Concise Total Synthesis of (±)-Marcfortine B

Diastereoselective Hydrogenation of Tetrasubstituted Olefins using a Heterogeneous Pt-Ni Alloy Catalyst

Stereoselective hydrogenation of tetrasubstituted olefins is an attractive method to access compounds with two contiguous stereocenters. However, homogeneous catalysts for enantio- and diastereoselective hydrogenation exhibit low reactivity toward tetrasubstituted olefins due to steric crowding between the ligand scaffold and the substrate. Monometallic heterogeneous catalysts, on the other hand, provide accessible surface active sites for hindered olefins but exhibit unpredictable and inconsistent stereoinduction. In this work, we develop a Pt-Ni bimetallic alloy catalyst that can diastereoselectively hydrogenate unactivated, sterically-bulky tetrasubstituted olefins, utilizing the more oxophilic Ni atoms to adsorb a hydroxyl directing group and direct facially-selective hydrogen addition to the olefin via the Pt atoms. Structure-activity studies on several Pt-Ni compositions underscore the importance of exposing a uniform PtNi alloy surface to achieve high diastereoselectivity and minimize side reactions. The optimized Pt-Ni/SiO2 catalyst exhibits good functional group tolerance and broad scope for tetrasubstituted olefins in a cyclopentene scaffold, generating cyclopentanol products with three contiguous stereocenters. The synthetic utility of the method is demonstrated in a four-step synthesis of (1R,2S)-(+)-cis-methyldihydrojasmonate with high yield and enantiopurity.

Recent advances in microwave-assisted multicomponent synthesis of spiro heterocycles

Spiro heterocycle frameworks are a class of organic compounds that possesses unique structural features making them highly sought-after targets in drug discovery due to their diverse biological and pharmacological activities. Microwave-assisted organic synthesis has emerged as a powerful tool for assembling complex molecular architectures. The use of microwave irradiation in synthetic chemistry is a promising method for accelerating reaction rates and improving yields. This review provides insights into the current state of the art and highlights the potential of microwave-assisted multicomponent reactions in the synthesis of novel spiro heterocyclic compounds that were reported between 2017 and 2023.

Synthesis of Bis-Heterocycles Bearing Methyleneindole Motifs by Pd-Catalyzed Domino Reaction

A highly robust, general, and practically simple palladium-catalyzed domino bicyclization strategy is presented to synthesize nitrogen-containing bis-heterocycles bearing methylene indole motifs from alkyne-tethered carbamoyl chlorides and β,γ- or γ,δ-unsaturated hydrazones. The salient features of this transformation include broad substrate scope, good functional group tolerance, ease for scale-up, and convenient conversion.

Highly Enantioselective Construction of the Polycyclic Piperazin-2-ones via NHC-Catalyzed [12 + 2] Cycloadditions

NHC-catalyzed diastereoselective and enantioselective [12 + 2] higher-order cycloadditions of 5H-benzo[a]-pyrrolizine-3-carbaldehydes with cyclic sulfonic imines are described. Various optically pure polycyclic piperazin-2-ones were successfully constructed under mild reaction conditions with excellent diastereoselectivities and enantioselectivities. The generated chiral polycyclic piperazin-2-ones could be transformed to planar-chiral nine-membered cyclic amides via squaramide-catalyzed asymmetric ring-expansion reactions.

Natural-Product-Inspired Microwave-Assisted Synthesis of Novel Spirooxindoles as Antileishmanial Agents: Synthesis, Stereochemical Assignment, Bioevaluation, SAR, and Molecular Docking Studies

Leishmaniasis is a neglected tropical disease, and there is an emerging need for the development of effective drugs to treat it. To identify novel compounds with antileishmanial properties, a novel series of functionalized spiro[indoline-3,2'-pyrrolidin]-2-one/spiro[indoline-3,3'-pyrrolizin]-2-one 23a-f, 24a-f, and 25a-g were prepared from natural-product-inspired pharmaceutically privileged bioactive sub-structures, i.e., isatins 20a-h, various substituted chalcones 21a-f, and 22a-c amino acids, via 1,3-dipolar cycloaddition reactions in MeOH at 80 °C using a microwave-assisted approach. Compared to traditional methods, microwave-assisted synthesis produces higher yields and better quality, and it takes less time. We report here the in vitro antileishmanial activity against Leishmania donovani and SAR studies. The analogues 24a, 24e, 24f, and 25d were found to be the most active compounds of the series and showed IC₅₀ values of 2.43 µM, 0.96 µM, 1.62 µM, and 3.55 µM, respectively, compared to the standard reference drug Amphotericin B (IC₅₀ = 0.060 µM). All compounds were assessed for Leishmania DNA topoisomerase type IB inhibition activity using the standard drug Camptothecin, and 24a, 24e, 24f, and 25d showed potential results. In order to further validate the experimental results and gain a deeper understanding of the binding manner of such compounds, molecular docking studies were also performed. The stereochemistry of the novel functionalized spirooxindole derivatives was confirmed by single-crystal X-ray crystallography studies.

Stereoselective Construction of Unsymmetrically Linked Heterocycles via Palladium-Catalyzed Alkyne Insertion/Cycloimidoylation Cascade

A novel strategy to access unsymmetrically linked heterocycles via palladium-catalyzed acylcycloimidoylation of alkyne-tethered carbamoyl chlorides with isocyanides has been developed. Functionalized isocyanides were successfully applied as imine-containing heterocycle precursors to capture the vinyl-Pd^II intermediate, which was generated from a syn-carbopalladation of alkyne, followed by subsequent intramolecular C-H bond activation/imidoylative Heck reactions. Methylene oxindoles within Z-tetrasubstituted olefins were obtained in high yields with excellent stereoselectivities. Broad functional groups were well tolerated under mild reaction conditions.

Tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles: construction of vicinal quaternary carbon centers

A novel tandem 1,6-addition/cyclopropanation/rearrangement reaction of vinylogous para-quinone methides with 3-chlorooxindoles has been developed, providing dispirooxindole–cyclopentane–cyclohexadienones with vicinal quaternary carbon centers.

A general approach to 2,2-disubstituted indoxyls: total synthesis of brevianamide A and trigonoliimine C

The indoxyl unit is a common structural motif in alkaloid natural products and bioactive compounds. Here, we report a general method that transforms readily available 2-substituted indoles into 2,2-disubstituted indoxyls via nucleophile coupling with a 2-alkoxyindoxyl intermediate and showcase its utility in short total syntheses of the alkaloids brevianamide A (7 steps) and trigonoliimine C (6 steps). The developed method is operationally simple and demonstrates broad scope in terms of nucleophile identity and indole substitution, tolerating 2-alkyl substituents and free indole N-H groups, elements beyond the scope of most prior approaches. Spirocyclic indoxyl products are also accessible via intramolecular nucleophilic trapping.

Enantioselective Construction of Spirooxindole-Fused Cyclopentanes

The present study reports an asymmetric organocatalytic cascade reaction of oxindole derivates with α,β-unsaturated aldehydes efficiently catalyzed by simple chiral secondary amine. Spirooxindole-fused cyclopentanes were produced in excellent isolated yields (up to 98%) with excellent enantiopurities (up to 99% ee) and moderate to high diastereoselectivities. The synthetic utility of the protocol was exemplified on a set of additional transformations of the corresponding spiro compounds. In addition, a study showing the promising biological activity of selected enantioenriched products was accomplished.

Cycloisomerization of Carbamoyl Chlorides in Hexafluoroisopropanol: Stereoselective Synthesis of Chlorinated Methylene Oxindoles and Quinolinones

Hexafluoroisopropanol (HFIP) was employed as an additive for the generation of 3-(chloromethylene)oxindoles via the chloroacylation of alkyne-tethered carbamoyl chlorides. This reaction avoids the use of a metal catalyst and accesses products in high yields and stereoselectivities. Additionally, this reaction is scalable and proved amenable to a series of product derivatizations, including the synthesis of nintedanib. The reactivity of alkene-tethered carbamoyl chlorides with hexafluoroisopropanol (HFIP) was harnessed towards the synthesis of 2-quinolinones.

Synthetic Strategies to Access Heteroatomic Spirocentres Embedded in Natural Products

The spirocyclic motif is abundant in natural products and provides an ideal three-dimensional template to interact with biological targets. With significant attention historically expended on the synthesis of flat-heterocyclic compound libraries, methods to access the less-explored three-dimensional medicinal-chemical space will continue to increase in demand. Herein, we highlight by reaction class the common strategies used to construct the spirocyclic centres embedded in a series of well-studied natural products.

1 Introduction

2 Cycloadditions

3 Palladium-Catalysed Coupling Reactions

4 Conjugate Additions

5 Imines, Aminals, and Hemiaminal Ethers

6 Mannich-Type Reactions

7 Oxidative Dearomatisation

8 Alkylation

9 Organometallic Additions

10 Conclusions

All-carbon [3 + 2] cycloaddition in natural product synthesis

Many natural products possess interesting medicinal properties that arise from their intriguing chemical structures. The highly-substituted carbocycle is one of the most common structural features in many structurally complicated natural products. However, the construction of highly-substituted, stereo-congested, five-membered carbocycles containing all-carbon quaternary center(s) is, at present, a distinct challenge in modern synthetic chemistry, which can be accessed through the all-carbon [3 + 2] cycloaddition. More importantly, the all-carbon [3 + 2] cycloaddition can forge vicinal all-carbon quaternary centers in a single step and has been demonstrated in the synthesis of complex natural products. In this review, we present the development of all-carbon [3 + 2] cycloadditions and illustrate their application in natural product synthesis reported in the last decade covering 2011-2020 (inclusive).

A unified strategy to reverse-prenylated indole alkaloids: total syntheses of preparaherquamide, premalbrancheamide, and (+)-VM-55599

A full account of our studies toward reverse-prenylated indole alkaloids that contain a bicyclo[2.2.2]core is described. A divergent route is reported which has resulted in the synthesis of preparaherquamide, (+)-VM-55599, and premalbrancheamide. An intramolecular Dieckmann cyclization between an enolate and isocyanate was used to forge the bicyclo[2.2.2]diazaoctane core that is characteristic of these molecules. The pentacyclic indole scaffold was constructed through a one-pot Hofmann rearrangement followed by Fischer indole synthesis. The utilization of our previously reported indole peripheral functionalization strategy also led to natural products including malbrancheamides B, C, stephacidin A, notoamides F, I and R, aspergamide B, and waikialoid A. Ultimately, the divergent route that we devised provided access to a wide range of prenylated indole alkaloids that are differently substituted on the cyclic amine core.

A metal-free aromative cascade for the synthesis of diverse heterocycles

A metal-free aromative cascade has been developed for the synthesis of diverse heterocycles from readily accessible hydroxy/aminochalcones and acid/alkyl halides. The cascade being by a base-mediated intramolecular aldol cyclization/dehydration sequence to provide a triene, which sets the stage for a 6π-electrocyclization/oxidative aromatization to access diverse heterocyclic scaffolds.

Pd-Catalyzed de Novo Assembly of Diversely Substituted Indole-Fused Polyheterocycles

Here we describe a facile, tandem synthetic route for indolo[3,2-c]quinolinones, a class of natural alkaloid analogues of high biological significance. A Ugi four-component reaction with indole-2-carboxylic acid and an aniline followed by a Pd-catalyzed cyclization yields tetracyclic indoloquinolines in good to moderate yields. Commercially available building blocks yield highly diverse analogues in just two simple steps.

Structural and stereochemical diversity in prenylated indole alkaloids containing the bicyclo[2.2.2]diazaoctane ring system from marine and terrestrial fungi

Covering: up to February 2017 Various fungi of the genera Aspergillus, Penicillium, and Malbranchea produce prenylated indole alkaloids possessing a bicyclo[2.2.2]diazaoctane ring system. After the discovery of distinct enantiomers of the natural alkaloids stephacidin A and notoamide B, from A. protuberus MF297-2 and A. amoenus NRRL 35660, another fungi, A. taichungensis, was found to produce their diastereomers, 6-epi-stephacidin A and versicolamide B, as major metabolites. Distinct enantiomers of stephacidin A and 6-epi-stephacidin A may be derived from a common precursor, notoamide S, by enzymes that form a bicyclo[2.2.2]diazaoctane core via a putative intramolecular hetero-Diels-Alder cycloaddition. This review provides our current understanding of the structural and stereochemical homologies and disparities of these alkaloids. Through the deployment of biomimetic syntheses, whole-genome sequencing, and biochemical studies, a unified biogenesis of both the dioxopiperazine and the monooxopiperazine families of prenylated indole alkaloids constituted of bicyclo[2.2.2]diazaoctane ring systems is presented.

Ru-Catalyzed dehydrogenative synthesis of antimalarial arylidene oxindoles

Ru(ii)-NHC catalyzes α-olefination of 2-oxindoles using diaryl methanols in the absence of an acceptor. A wide array of symmetrical and unsymmetrical diaryl methanols undergoes dehydrogenative coupling with 2-oxindole selectively to generate various substituted 3-(diphenylmethylene)indolin-2-one derivatives in good yields and produces environmentally benign by-products, H2 and H2O. This methodology was successfully applied for the synthesis of a bioactive drug i.e. TAS-301. The biological activities of the synthesized 3-(diphenylmethylene)indolin-2-one derivatives were screened against the Plasmodium falciparum parasite and found to exhibit a significant activity with IC50 = 2.24 μM.

Catalyst-free [3 + 2] cyclization of dihydroisoquinoline imines and isatin-derived Morita–Baylis–Hillman carbonates via 1,5-electrocyclization: synthesis of tetrahydroisoquinoline-fused spirooxindoles

The incorporation of tetrahydroisoquinoline and spirooxindole frameworks has been achieved.

NHC-catalyzed [4+2] cycloaddition reactions for the synthesis of 3′-spirocyclic oxindoles via a C–F bond cleavage protocol

A chiral NHC-catalyzed cycloaddition of γ-fluoroenals is developed. The nucleophilic γ-carbon generated via C–F bond cleavage undergoes highly enantioselective cycloaddition (up to >99% ee) to isatins and provides 3′-spirocyclic oxindoles in good yields (up to 91%).

Stereoselective Synthesis of 3-(1,3-Diarylallylidene)oxindoles via a Palladium-Catalyzed Tandem Reaction

We have developed an efficient three-component tandem reaction for the synthesis of 3-(1,3-diarylallylidene)oxindoles combining three palladium-catalyzed reactions: the Sonogashira, Heck, and Suzuki-Miyaura reactions. This method allows a stereoselective approach to each (E)- and (Z)-isomer by ligand change and controlling the reaction temperature.

Mechanistic insights on the Pd-catalyzed addition of C-X bonds across alkynes - a combined experimental and computational study

The Pd-catalyzed intramolecular addition of carbamoyl chlorides and aryl halides across alkynes is investigated by means of DFT calculations and mechanistic test experiments. The data suggest a mechanistic pathway that involves oxidative addition, alkyne insertion, cis → trans isomerization and reductive elimination. Our data indicate that oxidative addition is the reactivity limiting step in the addition of aryl chlorides and bromides across alkynes. However, for the corresponding addition of carbamoyl chlorides, alkyne insertion is found to be limiting. Full energetic reaction pathways for the intramolecular additions across alkynes are presented herein and the role of ligands, alkyne substituents and tether moieties are discussed. Notably, the calculations could rationalize a pronounced effect of the alkyne substituent, which accounts for the exceptional reactivity of TIPS-substituted alkynes. In particular, the bulky silyl moiety is shown to significantly destabilize the formed Pd(ii)-intermediates, thus facilitating both cis → trans isomerization and reductive elimination, which overall results in a flatter energetic landscape and a therefore increased catalytic efficiency.

Indium triflate catalysed 3-aza-Cope rearrangement of amino acid derived α,β-unsaturated esters to alkylidene oxindoles

The first examples of amino acid derived α,β-unsaturated esters to Z-alkylidene oxindoles via 3-aza-Cope rearrangement using In(OTf)₃ are reported.

N-heterocyclic carbene-catalyzed synthesis of spirocyclopentene-oxindoles from bromoenals

Spirocyclopentene-oxindoles were synthesized in good yields with good diastereo- and enantioselectivitiesviathe N-heterocyclic carbene-catalyzed reaction of bromoenals and oxindoles.

Oxidative radical cyclizations of diketopiperazines bearing an amidomalonate unit. Heterointermediate reaction sequences toward the asperparalines and stephacidins

A novel approach to the diazabicyclo[2.2.2]octane core of prenylated bridged diketopiperazine alkaloids is described by direct oxidative cyclizations of functionalized diketopiperazines mediated by ferrocenium hexafluorophosphate or the Mn(OAc)₃•2H₂O/Cu(OTf)₂ system. Divergent reaction pathways take place depending on the substitution pattern of the substrates and the oxidation conditions such as temperature or the presence or absence of persistent radical TEMPO. For ester-substituted diketopiperazines, the ester group exerts a significant influence on the reaction outcome and stereochemistry of the radical cyclizations.

Stereoselective Synthesis of Methylene Oxindoles via Palladium(II)-Catalyzed Intramolecular Cross-Coupling of Carbamoyl Chlorides

We report a highly robust, general and stereoselective method for the synthesis of 3-(chloromethylene)oxindoles from alkyne-tethered carbamoyl chlorides using PdCl₂(PhCN)₂ as the catalyst. The transformation involves a stereo- and regioselective chloropalladation of an internal alkyne to generate a nucleophilic vinyl Pd^II species, which then undergoes an intramolecular cross-coupling with a carbamoyl chloride. The reaction proceeds under mild conditions, is insensitive to the presence of moisture and air, and is readily scalable. The products obtained from this reaction are formed with >95:5 Z:E selectivity in nearly all cases and can be used to access biologically relevant oxindole cores. Through combined experimental and computational studies, we provide insight into stereo- and regioselectivity of the chloropalladation step, as well as the mechanism for the C-C bond forming process. Calculations provide support for a mechanism involving oxidative addition into the carbamoyl chloride bond to generate a high valent Pd^IV species, which then undergoes facile C-C reductive elimination to form the final product. Overall, the transformation constitutes a formal Pd^II-catalyzed intramolecular alkyne chlorocarbamoylation reaction.

Total synthesis of (-)-depyranoversicolamide B

Starting from easily prepared (R)-C3-isoprenylated pyrroloindoline, the C3-isoprenylated indolyl diketopiperazine is prepared by an efficient reductive opening of the pyrrolo ring, and undergoes biomimetic Diels-Alder reaction to generate an anti-adduct as a sole stereoisomer. Oxidation of the indoline moiety to oxindole completes the synthesis of (-)-depyranoversicolamide B.