Copper-Mediated Dimerization to Access 3a,3a′-Bispyrrolidinoindoline: Diastereoselective Synthesis of (+)-WIN 64821 and (−)-Ditryptophenaline

Copper-Catalyzed Tunable Oxygenative Rearrangement of Tetrahydrocarbazoles

Herein, we report a general copper-catalyzed method for the tunable oxygenative rearrangement of tetrahydrocarbazoles to cyclopentyl-bearing spiroindolin-2-ones and spiroindolin-3-ones. The method demonstrates excellent chemoselectivity, regioselectivity, and product control simply by using the H2O and O2 as oxygen source, respectively. This open-flask method is safe and simple to operate, and no other chemical oxidants are required. Besides, inspired from the unique pathway of 1, 2-migration rearrangement, a highly controllable hydroxylation of indoles for the construction of C3a-hydroxyl iminium indolines was also developed. Mechanistic experiments suggest that a single-electron transfer-induced oxidation process is responsible for the tunable selectivity control.

Copper/Iodine Co-catalyzed Oxygenative Transannulation of Tryptamines Enables Direct Synthesis of Donaxaridine and Its Derivatives

We report a general copper/iodine co-catalyzed oxygenative transannulation strategy using readily available tryptamines. Molecular oxygen and water are used as oxygen sources and provide direct access to the donaxaridine scaffold and its derivatives. This methodology is applied to the efficient synthesis of the natural products donaxaridine, chimonamidine, donaxanine, donaxarine, and aline in just one or two steps. The tryptamines, albeit with oxy-sensitive dialkyl N-H groups, are selectively oxidized through a single-electron transfer dioxygenation process.

Divergent synthesis of bis(indolyl)methanes via FeIII-catalysed regioselective dehydrogenative coupling reactions: a biomimetic approach to 6,6'-bis-(debromo)-gelliusine F

The divergent dehydrogenative coupling reactions of tryptamines with the catalysis of nontoxic Fe^III salts in the presence of DDQ as the co-oxidant have been developed. Remarkably, the transformations feature a rapid and regioselective assembly of diverse 2,8'- and N1,8'-bis(indolyl) methane derivatives from readily-available starting materials by simply changing the Fe^III salt and reaction temperature. Besides, the fast reaction rate, mild reaction conditions, low catalyst cost and easy operations make this methodology quite useful. The synthetic utility was further demonstrated in the biomimetic synthesis of 6,6'-bis-(debromo)-gelliusine F.

Bispyrrolidinoindoline Epi(poly)thiodioxopiperazines (BPI-ETPs) and Simplified Mimetics: Structural Characterization, Bioactivities, and Total Synthesis

Within the 2,5-dioxopiperazine-containing natural products generated by "head-to-tail" cyclization of peptides, those derived from tryptophan allow further structural diversification due to the rich chemical reactivity of the indole heterocycle, which can generate tetracyclic fragments of hexahydropyrrolo[2,3-b]indole or pyrrolidinoindoline skeleton fused to the 2,5-dioxopiperazine. Even more complex are the dimeric bispyrrolidinoindoline epi(poly)thiodioxopiperazines (BPI-ETPs), since they feature transannular (poly)sulfide bridges connecting C3 and C6 of their 2,5-dioxopiperazine rings. Homo- and heterodimers composed of diastereomeric epi(poly)thiodioxopiperazines increase the complexity of the family. Furthermore, putative biogenetically generated downstream metabolites with C11 and C11'-hydroxylated cores, as well as deoxygenated and/or oxidized side chain counterparts, have also been described. The isolation of these complex polycyclic tryptophan-derived alkaloids from the classical sources, their structural characterization, the description of the relevant biological activities and putative biogenetic routes, and the synthetic efforts to generate and confirm their structures and also to prepare and further evaluate structurally simple analogs will be reported.

Biosynthesis of pyrroloindoline-containing natural products

Covering: up to 2022Pyrroloindoline is a privileged tricyclic indoline motif widely present in many biologically active and medicinally valuable natural products. Thus, understanding the biosynthesis of this molecule is critical for developing convenient synthetic routes, which is highly challenging for its chemical synthesis due to the presence of rich chiral centers in this molecule, especially the fully substituted chiral carbon center at the C3-position of its rigid tricyclic structure. In recent years, progress has been made in elucidating the biosynthetic pathways and enzymatic mechanisms of pyrroloindoline-containing natural products (PiNPs). This article reviews the main advances in the past few decades based on the different substitutions on the C3 position of PiNPs, especially the various key enzymatic mechanisms involved in the biosynthesis of different types of PiNPs.

Copper-Catalyzed Aerobic Selective Oxidation of Tetrahydrocarbolines

We report a safe and convenient open-flask copper-catalyzed selective oxidation/functionalization methodology for tetrahydrocarbolines and tetrahydro-β-carbolines that employs atmospheric O₂ as the terminal oxidant. The system is applicable to oxidative rearrangement of tetrahydro-β-carbolines, tetrahydrocarboline oxidation to α-alkoxy carbazoles and spirooxindoles, and Witkop oxidation. Mechanistic experiments indicated that a single-electron oxidation process is responsible for the tunable selectivity control. This copper-catalysis protocol represents a significant advance in the field of indole oxidation.

Chemical synthesis in competition with global genome mining and heterologous expression for the preparation of dimeric tryptophan-derived 2,5-dioxopiperazines

Covering: up to the end of 2021Within the 2,5-dioxopiperazines-containing natural products, those generated from tryptophan allow further structural diversification due to the rich chemical reactivity of the indole heterocycle. The great variety of natural products, ranging from simple dimeric bispyrrolidinoindoline dioxopiperazines and tryptophan-derived dioxopiperazine/pyrrolidinoindoline dioxopiperazine analogs to complex polycyclic downstream metabolites containing transannular connections between the subunits, will be covered. These natural products are constructed by Nature using hybrid polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) assembly lines. Mining of microbial genome sequences has more recently allowed the study of the metabolic routes and the discovery of their hidden biosynthetic potential. The competition (ideally, also the combined efforts) between their isolation from the cultures of the producing microorganisms after global genome mining and heterologous expression and the synthetic campaigns, has more recently allowed the successful generation and structural confirmation of these natural products. Their biological activities as well as their proposed biogenetic routes and computational studies on biogenesis will also be covered.

Copper-Catalyzed Selective Oxidative Cross-Coupling of Tryptophols and Tryptamines To Access Heterocyclic 3a,3a'-Bisindolines

The first example of cyclization cross-coupling of tryptophols and tryptamines has been realized by copper catalysis with air or oxone as the terminal oxidant, resulting in the direct construction of a new class of heterocyclic 3a,3a'-bisindolines in moderate to good yields with high chemoselectivities. A series of mechanistic control experiments were also conducted, indicating that the copper catalyst selectively coordinates with the nitrogen moiety of the tryptamine to initiate the oxidation, and a nucleophilic-alkylation process is proposed for the carbon-carbon bond-forming in the reaction. The novel synthetic strategies and molecular skeletons outlined in this work provide new ideas and concepts for the design of other useful reaction and potential drugs.

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.

Copper-Catalyzed Cyclization/Dimerization of Tryptamines with O2/Air as the Sole Oxidant: Direct Access to Complex Bispyrrolidino[2,3-b]indoline

The first transition metal catalytic one-step synthesis of the 3a, 3a'-bispyrrolidino [2,3-b] indoline scaffold via tandem cyclization/dimerization of tryptamines has been realized with the environmentally friendly O₂/air as the sole oxidant. Different from the traditional direct oxidation of indole "N-H" group by excess amount of metal salts, a copper-catalyzed oxidative cyclization reaction is developed for the formation of the radical pyrrolidinoindoline intermediate in the current strategy. The robustness and practicality of this methodology is demonstrated by the step-economic, divergent total synthesis of natural products (±)-folicanthine and meso-folicanthine.

Total Synthesis of Homo- and Heterodimeric Bispyrrolidinoindoline Dioxopiperazine Natural Products

Total synthesis and structural confirmation of homo- and heterodimeric bispyrrolidinoindoline dioxopiperazine alkaloids isolated from fungi and bacteria, namely, ditryptoleucine A, ditryptoleucine B (11), the N,N'-bis-demethylated analogue (+)-12, (-)-dibrevianamide F (13), (-)-SF-5280-451 (14), tetratryptomycin A (15), (-)-tryprophenaline (17), and (-)-SF-5280-415 (18), has been carried out starting from the corresponding bispyrrolidinoindolines derived from tryptophan. Our efforts to synthesize all possible diastereomers of the natural ditryptoleucine isolates uncovered structural factors that determine the rate and efficiency of dioxopiperazine ring formation, leading in some cases to mixtures of diastereomers by concomitant epimerization, to the formation of their putative monomeric dioxopiperazine dipeptide biogenetic precursors, and to the alternative formation of a dimer with a fused 1,3,5-triazepan-6-one heterocycle.

Cu-Catalyzed Dimerization of Indole Derived Oxime Acetate for Synthesis of Biimidazo[1,2-a]indoles

A copper-mediated cyclization and dimerization of indole derived oxime acetate was developed to generate a series of biimidazo[1,2-a]indole scaffolds with two contiguous stereogenic quaternary carbons in one step.

Application of The Dess-Martin Oxidation in Total Synthesis of Natural Products

Dess-Martin periodinane (DMP), a commercially available chemical, is frequently utilized as a mild oxidative agent for the selective oxidation of primary and secondary alcohols to their corresponding aldehydes and ketones, respectively. DMP shows several merits over other common oxidative agents such as chromiumand DMSO-based oxidants; thus, it is habitually employed in the total synthesis of natural products. In this review, we try to underscore the applications of DMP as an effective oxidant in an appropriate step (steps) in the multi-step total synthesis of natural products.

Exploration of a KI-catalyzed oxidation system for direct construction of bispyrrolidino[2,3-b]indolines and the total synthesis of (+)-WIN 64821

A facile and environmentally benign KI(cat.)/NaBO₃·4H₂O oxidation system has been developed for the tandem oxidative aminocyclization/coupling of tryptamines, affording a series of 3a,3a′-bispyrrolidino[2,3-b]indolines with high efficiency (up to 94% yield).

Double the Chemistry, Double the Fun: Structural Diversity and Biological Activity of Marine-Derived Diketopiperazine Dimers

While several marine natural products bearing the 2,5-diketopiperazine ring have been reported to date, the unique chemistry of dimeric frameworks appears to remain neglected. Frequently reported from marine-derived strains of fungi, many naturally occurring diketopiperazine dimers have been shown to display a wide spectrum of pharmacological properties, particularly within the field of cancer and antimicrobial therapy. While their structures illustrate the unmatched power of marine biosynthetic machinery, often exhibiting unsymmetrical connections with rare linkage frameworks, enhanced binding ability to a variety of pharmacologically relevant receptors has been also witnessed. The existence of a bifunctional linker to anchor two substrates, resulting in a higher concentration of pharmacophores in proximity to recognition sites of several receptors involved in human diseases, portrays this group of metabolites as privileged lead structures for advanced pre-clinical and clinical studies. Despite the structural novelty of various marine diketopiperazine dimers and their relevant bioactive properties in several models of disease, to our knowledge, this attractive subclass of compounds is reviewed here for the first time.

Oxazoline-/Copper-Catalyzed Alkoxyl Radical Generation: Solvent-Switched to Access 3a,3a'-Bisfuroindoline and 3-Alkoxyl Furoindoline

We report the first example of oxazoline-/copper-catalyzed alcohol oxidation to generate the alkoxyl radical under additive-free conditions. The resulting alkoxyl radical addition to alkene enables useful C-O bond-forming and selective C(sp³)-C(sp³) radical-radical dimerization/radical-trapping reactions, providing direct access to the 3a,3a'-bisfuro[2,3-b]indoline scaffold for the first time and a wide range of 3-alkoxyl furoindolines with high efficiency.

Design of C2-symmetric alkaloidal chiral amphiphiles and configurational effects on self-assembly

Alkaloids are a cornerstone in the development of medicinal and synthetic compounds due to their capability of specific recognition of targeted biomacromolecules, and uses in optical resolution and asymmetric reactions. To explore the untapped potential of the rigid and densely functionalized structures of alkaloids with precisely regulated configurations as optically active core scaffolds of self-assembling molecules, here we report the design, syntheses, chiroptical properties and self-assemblies of C2-symmetric alkaloidal amphiphiles with anti/syn stereochemical variations. Bispyrrolidinoindoline (BPI) was chosen as the optically active core scaffold. It was synthetically modified with hydrophobic alkyl chains and hydrophilic tetraethylene glycol tails to provide amphiphilicity. The anti/syn configurational differences in the amphiphiles significantly influenced the chiroptical, dynamic and supramolecular properties. Amphiphiles with anti-configurations responded to a solvent polarity change by altering their conformations, while the conformational changes of the syn-type amphiphiles were largely restricted. Furthermore, the anti-type amphiphile having the highest structural flexibility showed a characteristic split Cotton effect in an organic medium and formed the largest aggregates upon addition of water with a significant change in the circular dichroism (CD) profile, while amphiphiles having conformational restriction by the syn-configuration or a macrocyclic structure showed monomodal CD signals and afforded significantly smaller aggregates upon addition of water. Hence, the C2-symmetric alkaloidal BPI structure is demonstrated to be a useful core scaffold for supramolecular chemistry to design amphiphiles with controllable configurational diversity, which allows for the customization of chiroptical properties, conformational flexibility and self-assembly.

Enantioselective Radical Cyclization of Tryptamines by Visible Light-Excited Nitroxides

Nitroxides can absorb both ultraviolet (UV) and visible light, and their electron can be excited from the π-bonding orbital to the antibonding π* orbital or the n-bonding orbital to the antibonding π* orbital, respectively. Despite the reported UV-induced hydrogen atom transfer (HAT) process, the potential of nitroxides for visible light-excited photosynthesis is underexplored. Here we demonstrate that nitroxide can convert indole to its radical through a visible light-induced HAT process. A chiral phosphoric acid-catalyzed cyclization of the in situ-formed imine radical, followed by trapping by another molecule of nitroxide, provides the product in high yield and enantioselectivity. To highlight the novelty and efficiency of this strategy, an asymmetric total synthesis of natural product (-)-verrupyrroloindoline was accomplished in 5 steps.

Access to 3a-Benzoylmethyl Pyrrolidino[2,3- b]indolines via CuII-Catalyzed Radical Annulation/C3-Functionalization Sequence

A Cu^II-catalyzed radical annulation/C3-functionalization cascade of tryptamine derivatives with aryl ethylene is reported. The mild catalytic system enables the facile construction of 3a-benzoylmethylpyrrolidino[2,3- b]indolines with excellent chemo- and regioselectivities. Remarkably, this novel method utilizes earth-abundant and inexpensive cupric salt as the catalyst and air as the co-oxidant, rendering the process highly environmentally friendly and atom economic. Presumably, the reaction proceeds through Cu^II-initiated formation of pyrrolidino[2,3- b]indolines radical intermediate I, which is successively trapped by aryl ethylene and O₂ to form the product. An ¹⁸O₂-labeling experiment and several control experiments were designed to support the mechanistic proposal.

Methods Utilizing First-Row Transition Metals in Natural Product Total Synthesis

First-row transition-metal-mediated reactions constitute an important and growing area of research due to the low cost, low toxicity, and exceptional synthetic versatility of these metals. Currently, there is considerable effort to replace existing precious-metal-catalyzed reactions with first-row analogs. More importantly, there are a plethora of unique transformations mediated by first-row metals, which have no classical second- or third-row counterpart. Herein, the application of first-row metal-mediated methods to the total synthesis of natural products is discussed. This Review is intended to highlight strategic uses of these metals to realize efficient syntheses and highlight the future potential of these reagents and catalysts in organic synthesis.

Recent advances on the total synthesis of alkaloids in mainland China

Alkaloids are a large family of natural products that mostly contain basic nitrogen atoms. Because of their intriguing structures and important functions, they have long been popular targets for synthetic organic chemists. China's chemists have made significant progress in the area of alkaloid synthesis over past decades. In this article, selected total syntheses of alkaloids from research groups in mainland China during the period 2011–16 are highlighted.

Regioselective synthesis of 2,3′-biindoles mediated by an NBS-induced homo-coupling of indoles

An efficient method for the synthesis of 2,3′-biindole and [3,2-a]carbazole derivatives via an NBS-induced homo-coupling of indoles with high regioselectivity.

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.

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.

Total Synthesis of (+)-Chimonanthine, (+)-Folicanthine, and (-)-Calycanthine

Facile, straightforward, and asymmetric total syntheses of (+)-chimonanthine (1), (+)-folicanthine (2), and (-)-calycanthine (3) were accomplished in four to five steps from commercially available tryptamine. The synthesis features copper-mediated asymmetric cyclodimerization of chiral tryptamine derivative, which established a new entry into constructing the sterically hindered vicinal quaternary stereogenic carbon centers of dimeric hexahydropyrroloindole alkaloids in one procedure. An unprecedented base-induced isomerization from the chimonanthine skeleton to the calycanthine skeleton was observed and facilitated the synthesis of (-)-calycanthine (3).

Copper catalyzed sequential arylation−oxidative dimerization of o-haloanilides: synthesis of dimeric HPI alkaloids

A copper catalyzed sequential arylation−oxidative dimerization reaction was developed as the key step for the synthesis of hexahydropyrroloindole alkaloids.

Synthesis of naked amino-pyrroloindoline via direct aminocyclization of tryptamine

Direct access to unprotected amino-pyrroloindolineviaaminocyclization of tryptamine and tryptophan catalyzed by Rh₂(esp)₂has been described usingO-(2,4-dinitrophenyl)hydroxylamine (DPH) as the nitrogen source.