Application of diazene-directed fragment assembly to the total synthesis and stereochemical assignment of (+)-desmethyl-meso-chimonanthine and related heterodimeric alkaloids

Unimolecular Fragment Coupling: A New Bond-Forming Methodology via the Deletion of Atom(s)

Unimolecular fragment coupling (UFC) is defined as a reaction format, wherein atom(s) located in the middle of a molecule are extruded, and the remaining fragments are coupled. UFC is a potentially powerful strategy that is an alternative to transition-metal-catalyzed cross-coupling because the target chemical bond is formed in an intramolecular fashion, which is inherently beneficial for chemoselectivity and stereoselectivity issues. In this Perspective, we will present an overview of the recent advances in UFC reactions, which encompass those proceeding through the elimination of CO2, CO, SO2, isocyanates, N2, or single atoms primarily via transition metal catalysis.

Total Synthesis of (+)- and (-)-Calycanthine by Means of Thio-Urea-Catalyzed Sequential Michael Reactions of Bis-oxindoles

A unified catalytic asymmetric approach to naturally occurring piperidinoindoline and pyrrololidinoindoline alkaloids has been realized via the development of a thio-urea-catalyzed sequential Michael addition of bis-oxindole onto nitroethylene (up to 93% ee and >20:1 dr). This strategy offers the total syntheses of either enantiomers of naturally occurring calycanthine.

Photoredox-Catalyzed Generation of Tertiary Anions from Primary Amines via a Radical Polar Crossover

A method for the generation of tertiary carbanions via a deaminative radical-polar crossover is reported using redox active imines from α-tertiary primary amines. A variety of benzylic amines and amino esters can be used in this approach, with the latter engaging in a novel "aza-Reformatsky" reaction. Electronic trends correlate the stability of the resulting carbanion with reaction efficiency. The anions can be trapped with different electrophiles including aldehydes, ketones, imines, Michael acceptors, and H2 O/D2 O. Selective anion formation can be achieved in the presence of another equivalent or more acidic C-H bond in both an inter- and intramolecular fashion. Mechanistic studies suggest the intermediacy of a discrete carbanion intermediate.

Transition-metal catalyzed C-H activation as a means of synthesizing complex natural products

Over the past few decades, the advent of C-H activation has led to a rethink among chemists about the synthetic strategies employed for multi-step transformations. Indeed, deploying innovative and masterful tricks against the numerous classical organic transformations has been the need of the hour. Despite this, the immense importance of C-H activation remains unfulfilled unless the methodology can be deployed for large-scale industrial processes and towards the concise, step-economic synthesis of prodigious natural products and pharmaceutical drugs. Lately, the growing potential of C-H activation methodology has indeed driven the pioneers of synthetic organic chemists into finding more efficient methods to accelerate the synthesis of such complex molecular scaffolds. This review aims to draw a general overview of the various C-H activation procedures that have been adopted for synthesizing these vast majority of structurally complicated natural products. Our objective lies in drawing a complete picture and taking the readers through the synthesis of a series of such complex organic compounds by simplified techniques, making it step-economic on a larger scale and thus instigating the readers to trigger the use of such methodology and uncover new, unique patterns for future synthesis of such natural products.

Stereoselective Routes to Hexahydropyrroloindoles and Tetrahydropyrroloquinolines from Activated Aziridines and Electron Deficient 3H-Indoles

An unprecedented and novel synthetic route to hexahydropyrrolo[2,3-b]indoles bearing cis-contiguous stereocenters with excellent stereoselectivities (ee of >99%, dr of ≤99:1) has been disclosed that proceeds through the ring opening of activated aziridines with electron deficient 4-substituted indoles followed by a novel cyclization in a domino fashion, thereby obviating the use of 3-substituted indoles as the prerequisite nucleophile. Another efficient synthetic route to tetrahydropyrrolo[4,3,2-de]quinolines in excellent yields (≤93%) and excellent enantioselectivity (ee of >99%) has been established via ring opening of activated aziridines with 4-bromo-1-methyl-1H-indole at relatively higher temperatures followed by Cu(I)-catalyzed intramolecular C-N cyclization in the same pot. The stability and the formation of products at different temperatures are explained by computational studies.

Copper-Mediated Single-Electron Approach to Indoline Amination: Scope, Mechanism, and Total Synthesis of Asperazine A

Pyrroloindolines bearing a C3-N linkage comprise the core of many biologically active natural products, but many methods toward their synthesis are limited by the sterics or electronics of the product. We report a single electron-based approach for the synthesis of this scaffold and demonstrate high-yielding aminations, regardless of electronic or steric demands. The transformation uses copper wire and isopropanol to promote the reaction. The broad synthetic utility of this heterogeneous copper-catalyzed approach to access pyrroloindolines, diketopiperazine, furoindoline, and (+)-asperazine is included, along with experiments to provide insight into the mechanism of this new process.

Synthesis of Natural Products by C-H Functionalization of Heterocycless

Total synthesis is considered by many as the finest combination of art and science. During the last decades, several concepts were proposed for achieving the perfect vision of total synthesis, such as atom economy, step economy, or redox economy. In this context, C-H functionalization represents the most powerful platform that has emerged in the last years, empowering rapid synthesis of complex natural products and enabling diversification of bioactive scaffolds based on natural product architectures. In this review, we present an overview of the recent strategies towards the total synthesis of heterocyclic natural products enabled by C-H functionalization. Heterocycles represent the most common motifs in drug discovery and marketed drugs. The implementation of C-H functionalization of heterocycles enables novel tactics in the construction of core architectures, but also changes the logic design of retrosynthetic strategies and permits access to natural product scaffolds with novel and enhanced biological activities.

Total Synthesis and Stereochemical Assignment of (-)-Psychotridine

We report the first enantioselective total synthesis and stereochemical assignment of (-)-psychotridine. The application of our diazene-directed assembly of enantiomerically enriched cyclotryptamines afforded a highly convergent synthesis of the pentameric alkaloid, allowing its detailed structural assignment. Highlights of the synthesis include the introduction of four quaternary stereocenters with complete stereochemical control in a single step via the photoextrusion of three molecules of dinitrogen from an advanced intermediate and metal-catalyzed C-H amination reactions in challenging settings.

Concise total syntheses of bis(cyclotryptamine) alkaloids via thio-urea catalyzed one-pot sequential Michael addition

Naturally occurring bis(cyclotryptamine) alkaloids feature vicinal all-carbon quaternary stereocenters with an elongated labile C-3a-C-3a' Sigma bond with impressive biological activities. In this report, we have developed a thio-urea catalyzed one-pot sequential Michael addition of bis-oxindole onto selenone to access enantioenriched dimeric 2-oxindoles with vicinal quaternary stereogenic centers at the pseudobenzylic position (up to 96% ee and >20 : 1 dr). This strategy has been successfully applied for the total syntheses of either enantiomers of chimonanthine, folicanthine, and calycanthine.

Strategies for total synthesis of bispyrrolidinoindoline alkaloids

Covering up to 2021Complex cyclotryptamine alkaloids with a bispyrrolidino[2,3-b]indoline (BPI) skeleton are an intriguing family of natural products, exhibiting wide systematic occurrences, large structural diversity, and multiple biological activities. Based on their structural characteristics, BPI alkaloids can be classified into chimonanthine-type BPI alkaloids, BPI diketopiperazines, and BPI epipolythiodiketopiperazines. These intricate molecules have captivated great attention soon after their isolation and identification in the 1960s. Due to the structural complexity, the total synthesis of these cyclotryptamine alkaloids is challenging. Nevertheless, remarkable progress has been achieved in the last six decades; in particular, several methods have been successfully established for the construction of vicinal all-carbon quaternary stereocenters. In this review, the structural diversity and chemical synthesis of these BPI alkaloids were summarized. BPI alkaloids are mainly synthesized by the methods of oxidative dimerization, reductive dimerization, and alkylation of bisoxindole. The purpose of this review is to present overall strategies for assembling the BPI skeleton and efforts towards controlling the stereocenters.

The applications of catalytic asymmetric halocyclization in natural product synthesis

Catalytic asymmetric halocyclization of olefinic substrate has evolved rapidly and been well utilized as a practical strategy for constructing enantioenriched cyclic skeletons in natural product synthesis.

Total Synthesis of (-)-Calycanthine via Iron-Catalyzed Stereoselective Oxidative Dimerization

Dimeric cyclotryptamine alkaloids typically feature vicinal all-carbon quaternary stereocenters and four nitrogen atoms. In comparison with the actual biosynthetic tryptophan derivatives, we designed the 2N-featured monomer 7, aiming to construct vicinal all-carbon quaternary stereocenters via a one-step dimerization process to access the 4N-featured isomeric members of this family. In this work, we disclose the first synthetic route to access the skeleton of (-)-isocalycanthine, featuring an iron-catalyzed oxidative dimerization reaction in a catalytic single-step operation with an overwhelming control of the absolute and relative stereochemistry. This strategy has been successfully applied to the total synthesis of (-)-calycanthine and 16 isocalycanthine derivatives, which demonstrates a new synthetic pathway for dimeric cyclotryptamine alkaloids.

Box-copper catalyzed cascade asymmetric amidation for chiral exo-methylene aminoindoline derivatives

Enantioselective copper-catalyzed cascade inter- and intramolecular amidation was achieved between ethynyl benzoxazinanones and α-halohydroxamates in the presence of an indapybox ligand. The one-pot cascade transformation was triggered by the attack of hydroxamates to dipolar copper-allenylidene intermediates, followed by a nucleophilic annulation reaction. Thus, a series of exo-methylene 3-aminoindoline derivatives were obtained in good yields with high enantioselectivities under mild reaction conditions.

In vitro antiplasmodial activity, targeted LC-MS metabolite profiling, and identification of major natural products in the bioactive extracts of Palicourea and Psychotria species from the Amazonia and Atlantic Forest biomes, Brazil

INTRODUCTION

A great variety of bioactive natural products has been reported for different Palicourea and Psychotria species (Rubiaceae). However, few of them as well as few of species of these botanical genera have been evaluated for antiplasmodial activity.

OBJECTIVE

To assess the antiplasmodial activity of 24 extracts from Palicourea and Psychotria genera, along with the targeted LC-MS metabolite profiling, as well as identification of the main metabolites in the bioactive extracts.

METHODS

Twenty four ethanol and acid-base extracts from Palicourea and Psychotria genera collected in the Amazonia and Atlantic Forest, Brazil, were evaluated against chloroquine-resistant Plasmodium falciparum W2 strain by PfLDH. The metabolite profiling and putative identification of metabolites from bioactive extracts were determined by LC-DAD-ESI-MS and LC-HRMS, respectively.

RESULTS

The ethanol extracts disclosed low antiplasmodial activity (% GI < 50%). High antiplasmodial effect was observed for the acid-base extracts from Psychotria apoda and Psychotria colorata with 100% inhibition of parasite growth inhibition. Fragment ions related to pyrrolidinoindoline alkaloids were observed by LC-DAD-ESI-MS mainly in the most bioactive extracts. The results of the in vitro screening associated with the LC-DAD-ESI-MS and LC-HRMSⁿ data allowed to predict, for the first time, the pyrrolidinoindoline alkaloids as possible antiplasmodial representing, then, new potential natural antimalarial hits. In addition, other metabolite classes such as flavanones, lignans and chalcones were also putatively identified in the bioactive extracts of Psychotria apoda, Psychotria capitata, and Psychotria poeppigiana.

CONCLUSION

The present results point to Palicourea and Psychotria species as sources of new antimalarial hits.

Phosphine-Catalyzed Asymmetric Allylic Alkylation of Achiral MBH Carbonates with 3,3'-Bisindolines: Enantioselective Construction of Quaternary Stereogenic Centers

The asymmetric allylic alkylation (AAA) of achiral Morita-Baylis-Hillman (MBH) carbonates with 3,3'-bisindolines under the catalysis of amino-acid-derived bifunctional phosphines was accomplished. With the AAA approach introduced herein, challenging 3,3'-bisindolines bearing an all-carbon quaternary stereocenter (C3a) have been constructed in good yields with good to excellent enantioselectivties. In addition, the synthetic value of this protocol was demonstrated by the facile synthesis of the core skeleton of gliocladin C.

Enantioselective [4 + 2] Cycloaddition/Cyclization Cascade Reaction and Total Synthesis of cis-Bis(cyclotryptamine) Alkaloids

The asymmetric catalytic synthesis of 3-cyclotryptamine substituted oxindoles through formal [4 + 2] cycloaddition/cyclization cascade is described. A wide range of cyclotryptamine derivatives were obtained in enantioenriched form under mild reaction conditions and were found to have potential anticancer activity. The strategy enables ready assembly of cyclotryptamine subunits at the C_3a-C_3a' positions with two quaternary stereogenic centers in cis-selectivity, leading to the concise synthesis of optically active cis-bis(hexahydropyrroloindole) and others of the cyclotryptamine alkaloid family.

The catalytic asymmetric dearomatization of tryptamine for accessing meso-contiguous quaternary carbon centers of oligomeric cyclotryptamine alkaloids: a formal synthesis of hodgkinsine B

Herein, we disclosed a catalytic asymmetric dearomatization (CADA) of tryptamine via tandem [4 + 2] cycloaddition/cyclization with o-azaxylylene in situ generated from functionalized 3-bromooxindole promoted by chiral N,N′-dioxide/Ni(BF₄)₂.

Ni-Catalyzed Iterative Alkyl Transfer from Nitrogen Enabled by the In Situ Methylation of Tertiary Amines

Current methods to achieve transition-metal-catalyzed alkyl carbon-nitrogen (C-N) bond cleavage require the preformation of ammonium, pyridinium, or sulfonamide derivatives from the corresponding alkyl amines. These activated substrates permit C-N bond cleavage, and their resultant intermediates can be intercepted to affect carbon-carbon bond-forming transforms. Here, we report the combination of in situ amine methylation and Ni-catalyzed benzalkyl C-N bond cleavage under reductive conditions. This method permits iterative alkyl group transfer from tertiary amines and demonstrates a deaminative strategy for the construction of Csp³-Csp³ bonds. We demonstrate PO(OMe)₃ (trimethylphosphate) to be a Ni-compatible methylation reagent for the in situ conversion of trialkyl amines into tetraalkylammonium salts. Single, double, and triple benzalkyl group transfers can all be achieved from the appropriately substituted tertiary amines. Transformations developed herein proceed via recurring events: the in situ methylation of tertiary amines by PO(OMe)₃, Ni-catalyzed C-N bond cleavage, and concurrent Csp³-Csp³ bond formation.

Discovery and Total Synthesis of a Bis(cyclotryptamine) Alkaloid Bearing the Elusive Piperidinoindoline Scaffold

Bis(cyclotryptamine) alkaloids have been popular topics of study for many decades. Five possible scaffolds for bis(cyclotryptamine) alkaloids were originally postulated in the 1950s, but only four of these scaffolds have been observed in natural products to date. We describe synthetic access to the elusive fifth scaffold, the piperidinoindoline, through syntheses of compounds now termed "dihydropsychotriadine" and "psychotriadine". The latter of these compounds was subsequently identified in extracts of the flower Psychotria colorata. Our synthetic route features a stereospecific solid-state photodecarbonylation reaction to introduce the key vicinal quaternary stereocenters.

Photochemically Mediated Nickel-Catalyzed Synthesis of N-(Hetero)aryl Sulfamides

A general method for the N-arylation of sulfamides with aryl bromides is described. The protocol leverages a dual-catalytic system, with [Ir(ppy)2(dtbbpy)]PF6 as a photosensitizer, NiBr2·glyme as a precatalyst, and 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) as a base, and proceeds at room temperature under visible light irradiation. Using these tactics, aryl boronic esters and aryl chlorides can be carried through the reaction untouched. The developed reactions efficiently engage simple bromoarenes and primary sulfamides in between 66% and quantitative yields. For more challenging substrates, such as secondary sulfamides, the reaction efficiency is documented. Thereby, these methods complement the known Buchwald-Hartwig coupling methods for N-arylation of sulfamides.

Total Synthesis and Anti-Cancer Activity of All Known Communesin Alkaloids and Related Derivatives

A unified enantioselective total synthesis and anticancer evaluation of all known epoxide-containing communesin alkaloids and related derivatives is described. Our synthesis is predicated on the convergent and modular diazene-directed assembly of two complex fragments to secure the critical C3a-C3a' linkage followed by a guided biomimetic aminal reorganization to deliver the heptacyclic core of these alkaloids. Concise enantioselective syntheses of the fragments were devised, with highlights including the application of a rationally designed sulfinamide chiral auxiliary, an efficient calcium trifluoromethanesulfonate promoted intramolecular amination, and a diastereoselective epoxidation that simultaneously converts the new chiral auxiliary to a versatile amine protective group. The modularity of our convergent approach enabled the rapid synthesis of all epoxide-containing members of the communesin family from a single heterodimeric intermediate, including the first total synthesis of communesins C-E, and G-I, and facilitated our stereochemical revision of (-)-communesin I, the most recently isolated communesin alkaloid. Furthermore, the generality of our biogenetically inspired heterodimer rearrangement was demonstrated in a guided synthesis of a communesin derivative with an unnatural topology. Finally, we report the first comparative analysis of the anticancer activities of all naturally occurring communesin alkaloids A-I and eight complex derivatives against five human cancer cell lines. From these data, we have identified (-)-communesin B as the most potent natural communesin and discovered that derivatives with N8'-sulfonamide substitution exhibit up to a 10-fold increase in potency over the natural alkaloids.

Recent applications of C-H functionalization in complex natural product synthesis

In this review, recent examples featuring C-H functionalization in the synthesis of complex natural products are discussed. A focus is given to the way in which C-H functionalization can influence the logical process of retrosynthesis, and the review is organized by the type and method of C-H functionalization.

Enantioselective Synthesis of Pyrroloindolines via Noncovalent Stabilization of Indole Radical Cations and Applications to the Synthesis of Alkaloid Natural Products

While interest in the synthetic chemistry of radical cations continues to grow, controlling enantioselectivity in the reactions of these intermediates remains a challenge. Based on recent insights into the oxidation of tryptophan in enzymatic systems, we report a photocatalytic method for the generation of indole radical cations as hydrogen-bonded adducts with chiral phosphate anions. These noncovalent open-shell complexes can be intercepted by the stable nitroxyl radical TEMPO· to form alkoxyamine-substituted pyrroloindolines with high levels of enantioselectivity. Further elaboration of these optically enriched adducts can be achieved via a catalytic single-electron oxidation/mesolytic cleavage sequence to furnish transient carbocation intermediates that may be intercepted by a wide range of nucleophiles. Taken together, this two-step sequence provides a simple catalytic method to access a wide range of substituted pyrroloindolines in enantioenriched form via a standard experimental protocol from a common synthetic intermediate. The design, development, mechanistic study, and scope of this process are presented, as are applications of this method to the synthesis of several dimeric pyrroloindoline natural products.

Concise total synthesis of (+)-asperazine A and (+)-pestalazine B

The highly convergent total synthesis of dimeric diketopiperazine alkaloids (+)-asperazine A and (+)-pestalazine B is described. A critical aspect of our expedient route was the development of a directed regio- and diastereoselective C3-N1' coupling of complex tetracyclic diketopiperazine components. This late-stage heterodimerization reaction was made possible by design of tetracyclic diketopiperazines that allow C3-carbocation coupling of the electrophilic component to the N1' locus of the nucleophilic fragment. The application of this new coupling reaction to the first total synthesis of (+)-asperazine A led to our revision of the sign and magnitude of the optical rotation for the reported structure.

Natural products as inspiration for the development of new synthetic methods

Natural products have played an important role in shaping modern synthetic organic chemistry. In particular, their complex molecular skeletons have stimulated the development of many new synthetic methods. We highlight in this article some recent examples of synthetic design inspired by the biosynthesis of natural products.

Concise Synthesis of (-)-Hodgkinsine, (-)-Calycosidine, (-)-Hodgkinsine B, (-)-Quadrigemine C, and (-)-Psycholeine via Convergent and Directed Modular Assembly of Cyclotryptamines

The enantioselective total synthesis of (-)-hodgkinsine, (-)-calycosidine, (-)-hodgkinsine B, (-)-quadrigemine C, and (-)-psycholeine through a diazene-directed assembly of cyclotryptamine fragments is described. Our synthetic strategy enables multiple and directed assembly of intact cyclotryptamine subunits for convergent synthesis of highly complex bis- and tris-diazene intermediates. Photoextrusion of dinitrogen from these intermediates enables completely stereoselective formation of all C3a-C3a' and C3a-C7' carbon-carbon bonds and all the associated quaternary stereogenic centers. In a representative example, photoextrusion of three dinitrogen molecules from an advanced intermediate in a single-step led to completely controlled introduction of four quaternary stereogenic centers and guided the assembly of four cyclotryptamine monomers en route to (-)-quadrigemine C. The synthesis of these complex diazenes was made possible through a new methodology for synthesis of aryl-alkyl diazenes using electronically attenuated hydrazine-nucleophiles for a silver-promoted addition to C3a-bromocyclotryptamines. The application of Rh- and Ir-catalyzed C-H amination reactions in complex settings were used to gain rapid access to C3a- and C7-functionalized cyclotryptamine monomers, respectively, used for diazene synthesis. This convergent and modular assembly of intact cyclotryptamines offers the first solution to access these alkaloids through completely stereoselective union of monomers at challenging linkages and the associated quaternary stereocenters as illustrated in our synthesis of five members of the oligocyclotryptamine family of alkaloids.

An asymmetric acetate-Mannich reaction of chiral isatin derived ketimines and its applications

A highly efficient TMSOTf-mediated asymmetric acetate-Mannich reaction of isatin derived tert-butylsulfinyl ketimines and S-phenyl thioacetate was developed to afford the direct synthesis of indole-based β^3,3-amino acid thioester with excellent selectivity (dr > 98 : 2). Syntheses of (+)-AG-041R and 3-aminopyrroloindoline have been accomplished utilizing the developed method.

Nickel-Catalyzed Barton Decarboxylation and Giese Reactions: A Practical Take on Classic Transforms

Two named reactions of fundamental importance and paramount utility in organic synthesis have been reinvestigated, the Barton decarboxylation and Giese radical conjugate addition. N-hydroxyphthalimide (NHPI) based redox-active esters were found to be convenient starting materials for simple, thermal, Ni-catalyzed radical formation and subsequent trapping with either a hydrogen atom source (PhSiH3 ) or an electron-deficient olefin. These reactions feature operational simplicity, inexpensive reagents, and enhanced scope as evidenced by examples in the realm of peptide chemistry.

Appreciation of symmetry in natural product synthesis

This review defines symmetric molecules from a synthetic perspective and shows various strategies that take advantage of molecular symmetry to construct them.

Transition metal-catalyzed iodine(iii)-mediated nitrene transfer reactions: efficient tools for challenging syntheses

The main synthetic applications of catalytic C(sp³)–H amination and alkene aziridination reactions are discussed in the context of natural product synthesis. The examples highlight that these synthetic methods now firmly belong in the organic chemist's toolbox.

Catalytic C–H amination at its limits: challenges and solutions

Pushing C–H amination to its limits fosters innovative synthetic solutions and offers a deeper understanding of the reaction mechanism and scope.

Convergent and Biomimetic Enantioselective Total Synthesis of (-)-Communesin F

The first biomimetic enantioselective total synthesis of (-)-communesin F based on a late-stage heterodimerization and aminal exchange is described. Our synthesis features the expedient diazene-directed assembly of two advanced fragments to secure the congested C3a-C3a' linkage in three steps, followed by a highly efficient biogenetically inspired aminal reorganization to access the heptacyclic communesin core in only two additional steps. Enantioselective syntheses of the two fragments were developed, with highlights including the catalytic asymmetric halocyclization and diastereoselective oxyamination reactions of tryptamine derivatives, a stereoselective sulfinimine allylation, and an efficient cyclotryptamine-C3a-sulfamate synthesis by either a new silver-promoted nucleophilic amination or a rhodium-catalyzed C-H amination protocol. The versatile syntheses of the fragments, their stereocontrolled assembly, and the efficient aminal exchange as supported by in situ monitoring experiments, in addition to the final stage N1'-acylation of the communesin core, provide a highly convergent synthesis of (-)-communesin F.

Total Synthesis of Dimeric HPI Alkaloids

In this paper, we report a full account of the synthesis of dimeric hexahydropyrroloindole alkaloids and its analogues. The key feature of our new strategy is the novel catalytic copper (10 %) mediated intramolecular arylations of o-haloanilides followed by intermolecular oxidative dimerization of the resulting oxindoles in one pot. This sequential reaction leads to the key intermediates for the synthesis of (+)-chimonanthine, (+)-folicanthine, (-)-calycanthine and (-)-ditryptophenaline. In the presence of catalytic amount of cuprous iodide (10 %), an intramolecular arylation of o-haloanilides followed by an intermolecular oxidative dimerization of the resulting oxindoles leads to a common intermediate for the synthesis of (+)-chimonanthine, (+)-folicanthine and (-)-calycanthine. Based on this cascade sequence, we also developed a flexible strategy towards the asymmetric syntheses of dimeric HPI alkaloids (-)-ditryptophenaline and its analogues.

Concise Total Synthesis of (+)-Asperazine, (+)-Pestalazine A, and (+)-iso-Pestalazine A. Structure Revision of (+)-Pestalazine A

The concise, enantioselective total syntheses of (+)-asperazine (1), (+)-iso-pestalazine A (2), and (+)-pestalazine A (3) have been achieved by the development of a late-stage C3-C8' Friedel-Crafts union of polycyclic diketopiperazines. Our modular strategy enables the union of complex polycyclic diketopiperazines in virtually their final forms, thus providing rapid and highly convergent assembly at the challenging quaternary stereocenter of these dimeric alkaloids. The significance of this carbon-carbon bond formation can be gauged by the manifold constraints that were efficiently overcome, namely the substantial steric crowding at both reactive sites, the nucleophilic addition of C8' over N1' to the C3 carbocation, and the multitude of reactivity posed by the use of complex diketopiperazine fragments in the coupling event. The success of the indoline π-nucleophile that evolved through our studies is notable given the paucity of competing reaction pathways observed in the presence of the highly reactive C3 carbocation generated. This first total synthesis of (+)-pestalazine A also allowed us to revise the molecular structure for this natural alkaloid.