α-Arylation of Silyl Enol Ethers via Rhodium(III)-Catalyzed C–H Functionalization

Cu-Catalyzed [3+1+1] Cascade Cyclization of O-Acyl Oximes with Sulfur and Silyl Enol Ethers: Rapid Access to Naphthothiazoles

A three-component cyclization reaction of O-acyl oximes, silyl enol ethers and elemental sulfur has been developed, in which silyl enol ether acts as a C1 synthon to participate in cyclization reaction and build a series of 2-aroylnaphthothiazoles and 2-aroylbenzothienothiazoles. The preliminary exploration of the reaction mechanism indicated that this transformation probably proceeded through a radical process, involving S3⋅- as a key intermediate, enabling subsequent nucleophilic substitution with O-acyl oximes to afford iminosulfur radical, which undergoes 1,3-H shift to yield sulfur-centered radical intermediate. And then this intermediate undergoes radical addition with silyl enol ether, leading to the formation of the titled products through intramolecular cyclization and oxidation. Moreover, the products obtained exhibit favorable fluorescence properties, which indicates their potential application as functional materials.

Rhodium-Catalyzed C-H Arylation of Indoles with Arylsilanes at Room Temperature

The Cp*Rh-catalyzed C-H arylation of indoles with arylsilanes is developed. This C-H activation transformation allows for the Rh-catalyzed indole C2 arylation to overcome the limitations of requiring strong directing group assistance and high-temperature conditions, achieving a room-temperature transformation driven by a weak directing group. Cp*Rh/MeOH catalytic media are considered a key factor enabling this transformation to occur under mild conditions, and experimental studies and theoretical calculations were performed to rationalize the reaction mechanisms and the influence of methanol as a solvent in promoting the reaction.

Access to Carbonyl Azides via Iodine(III)-Mediated Cross-Coupling

Herein, we present a prominent metal-free C-N cross-coupling platform that enables access to carbamoyl- and ketoazides from isocyanides or silyl enol ethers and trimethylsilyl azide (TMSN3) with an aid of iodine(III) promoter. This offers a rapid route to a diverse set of synthetically valuable azide decorated fragments with excellent substrate scope and good to excellent yields. The disclosed platform exemplifies the use of TMSN3 for incorporation of the azide fragment without the loss of N2.

Rhodium(III)-Catalyzed Oxidative Cross-Coupling of N-Pyrimidylindoles with Cyclic β-Keto Esters for Accessing All-Carbon Quaternary Centers

Rh(III)-catalyzed direct oxidative C-H/C-H cross-coupling between N-pyrimidylindoles and β-ketoesters is presented. Easily available β-ketoesters are used as an alkylating agent for the facile construction of all-carbon quaternary centers under mild conditions. The ester group in the product can undergo decarboxylation or decarboxylative amination.

DFT Study on the Mechanisms and Selectivities in Rh (III)-Catalyzed [5 + 1] Annulation of 2-Alkenylanilides and 2-Alkylphenols with Allenyl Acetates

The mechanisms and regio-, chemo-, and stereoselectivity were theoretically investigated in the Rh(III)-catalyzed [5 + 1] annulation of 2-alkenylanilides and 2-alkylphenols with allenyl acetates. Two different reactants, 2-alkenylanilides and 2-alkylphenols, were selected as model systems in the density functional theory calculations. The obtained theoretical results show that both these reactants exhibit similar steps, namely, (1) N-H/O-H deprotonation and C-H activation, (2) allenyl acetate migratory insertion, (3) β-oxygen elimination, (4) intramolecular nucleophilic addition of the nitrogen/oxygen-rhodium bond resulting in [5 + 1]-annulation, and (5) protonation with the formation of the desired product and regeneration of the Rh(III) catalyst. The theoretical evidence suggests that the selectivity is determined at the step of allenyl acetate's migratory insertion. Moreover, the regioselectivity is driven by electronic effects, while the interaction energies (C-H···π and C-H···O interactions) play a more imperative role in controlling the stereoselectivity. The obtained theoretical results not only well rationalize the experimental observations but also provide important mechanistic insights for related types of [5 + 1]-annulation reactions.

Exploring the synthetic potential of epoxide ring opening reactions toward the synthesis of alkaloids and terpenoids: a review

Epoxides are oxygen containing heterocycles which are significantly employed as crucial intermediates in various organic transformations. They are considered highly reactive three-membered heterocycles due to ring strain and they undergo epoxide ring opening reactions with diverse range of nucleophiles. Epoxide ring-opening reactions have gained prominence as flexible and effective means to obtain various functionalized molecules. These reactions have garnered substantial attention in organic synthesis, driven by the need to comprehend the synthesis of biologically and structurally important organic compounds. They have also found applications in the synthesis of complex natural products. In this review article, we have summarized the implementation of epoxide ring opening reactions in the synthesis of alkaloids and terpenoids based natural products reported within the last decade (2014-2023).

Synthesis of chiral functional polyhydroxylated arenes via Mukaiyama aldol reaction from Perlin aldehydes

Functionalized aryl polyhydroxylated compounds could be of great synthetic value for natural product synthesis. However, the synthesis of such compounds usually requires multi-step synthesis or the usage of sensitive reagents. We present here a practically simple route for the synthesis of such functionalized arylpolyols from glycal derived α,β-unsaturated 2,3-dideoxy aldehyde as well as α,β-saturated 2,3-dideoxy aldehyde (Perlin aldehydes) via Mukaiyama cross aldol condensation in the presence of silyl enol ether and TiCl4. It was observed that the nature of the electronic substitution of the silyl enol ether does not play any role in the yield of the desired products. Further functionalization of the products has also been shown.

Cp*Co(III)-Catalyzed Regioselective [4 + 2]-Annulation of N-Chlorobenzamides with Vinyl Acetate/Vinyl Ketones

An efficient and straightforward strategy for the synthesis of isoquinolones through [4 + 2]-annulation of N-chlorobenzamides with vinyl acetate in the presence of CoCp*(III) catalyst in a regioselective manner is described. Furthermore, the annulation reaction was diversified by using vinyl ketones. By utilizing this strategy, biologically valuable isoquinolone derivatives were prepared in good yields. Subsequently, isoquinolone derivatives were further transformed into 1-chloroisoquinolines in the presence of POCl₃. Furthermore, mechanistic investigations such as deuterium labeling study and competition experiment were performed to support the proposed reaction mechanism.

Synthetic Access to α-Oxoketene Aminals by the Nucleophilic Addition of Enol Silane-Derived Palladium(II) Enolates to Carbodiimides

Synthetically important α-oxoketene aminal intermediates can now be accessed from readily available and inexpensive carbodiimides as starting materials via the nucleophilic addition of palladium enolates derived from enol silane precursors. This operationally simple method features mild reaction conditions, including open air atmosphere, ligand-free metal catalysis, broad substrate scope, and multi-gram scalability. Select synthetic applications that take advantage of the enamine character of α-oxoketene aminals and involve C-nucleophilic additions to electrophilic systems, including an α,β-unsaturated ester, an azo dicarboxylate, an aralkyl halide, and an aldehyde, are demonstrated.

Synthesis of Isoquinolone, 1,2-Benzothiazine, and Naphtho[1',2':4,5]imidazo[1,2-a]pyridine Derivatives via Rhodium(III)-Catalyzed (4 + 2) Annulation

In this study, we report a novel and efficient synthetic method to construct isoquinolone scaffold via the Rh(III)-catalyzed (4 + 2) annulation of benzamide with an unreported coupling reagent methyl 2-chloroacrylate. Accordingly, other valuable 1,2-benzothiazine and naphtho[1',2':4,5]imidazo[1,2-a]pyridine derivatives are also obtained through a similar synthetic protocol. Thus, our developed method is highlighted by high yield and reaction versatility.

Silver-Catalyzed Controlled Intermolecular Cross-Coupling of Silyl Enol Ethers: Scalable Access to 1,4-Diketones

A protocol for silver-catalyzed controlled intermolecular cross-coupling of silyl enolates is disclosed. The protocol displays good functional group tolerance and allows efficient preparation of a series of synthetically useful 1,4-diketones. Preliminary mechanistic investigations suggest that the reaction proceeds through a one-electron process involving free radical species in which PhBr acts as the oxidant.

Recent advances on synthesis and biological activities of C-17 aza-heterocycle derived steroids

Steroids modification for improving their biological activities is one of the most efficient and fruitful methods to develop novel medicines. Steroids with aza-heterocycles attaching to the C-17 owing various biological activities have received great attentions and some of the compounds are developed successfully as drugs. In this review, the research of the syntheses and biological activities of steroids bearing various aza-heterocycles published in the last 8 years is assembled, and some important structure-activity relationships (SARs) of active compounds are presented. According to the analysis of the literatures and our experiences in this field, the potential of aza-heterocyclic steroids as medicinal drugs is proposed.

Synthesis of dihydroisoquinolinone-4-methylboronic esters via domino Heck/borylation using a structurally characterized palladacycle as a catalyst

Synthesis of dihydroisoquinolinone-4-methylboronic esters from N-allylcarboxamides and B₂(Pin)₂via domino Heck/borylation approach is reported. A quinoxaline-based NHC-palladacycle [Pd(C∧C:)PPh₃Cl], which has been structurally characterized, is used as a catalyst. The scope of the substrate with a wide range of substituents is explored. In addition to the synthesis of title compounds, a few examples of methylboronic esters of indoline and benzofuran motifs have also been prepared using the same protocol.

Synthesis of dihydroisoquinolinone-4-methylboronic esters from N-allylcarboxamides and B₂(Pin)₂via domino Heck/borylation has been achieved by using a quinoxaline-based NHC-palladacycle [Pd(C∧C:)PPh₃Cl].

Mild Synthesis of 3,4-Dihydroisoquinolin-1(2H)-ones via Rh(III)-Catalyzed Tandem C-H-Allylation/N-Alkylation Annulation with 2-Methylidenetrimethylene Carbonate

A tandem rhodium(III)-catalyzed system was established to access 3,4-dihydroisoquinolin-1(2H)-one by coupling of N-methoxy-3-methylbenzamide with 2-methylidenetrimethylene carbonate. This one-pot synthesis protocol processed smoothly under mild reaction conditions. Moreover, a total of 28 examples, broad substrate scope, and high functional-group compatibility were observed. Preliminary mechanism studies were also conducted and demonstrated that the rhodium(III) catalyst played a vital role in the C-H-allylation and N-alkylation cyclization process.

Electrodimerization of N-Alkoxyamides for the Synthesis of Hydrazines

An efficient and valuable N-N dimerization reaction of N-alkoxyamides is reported under undivided electrolytic conditions. This electrochemical strategy provides a powerful way to access a wide range of advanced, highly functionalized hydrazines. Remarkably, an N-centered radical generated from the cleavage of the N-H bond under electrolytic conditions plays a crucial role in this transformation. Furthermore, various N-alkoxyamides bearing different substituents are suitable in this transformation, furnishing the corresponding hydrazines in up to 92% yield.

The Ligand Free Palladium(II)-Catalyzed Regioselective 1,2-Addition of Enol Silanes to Quinones to Access 4-Hydroxy-4-(2-oxo-2-arylethyl)cyclohexadien-1-ones and Synthetic Applications

In contrast to the conventional 1,4-addition process, regioselective 1,2-addition of silyl enol ethers to quinones can now be achieved via a palladium(II) enolate pathway that provides access to 4-hydroxy-4-(2-oxo-2-arylethyl)cyclohexa-2,5-dien-1-one derivatives. This quinone alkylation protocol proceeds under mild reaction conditions at ambient temperature under open air and does not require either an external ligand for the palladium or the use of a base. Additionally, the cyclohexadienone products have been exploited as synthetic precursors for the construction of fused heteroaryl systems.

Direct synthesis of indazole derivatives via Rh(III)-catalyzed C-H activation of phthalazinones and allenes

A novel Rh(III)-catalyzed annulation of phthalazinones or pyridazinones with various allenes was developed, leading to the formation of indazole derivatives bearing a quaternary carbon in moderate to good yields. The targeted products were synthesized via sequential C-H activation and olefin insertion, followed by β-hydride elimination and intramolecular cyclization. The synthetic protocol proceeded efficiently with broad functional group tolerance, high atom efficiency and high Z-selectivity. The practicability of this method was proved by synthetic transformation.

N-Alkenylation of hydroxamic acid derivatives with ethynyl benziodoxolone to synthesize cis-enamides through vinyl benziodoxolones

The stereoselective synthesis of cis-β-N-alkoxyamidevinyl benziodoxolones (cis-β-N-RO-amide-VBXs) from O-alkyl hydroxamic acids in the presence of an ethynyl benziodoxolone-acetonitrile complex (EBX-MeCN) is reported herein. The reaction was performed under mild conditions including an aqueous solvent, a mild base, and room temperature. The reaction tolerated various O-alkyl hydroxamic acids derived from carboxylic acids, such as amino acids, pharmaceuticals, and natural products. Vinyl dideuterated cis-β-N-MeO-amide-VBXs were also synthesized using deuterium oxide as the deuterium source. Valine-derived cis-β-N-MeO-amide-VBX was stereospecifically derivatized to hydroxamic acid-derived cis-enamides without the loss of stereoselectivity or reduction in the deuterium/hydrogen ratio.

Pd(II)-Catalyzed Annulation Reactions of Epoxides with Benzamides to Synthesize Isoquinolones

Epoxides as alkylating reagents are unprecedentedly applied in Pd(II)-catalyzed C-H alkylation and oxidative annulation of substituted benzamides to synthesize isoquinolones rather than isochromans, which is accomplished through alerting the previously reported reaction mechanism by the addition of oxidant and TEA. Under these conditions, various isoquinolones have been prepared with yields up to 92%. In addition, this methodology has been successfully employed in the total syntheses of rupreschstyril, siamine, and cassiarin A in an expedient fashion.

Rh(iii)-Catalyzed C–H allylation/annulative Markovnikov addition with 5-methylene-1,3-dioxan-2-one: formation of isoquinolinones containing a C3 quaternary centre

Rh(iii)-Catalyzed C–H allylation/annulative Markovnikov addition reaction was disclosed, offering isoquinolinones containing a C3 quaternary centre. By using this method as the key step, the US28 inverse agonist analogs were synthesized.

Lossen Rearrangement vs C-N Reductive Elimination Enabled by Rh(III)-Catalyzed C-H Activation/Selective Lactone Ring-Opening: Chemodivergent Synthesis of Quinolinones and Dihydroisoquinolinones

An unprecedented Rh(III)-catalyzed cascade C-H activation/Lossen rearrangement of aromatic amides with methyleneoxetanones has been realized along with a tunable C-N bond reductive elimination/trans esterification, giving divergent access to quinolinones and dihydroisoquinolinones via selective ring-opening of the four-membered lactone unit. Combined computational and experimental mechanistic studies defined the solvent-involved distinguished reaction paths, the origin of the observed chemodivergence, as well as the role of the substituent attached at the oxidizing directing group in tuning the reaction outcomes.

Vinylogous Elimination/C-H Functionalization/Allylation Cascade Reaction of Allenoate Adducts: Synthesis of Ring-Fused Dihydropyridinones

A palladium-catalyzed cascade reaction of β'-allenoate adducts with aryl/heteroaryl carboxamides through a vinylogous elimination/C-H functionalization/intramolecular allylation reaction sequence has been developed with high Z stereoselectivity. Various ring-fused dihydropyridinones bearing an α,β-unsaturated ester substituent are obtained. It is the first example of application of the allenoate adducts to C-H functionalization annulations as practical precursors of hard-to-get functionalized electron-deficient 1,3-butadienes. Using air as the terminal oxidant also shows a great advantage in environmental friendliness.