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

A General Synthetic Strategy toward the Truxillate Natural Products via Solid-State Photocycloadditions

The truxillates constitute a large class of dimeric natural products featuring a central, highly substituted cyclobutane core. In principle, these structures could be efficiently synthesized via [2 + 2] photocycloaddition. However, the difficulty in controlling the high-energy electronically excited reactive intermediates in the solution state can lead to poor regio- and diastereocontrol. This has limited the use of photocycloaddition methodology toward the synthesis of this important class of natural products. Herein, we demonstrate that acid-controlled precipitation of C-acyl imidazoles promotes a highly selective solid-state photocycloaddition, and the products of this reaction can be quickly transformed into truxillate natural products.

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.

Controllable skeletal reorganizations in natural product synthesis

Covering: 2016 to 2023The synthetic chemistry community is always in pursuit of efficient routes to natural products. Among the many available general strategies, skeletal reorganization, which involves the formation, cleavage, and migration of C-C and C-heteroatom bonds, stands out as a particularly useful approach for the efficient assembly of molecular skeletons. In addition, it allows for late-stage modification of natural products for quick access to other family members or unnatural derivatives. This review summarizes efficient syntheses of steroid, terpenoid, and alkaloid natural products that have been achieved by means of this strategy in the past eight years. Our goal is to illustrate the strategy's potency and reveal the spectacular human ingenuity demonstrated in its use and development.

Single-atom logic for heterocycle editing

Medicinal chemistry continues to be impacted by new synthetic methods. Particularly sought after, especially at the drug discovery stage, is the ability to enact the desired chemical transformations in a concise and chemospecific fashion. To this end, the field of organic synthesis has become captivated by the idea of 'molecular editing'-to rapidly build onto, change or prune molecules one atom at a time using transformations that are mild and selective enough to be employed at the late stages of a synthetic sequence. In this Review, the definition and categorization of a particularly promising subclass of molecular editing reactions, termed 'single-atom skeletal editing', are proposed. Although skeletal editing applies to both cyclic and acyclic compounds, this Review focuses on heterocycles, both for their centrality in medicinal chemistry and for the definitional clarity afforded by a focus on ring systems. A classification system is presented by highlighting methods (both historically important examples and recent advances) that achieve such transformations, with the goal to spark interest and inspire further development in this growing field.

Light-driven transition-metal-free direct decarbonylation of unstrained diaryl ketones via a dual C–C bond cleavage

The cleavage and formation of carbon−carbon bonds have emerged as powerful tools for structural modifications in organic synthesis. Although transition−metal−catalyzed decarbonylation of unstrained diaryl ketones provides a viable protocol to construct biaryl structures, the use of expensive catalyst and high temperature (>140 ^oC) have greatly limited their universal applicability. Moreover, the direct activation of two inert C − C bonds in diaryl ketones without the assistance of metal catalyst has been a great challenge due to the inherent stability of C − C bonds (nonpolar, thermo-dynamically stable, and kinetically inert). Here we report an efficient light-driven transition-metal-free strategy for decarbonylation of unstrained diaryl ketones to construct biaryl compounds through dual inert C − C bonds cleavage. This reaction featured mild reaction conditions, easy-to-handle reactants and reagents, and excellent functional groups tolerance. The mechanistic investigation and DFT calculation suggest that this strategy proceeds through the formation of dioxy radical intermediate via a single-electron-transfer (SET) process between photo-excited diaryl ketone and DBU mediated by DMSO, followed by removal of CO₂ to construct biaryl compounds.

The cleavage and formation of carbon−carbon bonds is an important strategy for structural modifications in organic syntheses. Herein, the authors present a photoinduced method to construct biaryl compounds through dual inert C−C bond cleavage.

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.

Unexpected chiral vicinal tetrasubstituted diamines via borylcopper-mediated homocoupling of isatin imines

Addressing the asymmetric synthesis of oxindole-based α-aminoboronic acids, instead of the expected products we disclosed the efficient homocoupling of oxindole-based N-tert-butanesulfinyl imines, with the generation of chiral, quaternary 1,2-diamines in a mild and completely stereoselective way. The obtained 3,3'-bisoxindole derivatives were fully characterized by NMR and single-crystal X-ray diffraction analysis and proved to be single diastereoisomers and atropisomers. A plausible mechanism for the one-pot Cu(II)-catalyzed Bpin addition to the isatin-derived ketimine substrate and subsequent homocoupling is described.

Natural product anticipation through synthesis

Natural product synthesis remains one of the most vibrant and intellectually rewarding areas of chemistry, although the justifications for pursuing it have evolved over time. In the early years, the emphasis lay on structure elucidation and confirmation through synthesis, as exemplified by celebrated studies on cocaine, morphine, strychnine and chlorophyll. This was followed by a phase where the sheer demonstration that highly complex molecules could be recreated in the laboratory in a rational manner was enough to justify the economic expense and intellectual agonies of a synthesis. Since then, syntheses of natural products have served as platforms for the demonstration of elegant strategies, for inventing new methodology 'on the fly' or to demonstrate the usefulness and scope of methods established with simpler molecules. We now add another aspect that we find fascinating, viz. 'natural product anticipation'. In this Review, we survey cases where the synthesis of a compound in the laboratory has preceded its isolation from nature. The focus of our Review lies on examples where this anticipation of a natural product has triggered a successful search or where synthesis and isolation have occurred independently. Finally, we highlight cases where a potential natural product structure has been suggested as a result of synthetic endeavours but not yet confirmed by isolation, inviting further collaborations between synthetic and natural product chemists.

An NHC-Catalyzed [3+2] Cyclization of β-Disubstituted Enals with Benzoyl Cyanides

The NHC-catalyzed asymmetric [3+2] cyclization of benzoyl cyanides to homoenolate generated in-situ from enals was reported. This methodology leads to the efficient construction of a series of chiral cyclic compounds...

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.

Applications of Norrish type I and II reactions in the total synthesis of natural products: a review

Natural products and their analogue have played a key role in the drug discovery and development process. In the laboratory, the total synthesis of secondary metabolites is very useful in ascertaining the hypothetical complex structure of molecules of natural origin. Total synthesis of natural products using Norrish type I and II reactions as a crucial step has been explored in this overview. Norrish reactions are important photo-induced transformations of carbonyl compounds in organic synthetic chemistry and are connected in numerous industrially and biologically relevant procedures and the processing of carbonyl compounds in the atmosphere. The present review tries to focus on the brilliant applications of Norrish type I and II photochemical reactions as a key step in the total synthesis of natural products and highlights on natural sources, structures, and biological activities of the promising natural products for the first time elegantly.

Cross-Dehydrogenative Cyclization-Dimerization Cascade Sequence for the Synthesis of Symmetrical 3,3'-Bisoxindoles

The synthesis of symmetrical 3,3'-bisoxindoles from simple acyclic β-oxoanilides is reported. The described method forges three new C-C bonds in a single step via a sequential Mn(OAc)₃·2H₂O mediated oxidative radical cyclization-fragmentation-dimerization process. The scope of this reaction is demonstrated in the preparation of a variety of 3,3'-bisoxindoles, as well as its application toward the formal synthesis of the Calycanthaceae alkaloid, (±)-folicanthine.

Taming Radical Pairs in the Crystalline Solid State: Discovery and Total Synthesis of Psychotriadine

Solid-state photodecarbonylation is an attractive but underutilized methodology to forge hindered C-C bonds in complex molecules. This study discloses the use of this reaction to assemble the vicinal quaternary stereocenter motif present in bis(cyclotryptamine) alkaloids. Our strategy was enabled by experimental and computational investigations of the role of substrate conformation on the success or failure of the solid-state photodecarbonylation reaction. This informed a crystal engineering strategy to optimize the key step of the total synthesis. Ultimately, this endeavor culminated in the successful synthesis of the bis(cyclotryptamine) alkaloid "psychotriadine," which features the elusive piperidinoindoline framework. Psychotriadine, a previously unknown compound, was identified in the extracts of the flower Psychotria colorata, suggesting it is a naturally occurring metabolite.

Cephalodiones A-D: Compound Characterization and Semisynthesis by [6+6] Cycloaddition

Cephalodiones A-D (1-4), the first example of C₁₉ -norditerpenoid dimers, were isolated and fully characterized from a Cephalotaxus plant. These new skeletal natural products shared a unique tricyclo[6.4.1.1^2,7 ]tetradeca-3,5,9,11-tetraene-13,14-dione core that was capped in both ends with rigid multicyclic ring systems either C₂ -symmetrically or asymmetrically. Compounds 1-4 were proposed to be biosynthetically produced by the [6+6]-cycloaddition of two identical C₁₉ -norditerpenoid troponoids, which was validated by the semisyntheses of dimers 2-4. Moreover, some compounds showed significant inhibition on Th17 cell differentiation.

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.

Concise Total Synthesis of Tronocarpine

A concise total synthesis of tronocarpine, a chippiine-type indole alkaloid, was accomplished. The key feature of this total synthesis is a one-pot construction of the pentacyclic skeleton containing an azabicyclo[3.3.1]nonane core by tandem cyclization from an indole derivative with all carbon side chains and functional groups. This tandem cyclization consists of α,β-unsaturated aldehyde formation, intramolecular aldol reaction, six-membered lactamization, azide reduction, and seven-membered lactamization. The stereochemical outcome in this tandem cyclization is controlled by the stereocenter at the C14 position. This strategy can be utilized to synthesize other chippiine-type alkaloids with azabicyclo[3.3.1]nonane skeletons.

Scalable Synthesis of Vicinal Quaternary Stereocenters via the Solid-State Photodecarbonylation of a Crystalline Hexasubstituted Ketone

We report the synthesis and stereospecific solid-state photodecarbonylation of a hexasubstituted ketone featuring six distinct α-substituents. The photoproduct of the solid-state transformation features vicinal all-carbon quaternary stereocenters. While reactions carried out in bulk powders and aqueous crystalline suspensions were complicated by secondary photochemistry of the primary photoproduct, optimal conditions provided good yields and recyclable starting material. Subsequent transformations of the α-substituents having orthogonal chemical reactivity illustrate the potential of this transformation toward constructing complex architectures.