wide use in organic chemistry and the pharmaceutical industry.

Towards the total synthesis of chondrochloren A: synthesis of the (Z)-enamide fragment

The stereoselective synthesis of the (Z)-enamide fragment of chondrochloren (1) is described. A Buchwald-type coupling between amide 3 and (Z)-bromide 4 was used to generate the required fragment. The employed amide 3 comprising three chiral centers was obtained through a seven-step sequence starting from ᴅ-ribonic acid-1,4-lactone. The (Z)-vinyl bromide 4 is accessible in four steps from 4-hydroxybenzaldehyde. The pivotal cross coupling between both fragments was achieved after extensive experimentation with copper(I) iodide, K₂CO₃ and N,N′-dimethylethane-1,2-diamine.

A Highly Regio- and Stereoselective Syntheses of α-Halo Enamides, Vinyl Thioethers, and Vinyl Ethers with Aqueous Hydrogen Halide in Two-Phase Systems

A metal-free regio- and stereoselective method is achieved for the preparation of ( E)-configured α-halo enamides, vinyl thioethers, and vinyl ethers using aqueous HX (X = F, Cl, Br, I), which features high functional group compatibility and regio- and stereoselectivity, mild conditions, high efficiency, and rapid transformation. Additionally, the isomers could be yielded readily from the ( E)-configured α-halo enamides via photocatalysis or under Sonogashira coupling conditions.

Cp*Co(iii)-catalyzed amidation of olefinic and aryl C-H bonds: highly selective synthesis of enamides and pyrimidones

A highly efficient and selective synthesis of enamides via C-H amidation of N-methoxy acrylamides with dioxazolones is realized under [Cp*CoIII] catalysis. The resulting enamide can further selectively cyclize to form pyrimidones, which can also act as a directing group for a second C-H amidation. All these three classes of products were selectively delivered under controlled conditions.

Total Synthesis of Kanamienamide

Kanamienamide is a novel enol ether containing enamide with a single digit micromolar inhibitory activity against cancer cell lines. An efficient and convergent total synthesis of kanamienamide has been developed for the first time, which features a Cu-mediated amide coupling with vinyl iodide at the late stage. Other key transformations include Evans asymmetric alkylation, CBS asymmetric reduction, ring-closing metathesis reaction, and Stork-Zhao-Wittig olefination. This strategy is amenable for facile analogue preparation and SAR studies.

Isomerization of N-Allyl Amides To Form Geometrically Defined Di-, Tri-, and Tetrasubstituted Enamides

Enamides represent bioactive pharmacophores in various natural products, and have become increasingly common reagents for asymmetric incorporation of nitrogen functionality. Yet the synthesis of the requisite geometrically defined enamides remains problematic, especially for highly substituted and Z-enamides. Herein we wish to report a general atom economic method for the isomerization of a broad range of N-allyl amides to form Z-di-, tri-, and tetrasubstituted enamides with exceptional geometric selectivity. This report represents the first examples of a catalytic isomerization of N-allyl amides to form nonpropenyl disubstituted, tri- and tetrasubstituted enamides with excellent geometric control. Applications of these geometrically defined enamides toward the synthesis of cis vicinal amino alcohols and tetrasubstituted α-borylamido complexes are discussed.

Synthesis and biological evaluation of (-)-kunstleramide and its derivatives

Stereoselective total synthesis of (-)-kunstleramide, a cytotoxic dienamide from the bark of Beilschmiedia kunstleri gamble, has been accomplished by using Keck's asymmetric allylation and Trost isomerization as key reactions. Application of the developed strategy for the synthesis of a series of amide analogues (8-22) was also reported. Furthermore, the synthesized compounds were evaluated for their in vitro anti-proliferative activities against human epithelial lung carcinoma (A549), human epithelial cervical cancer (HeLa), human breast adenocarcinoma (MCF7) and human neuroblastoma (IMR32) cell lines using the SRB assay. All the compounds show moderate anti-proliferative activity against all cell lines. Some of the piperazine derivatives (17-22) strongly inhibit the growth of breast cancer cells with IC50 values of 8-20 μM.

Synthesis of polyfunctional secondary amines by the addition of functionalized zinc reagents to nitrosoarenes

Addition of functionalized aryl, heteroaryl or adamantyl zinc reagents to various nitroso-arenes in the presence of magnesium salts and LiCl in THF produces after a reductive work-up with FeCl2 and NaBH4 in ethanol the corresponding polyfunctional secondary amines in high yields.

Iridium-Catalyzed Enantioselective Hydroalkynylation of Enamides for the Synthesis of Homopropargyl Amides

Reported is an iridium-catalyzed asymmetric hydroalkynylation of enamides with terminal alkynes. The reaction occurs regioselectively at the β-position of an enamide to produce homopropargyl amides. Good to high enantioselectivity was observed with an iridium complex ligated by a chiral bis(phosphine) ligand. This method provides a straightforward route to synthesize chiral homopropargyl amides with a stereocenter β to the amide.

Studies on the synthesis of peptides containing dehydrovaline and dehydroisoleucine based on copper-mediated enamide formation

The preparation of peptide fragments containing dehydrovaline and dehydroisoleucine moieties present in the antibiotic myxovalargin is reported. Peptide formation is based on a copper-mediated C–N cross-coupling protocol between an acyl amide and a peptidic vinyl iodide. The presence of a neighboring arginine in the vinyl iodide posed a challenge with respect to the choice of the protecting group and the reaction conditions. It was found that ornithine – a suitable precursor – is better suited than arginine for achieving good yields for the C–N cross-coupling reaction. The optimized conditions were utilized for the synthesis of peptides 32, 33, 39 and 40 containing a neighboring ornithine as well as for the tripeptide 44 containing dehydroisoleucine with the correct stereochemistry.

Palladium-catalyzed cross-coupling of enamides with sterically hindered α-bromocarbonyls

Palladium-catalyzed intermolecular alkylation of enamides with α-bromo carbonyls was developed. Under mild reaction conditions, various cyclic and acyclic enamides reacted well with α-bromo carbonyls to afford the corresponding multi-substituted alkene products in good yields. The coupling products could be converted into very useful γ-amino acid, δ-amino alcohol, 1,4-dicarbonyl and γ-lactam derivatives.

Exploring tertiary enamides as versatile synthons in organic synthesis

Tertiary enamides have long been thought of as stable and marginally valuable enamine variants in synthesis. This notion has been challenged, however, in recent years. Enabling the regulation of the cross-conjugation system of the tertiary enamides has been successfully shown to enhance delocalization of the nitrogen lone-pair electrons into a carbon-carbon double, thereby reinvigorating the enaminic reactivity of the tertiary enamides. In this article, I summarize the recent advances in the exploration of the nucleophilic reactions of tertiary enamides and their applications in the synthesis of natural products and heterocyclic compounds of biological and pharmaceutical relevance, with a primary focus on our own work.

Cu-mediated enamide formation in the total synthesis of complex peptide natural products

Cu-mediated C(sp(2))-N bond formation has received intense interest recently, and has been applied to the total synthesis of a wide variety of structurally complex natural products. This review covers the synthetic assembly of peptide natural products in which Cu-mediated enamide formation is the key transformation. The total syntheses of cyclopeptide alkaloids, pacidamycin D, and yaku'amide A exemplify the versatility of the Cu-catalyzed cross-coupling reaction in comparison to other synthetic methods.