Domino Michael/Mannich/N-Alkylation Route to the Tetrahydrocarbazole Framework of Aspidosperma Alkaloids: Concise Total Syntheses of (−)-Aspidospermidine, (−)-Tabersonine, and (−)-Vincadifformine

Aza-Achmatowicz rearrangement coupled with intermolecular aza-Friedel-Crafts enables total syntheses of uleine and aspidosperma alkaloids

Aspidosperma and uleine alkaloids belong to the large family of monoterpene indole alkaloids with diverse biological activities and thus have attracted extensive synthetic interest. Reported is the development of a new synthetic strategy that allows direct C3-C2' linkage of indoles with functionalized 2-hydroxypiperidines to construct the core common to all aspidoserma and uleine alkaloids. Such indole-piperidine linkage is enabled by coupling aza-Achmatowicz rearrangement (AAR) with indoles via an intermolecular aza-Friedel-Crafts (iAFC) reaction. This AAR-iAFC reaction proceeds under mild acidic conditions with wide tolerance of functional groups (33 examples). The synthetic application of the AAR-iAFC method was demonstrated with collective total syntheses of 3 uleine-type and 6 aspidosperma alkaloids: (+)-3-epi-N-nor-dasycarpidone, (+)-3-epi-dasycarpidone, (+)-3-epi-uleine, 1,2-didehydropseudoaspidospermidine, 1,2-dehydroaspidospermidine, vincadifformine, winchinine B, aspidospermidine, and N-acetylaspidospermidine. We expect that this AAR-iAFC strategy is applicable to other monoterpene indole alkaloids with the C3-C2' linkage of indoles and piperidines.

Total Synthesis of Strempeliopidine and Non-Natural Stereoisomers through a Convergent Petasis Borono-Mannich Reaction

Strempeliopidine is a member of the monoterpenoid bisindole alkaloid family, a class of natural products that have been shown to elicit an array of biological responses including modulating protein-protein interactions in human cancer cells. Our synthesis of strempeliopidine leverages palladium-catalyzed decarboxylative asymmetric allylic alkylations to install the requisite all-carbon quaternary centers found in each of the two monomeric natural products, aspidospermidine and eburnamine. Initial studies employing Suzuki-Miyaura cross-coupling followed by diastereoselective hydrogenation provided evidence for a structural reassignment of the natural product. Our final synthetic sequence employs a diastereoselective Petasis borono-Mannich reaction to couple eburnamine to a trifluoroborate aspidospermidine derivative. These convergent approaches enabled the synthesis of eight diastereomers of this heterodimer and offer support for the reassignment of the absolute configuration of strempeliopidine.

Asymmetric Total Synthesis of (+)-Epiibogamine Enabled by Three-Component Domino Michael/Michael/Mannich Annulation of N-Sulfinyl Metallosilylenamines

The iboga alkaloids are promising antiaddictive and neuroregeneration candidates for medical treatment. There is a lack of studies for C20-epi iboga alkaloids due to the synthetic difficulties. Herein we report the shortest total synthesis of (+)-epiibogamine in seven steps from trimethyl orthobutyrate. The novel N-sulfinyl silylenamine reagent enabled the key step, with three-component domino Michael/Michael/Mannich annulation providing the 1-amino-2,4-diester scaffold with four new chiral centers, and access to the isoquinuclidine in high yield (84%) and diastereoselectivity (>95:5 dr).

Indole-Containing Natural Products 2019-2022: Isolations, Reappraisals, Syntheses, and Biological Activities

Indole alkaloids represent a large subset of natural products, with more than 4100 known compounds. The majority of these alkaloids are biologically active, with some exhibiting excellent antitumor, antibacterial, antiviral, antifungal, and antiplasmodial activities. Consequently, the natural products of this class have attracted considerable attention as potential leads for novel therapeutics and are routinely isolated, characterized, and profiled to gauge their biological potential. However, data on indole alkaloids, their various structures, and bioactivities are complex due to their diverse sources, such as plants, fungi, bacteria, sponges, tunicates, and bryozoans; thus, isolation methods produce an incredible trove of information. The situation is exacerbated when synthetic derivatives, as well as their structures, bioactivities, and synthetic schemes, are considered. Thus, to make such data comprehensive and inform researchers about the current field's state, this review summarizes recent reports on novel indole alkaloids. It deals with the isolation and characterization of 250 novel indole alkaloids, a reappraisal of previously reported compounds, and total syntheses of indole alkaloids. In addition, several syntheses and semi-syntheses of indole-containing derivatives and their bioactivities are reported between January 2019 and July 2022.

Enantioselective Total Synthesis of (-)-Limaspermidine and (-)-Kopsinine by a Nitroaryl Transfer Cascade Strategy

We report an intramolecular conjugate addition/Truce-Smiles/E1cb cascade of 2-nitrobenzenesulfonamide-functionalized cyclohexenones as a new entry to the core scaffold of monoterpene indole alkaloids. The method was applied to the asymmetric total synthesis of (-)-limaspermidine, (-)-kopsinilam, and (-)-kopsinine, as well as the framework of the kopsifoline alkaloids, thus highlighting its complementarity to existing approaches involving the use of indole-based starting materials or the interrupted Fischer indole synthesis. Furthermore, we show that the cascade tolerates various substituents on the nitroarene, opening the way to other natural products as well as non-natural analogues.

Site directed mutagenesis of Catharanthus roseus (+)-vincadifformine 19-hydroxylase (CYP71BY3) results in two distinct enzymatic functions

The most abundant monoterpenoid indole alkaloids (MIAs) in Catharanthus roseus roots include lochnericine and (+)-echitovenine. The formation of (+)-echitovenine involves a 3-step pathway including (+)-vincadifformine-19-hydroxylase (V19H) that differentiates it from a parallel pathway involved in the formation of lochnericine, hörhammericine and its O-acetylated derivative. Homology based modeling and docking experiments in the present study show that (+) and (-) vincadifformine can occupy the V19H active site and is proven experimentally by showing that (-)-vincadifformine is a competitive inhibitor of V19H. Comparative modeling of V19H with tabersonine 3-oxidase (T3O) and tabersonine 19-hydroxylase (T19H) that accept (-)-aspidosperma MIAs identified four conserved amino acid residues in T3O and T19H that were different in the V19H binding site and were used to generate a series of single-, double-, or four-point mutations in V19H. While all mutants retained their ability to convert (+)-vincadifformine to (+)-minovincinine only the four-point mutant gained T3O activity enabling it to convert (-)-tabersonine to tabersonine 2,3-epoxide. The gain of T3O-like activity following mutagenesis without the loss of V19H activity supports the hypothesis that V19H shares a common ancestor to T3O which is involved in vindoline biosynthesis in C. roseus leaves.

Diastereoselective Hydroxylation of N-tert-Butanesulfinyl Imines with 2-(Phenylsulfonyl)-3-phenyloxaziridine (Davis Oxaziridine)

The diastereoselective α-hydroxylation of N-tert-butanesulfinyl metallodienenamine and metalloenamines with Davis oxaziridine affords α-hydroxy N-sulfinyl imines with 50-88% yield and up to 98:2 diastereomeric ratio. Dramatic changes in diastereoselectivity and stereoselectivity were observed by choice of metal bases. The mechanistic understanding for the switch in diastereoselectivity was assisted by DFT computational modeling, which suggests the facial approach is governed by aza-enolate geometry. A one-pot protocol for the asymmetric synthesis of 1,2-amino alcohols is described.

Semi-syntheses and interrogation of indole-substituted Aspidosperma terpenoid alkaloids

We demonstrated here a series of Aspidosperma terpenoid alkaloids can be quickly prepared using semisynthesis from naturally sourced tabersonine, featuring multiple oxygen-based substituents on the indole ring such as hydroxy and methoxy groups. This panel of complex compounds enabled the exploration of indole modifications to optimize the indole alkaloids' anticancer activity, generating lead compounds (e.g., with C15-hydroxy, C16-methoxy, and/or C17-methoxy derivatizations) that potently inhibit cancer cell line growth in the single-digit micromolar range. These results can help guide the development of Aspidosperma terpenoid alkaloid therapeutics. Furthermore, this synthetic approach features late-stage facile derivatization on complex natural product molecules, providing a versatile path to indole derivatization of this family of alkaloids with diverse chemical functionalities for future medicinal chemistry and chemical biology discoveries.

Aspidosperma and Strychnos alkaloids: Chemistry and biology

Of Nature's nearly 3000 unique monoterpene indole alkaloids derived from tryptophan, those members belonging to the Aspidosperma and Strychnos families continue to impact the fields of natural products (i.e., isolation, structure determination, biosynthesis) and organic chemistry (i.e., chemical synthesis, methodology development) among others. This review covers the biological activity (Section 2), biosynthesis (Section 3), and synthesis of both classical and novel Aspidosperma (Section 4), Strychnos (Section 5), and selected bis-indole (Section 6) alkaloids. Technological advancements in genetic sequencing and bioinformatics have deepened our understanding of how Nature assembles these intriguing molecules. The proliferation of innovative synthetic strategies and tactics for the synthesis of the alkaloids covered in this review, which include contributions from over fifty research groups from around the world, are a testament to the creative power and technical skills of synthetic organic chemists. To be sure, Nature-the Supreme molecular architect and source of a dazzling array of irresistible chemical logic puzzles-continues to inspire scientists across multiple disciplines and will certainly continue to do so for the foreseeable future.

Domino Michael/Mannich Annulation Reaction of N-Sulfinyl Lithiodienamines

The domino Michael/Mannich (DMM) annulation reaction between an N-sulfinyl lithiodienamine and an electrophilic alkene is developed for the synthesis of chiral 2-amino cyclohexenes, a key building block in asymmetric synthesis. The DMM reaction proceeds at low temperature while maintaining the stereochemical fidelity. The product functionalized amino cyclohexenes, here obtained in 55-82% yield with diastereomeric ratios as high as >19:1.

Catalyzed and uncatalyzed procedures for the syntheses of isomeric covalent multi-indolyl hetero non-metallides: an account

Two or more indole molecules tailored to a single non-metal central atom, through any of their C2–7 positions are not only structurally engaging but also constitute a class of important pharmacophores. Although the body of such multi-indolyl non-metallide molecules are largely shared to the anticancer agent bis(indolyl)methane, other heteroatomic analogs also possess similar medicinal properties. This concise review will discuss various catalytic and uncatalytic synthetic strategies adopted for the synthesis of the non-ionic (non-metallic) versions of these important molecules till date.

Total Syntheses of Aspidospermidine, N-Methylaspidospermidine, N-Acetylaspidospermidine, and Aspidospermine via a Tandem Cyclization of Tryptamine-Ynamide

The total syntheses of aspidospermidine, N-methylaspidospermidine, N-acetylaspidospermidine, and aspidospermine were achieved from a common pentacyclic indoline intermediate. The common pentacyclic indoline intermediate was synthesized on a gram scale through a Stork-enamine alkylation of 1H-pyrrolo[2,3-d]carbazole derivatives, which were prepared through a Brønsted acid-catalyzed tandem cyclization of tryptamine-ynamide. The scalable synthesis of 1H-pyrrolo[2,3-d]carbazole afforded facile access and a practical approach to the Aspidosperma indole alkaloid family.

Water Modulated Diastereoselective Synthesis of cis/trans-Spiro[indoline-3,6'-naphtho[2,3-c]carbazoles]

p-TsOH catalyzed Diels-Alder reaction of 2-(1-alkylindol-3-yl)naphthalene-1,4-diones and 3-phenacylideneoxindoles showed fascinating diastereoselectivity. The reaction with the hydrated p-TsOH afforded trans-isomers of dihydrospiro[indoline-3,6'-naphtho[2,3-c]carbazoles] as major products. Alternatively, the reaction with anhydrous p-TsOH under a Dean and Stark apparatus predominately gave cis-isomer of dihydrospiro[indoline-3,6'-naphtho[2,3-c]carbazoles]. On the other hand, the similar p-TsOH catalyzed reaction of 2-(indol-3-yl)naphthalene-1,4-diones with 3-arylideneindolin-2-ones afforded cis/trans-isomers of dihydrospiro[indoline-3,6'-naphtho[2,3-c]carbazoles]. Additionally, the p-TsOH catalyzed reaction of 2-(indol-3-yl)naphthalene-1,4-diones with 2-arylidene-1,3-indanediones gave the expected spiro[indene-2,6'-naphtho[2,3-c]carbazoles] in satisfactory yields.

Multicomponent Reaction for Diastereoselective Synthesis of Spiro[carbazole-3,4'-pyrazoles] and Spiro[carbazole-3,4'-thiazoles]

In the presence of copper sulfate, the three-component reaction of aromatic aldehydes, ethylindole-3-acetate and 4-arylidene-5-methyl-2-phenylpyrazol-3-ones, in refluxing toluene afforded spiro[carbazole-3,4'-pyrazoles] in good yields with high diastereoselectivity. More importantly, the similar CuSO₄ promoted the four-component reaction of two molecular aromatic aldehydes with ethylindole-3-acetate and 5-methyl-2-phenyl-pyrazol-3-one resulted in 2,4-diarylspiro[carbazole-3,4'-pyrazoles] in satisfactory yields. Additionally, CuSO₄ promoted the four-component reaction of two molecular aromatic aldehydes, ethylindole-3-acetate and 2-phenylthiazol-4-one, in refluxing toluene gave 2,4-diarylspiro[carbazole-3,4'-thiazoles] with diastereomeric ratios in the range of 3:1 to 20:1.

Natural spirocyclic alkaloids and polyphenols as multi target dementia leads

The U rhynchophylla, U tomentosa, Isatis indigotica Fortune, Voacanga Africana, herbal constituents, fungal extracts from Aspergillus duricaulis culture media, include spirooxindoles, polyphenols or bridged spirocyclic alkaloids. Their constituents exhibit specific and synergistic multiple neuroprotective properties including inhibiting of Aβ fibril induced cytotoxicity, NMDA receptor inhibition in mice models of Alzheimer's disease (AD). The pioneering research from Woodward to Waldmann has advanced the synthesis of spirocyclic alkaloids. Furthermore, the elucidation of the genetic analysis, biochemical pathways that links strictosidine to the alkaloids akuammicine, stemmadenine, tabersonine, catharanthine, will now enable the biotechnological generation, also stimulate synthesis of related bridged spirocyclic alkaloids for medicinal investigations. From the value of spirocyclic structures as multi target dementia leads, we hypothesise that simpler Lipinski-like natural/synthetic alkaloid analogues may likewise be discovered that provide neurocognitive enhancing activities against dementia and AD.

N-tert-Butanesulfinyl imines in the asymmetric synthesis of nitrogen-containing heterocycles

The synthesis of nitrogen-containing heterocycles, including natural alkaloids and other compounds presenting different types of biological activities have proved to be successful employing chiral sulfinyl imines derived from tert-butanesulfinamide. These imines are versatile chiral auxiliaries and have been extensively used as eletrophiles in a wide range of reactions. The electron-withdrawing sulfinyl group facilitates the nucleophilic addition of organometallic compounds to the iminic carbon with high diastereoisomeric excess and the free amines obtained after an easy removal of the tert-butanesulfinyl group can be transformed into enantioenriched nitrogen-containing heterocycles. The goal of this review is to the highlight enantioselective syntheses of heterocycles involving the use of chiral N-tert-butanesulfinyl imines as reaction intermediates, including the synthesis of several natural products. The synthesis of nitrogen-containing heterocycles in which the nitrogen atom is not provided by the chiral imine will not be considered in this review. The sections are organized according to the size of the heterocycles. The present work will comprehensively cover the most pertinent contributions to this research area from 2012 to 2020. We regret in advance that some contributions are excluded in order to maintain a concise format.

A Concise Enantioselective Total Synthesis of (-)-Deoxoapodine

We have established a highly convergent 10-step route for the total synthesis of (-)-deoxoapodine, which is a hexacyclic aspidosperma alkaloid. The quaternary C5 center of the characteristic tetrahydrofuran ring was constructed by a chiral-phosphoric-acid-catalyzed enantioselective bromocycloetherification in a 5-endo fashion and subsequent allylation by using the Keck protocol. Construction of the aspidosperma skeleton features the formation of a nine-membered lactam by a catalytic C-H palladation/alkylation cascade at the indole 2-position and an iron-catalyzed oxidative transannular reaction at a late-stage of the synthesis.

Synthetic Approaches to Tricyclic Aminoketones in the Total Synthesis of Aspidosperma and Kopsia Alkaloids

Aspidosperma and kopsia alkaloids are significant functional molecules because of their potent biological activities. Their intricate structures present an intrinsic synthetic challenge and thus attract significant attention from synthetic organic academic community. Over the past decades, a series of elegant strategies has been developed, in particular, the Stork's original Fischer indolization of tricyclic aminoketones 1. Herein, we report a comprehensive review on various synthetic approaches access to tricyclic aminoketones 1 and provide a practical guidance to readers whose are interested in employing tricyclic aminoketones 1 as versatile building blocks in the realm of total synthesis of aspidosperma, kopsia and structurally related alkaloids.

Chiral N-tert-Butylsulfinyl Imines: New Discoveries

In this account the reactions of chiral N-tert-butylsulfinyl imines with organometallic reagents such as organoalkaline (lithium, sodium, potassium and cesium derivatives), organomagnesium, organozinc, organoboron, organoaluminium, organoindium and organosilicon compounds is comprehensively described. The reactivity in all cases is derived to synthetic applications in order to prepare interesting organic nitrogenated molecules, especially in the field of alkaloid compounds.

Identifying Genes Involved in alkaloid Biosynthesis in Vinca minor Through Transcriptomics and Gene Co-Expression Analysis

The lesser periwinkle Vinca minor accumulates numerous monoterpene indole alkaloids (MIAs) including the vasodilator vincamine. While the biosynthetic pathway of MIAs has been largely elucidated in other Apocynaceae such as Catharanthus roseus, the counterpart in V. minor remains mostly unknown, especially for reactions leading to MIAs specific to this plant. As a consequence, we generated a comprehensive V. minor transcriptome elaborated from eight distinct samples including roots, old and young leaves exposed to low or high light exposure conditions. This optimized resource exhibits an improved completeness compared to already published ones. Through homology-based searches using C. roseus genes as bait, we predicted candidate genes for all common steps of the MIA pathway as illustrated by the cloning of a tabersonine/vincadifformine 16-O-methyltransferase (Vm16OMT) isoform. The functional validation of this enzyme revealed its capacity of methylating 16-hydroxylated derivatives of tabersonine, vincadifformine and lochnericine with a Km 0.94 ± 0.06 µM for 16-hydroxytabersonine. Furthermore, by combining expression of fusions with yellow fluorescent proteins and interaction assays, we established that Vm16OMT is located in the cytosol and forms homodimers. Finally, a gene co-expression network was performed to identify candidate genes of the missing V. minor biosynthetic steps to guide MIA pathway elucidation.

Synthesis of (-)-Melodinine K: A Case Study of Efficiency in Natural Product Synthesis

Efficiency is a key organizing principle in modern natural product synthesis. Practical criteria include time, cost, and effort expended to synthesize the target, which tracks with step-count and scale. The execution of a natural product synthesis, that is, the sum and identity of each reaction employed therein, falls along a continuum of chemical (abiotic) synthesis on one extreme, followed by the hybrid chemoenzymatic approach, and ultimately biological (biosynthesis) on the other, acknowledging the first synthesis belongs to Nature. Starting materials also span a continuum of structural complexity approaching the target with constituent elements on one extreme, followed by petroleum-derived and "chiral pool" building blocks, and complex natural products (i.e., semisynthesis) on the other. Herein, we detail our approach toward realizing the first synthesis of (-)-melodinine K, a complex bis-indole alkaloid. The total syntheses of monomers (-)-tabersonine and (-)-16-methoxytabersonine employing our domino Michael/Mannich annulation is described. Isolation of (-)-tabersonine from Voacanga africana and strategic biotransformation with tabersonine 16-hydroxylase for site-specific C-H oxidation enabled a scalable route. The Polonovski-Potier reaction was employed in biomimetic fragment coupling. Subsequent manipulations delivered the target. We conclude with a discussion of efficiency in natural products synthesis and how chemical and biological technologies define the synthetic frontier.

Structurally Diverse Indole Alkaloids with Vasorelaxant Activity from Melodinus hemsleyanus

Six new structurally diverse indole alkaloids, melohemsines J-M (1-4), 11-hydroxy-Δ¹⁴-vincamine (5), and 11-hydroxy-16-epi-Δ¹⁴-vincamine (6), and 15 known alkaloids were isolated from the leaves and twigs of Melodinus hemsleyanus Diels. These new compounds and their absolute configurations were determined through spectroscopic data analyses, X-ray diffraction, and computational methods. Melohemsine J (1) is the first example of a melodinus-type alkaloid possessing a 6/6/5/5/6/5 hexacyclic skeleton and containing a tetrahydrofuro[2,3-b]pyridine-2(3H)-one unit. Melohemsine K (2) is an unusual aspidosperma-type alkaloid possessing a 6/5/6/5/5 pentacyclic architecture with a contracted E ring (loss of CH₂). Compounds 5-10 and 16 exhibited vasorelaxant activities with EC₅₀ values of 0.8-3.8 μM. In addition, compound 4 displayed moderate cytotoxicity toward the tumor cell lines HepG2 and A-549 with EC₅₀ values of 18.7 and 28.7 μM, respectively.

Total Syntheses of Dihydroindole Aspidosperma Alkaloids: Reductive Interrupted Fischer Indolization Followed by Redox Diversification

We report a novel reductive interrupted Fischer indolization process for the concise assembly of the 20-oxoaspidospermidine framework. This rapid complexity generating route paves the way toward various dihydroindole Aspidosperma alkaloids with different C-5 side chain redox patterns. The end-game redox modulations were accomplished by modified Wolff-Kishner reaction and photo-Wolff rearrangement, enabling the total synthesis of (-)-aspidospermidine, (-)-limaspermidine, and (+)-17-demethoxy-N-acetylcylindrocarine and the formal total synthesis of (-)-1-acetylaspidoalbidine.

Palladium-catalyzed asymmetric dearomative cyclization in natural product synthesis

Asymmetric catalysis is a rapidly growing field in modern organic chemistry and has been indispensable for the synthesis of enantioenriched materials to meet demands from the academies to pharmaceutical industries. Asymmetric dearomative cyclization catalyzed by transition metals has been a hot research area in the last decade. Fascinated by its ability to construct sterically hindered quaternary stereogenic center(s) through dearomatization and simultaneously forging new ring structure(s) through cyclization, palladium-catalyzed asymmetric dearomative cyclization has been applied to the synthesis of structurally complicated natural products and it is increasingly prevalent in the literature. In particular, the resultant product from dearomative cyclization, which usually carries one or more unsaturated C-C bond(s), allows further functional group transformations. Previously reported applications of palladium-catalyzed asymmetric dearomative cyclization in natural product synthesis are presented here and discussed in depth.

Asymmetric Total Syntheses of Kopsane Alkaloids via a PtCl2 -Catalyzed Intramolecular [3+2] Cycloaddition

A concise and asymmetric total synthesis of five kopsane alkaloids that share a unique heptacyclic caged ring system was accomplished. The key transformation in the sequence involved a remarkable PtCl₂ -catalyzed intramolecular [3+2] cycloaddition, which allowed for the rapid assembly of pentacyclic carbon skeletons bearing 2,3-quaternary functionalized indoline. Expeditious construction of diverse indoline scaffolds with excellent control of diastereoselectivity demonstrated the broad scope and versatility of this key transformation.

Copper-catalyzed tandem cis-carbometallation/cyclization of imine-ynamides with arylboronic acids

An efficient copper-catalyzed tandem regioselective cis-carbometallation/cyclization of imine-ynamides with arylboronic acids has been developed. This method leads to a facile and practical synthesis of valuable 2,3-disubstituted indolines in moderate to excellent yields and features a broad substrate scope and wide functional group tolerance. Other significant features of this protocol include the use of readily available starting materials, high flexibility, simple procedure and mild reaction conditions.

The indole-based subincanadine alkaloids and their biogenetic congeners

The tryptamine-derived polycyclic bridged bioactive indole alkaloids subincanadines A-G were isolated in 2002 by Ohsaki and coworkers from the bark of the Brazilian medicinal plant Aspidosperma subincanum. Kobayashi proposed that subincanadines D-F could be biosynthetically resulting from stemmadenine via two different pathways and, furthermore, that the subincanadines A-C could be biogenetically resulting from subincanadines D and E. Kam and coworkers, in their focused efforts, isolated five indole alkaloids from Malaysian Kopsia arborea species, namely valparicine, apparicine, arboridinine, arborisidine, and arbornamine in combination with subincanadine E. On the basis of structural features, it has been proposed and proved in some examples that subincanadine E is a biogenetic precursor of these five different bioactive indole alkaloids bearing complex structural architectures. All important information on isolation, characterization, bioactivity, probable biogenetic pathways, and more specifically racemic and enantioselective total synthesis of subincanadine alkaloids and their biogenetic congeners are summarized in the present chapter. Special importance is given to the total synthesis and the synthetic strategies intended therein, comprising a set of main reactions.

A Method for the Preparation of β-Amino-α,β-unsaturated Carbonyl Compounds: Study of Solvent Effect and Mechanism

An efficient method for the preparation of β-amino-α,β-unsaturated carbonyl compounds is demonstrated. Bench-stable sodium 3-oxo-enolates were prepared from carbonyl compounds, and reacted with amines in the presence of an acid and a desiccant. DFT studies revealed contrasting mechanisms toward the reactivity of aliphatic amines in protic solvents and aromatic amines in aprotic solvents. While the former proceeds through the formation of an imine, the latter passes through the Michael addition-elimination mechanism.

Synthesis of 2,3-disubstituted indoles via a tandem reaction

A wide array of 2,3-disubstituted indoles were accessed in modest to good yields via a tandem reduction/condensation/fragmentation/cyclization sequence. Differential fragmentation made the reaction more complicated.

Diastereoselective construction of carbazole-based spirooxindoles via the Levy three-component reaction

The CuSO₄ catalyzed three-component reaction of indole-2-acetate, aromatic aldehydes and 3-methyleneoxindoles in toluene at 130 °C afforded polysubstituted spiro[carbazole-3,3′-indolines] in good yields and with high diastereoselectivity.

Total Synthesis of (+)-epi-Condyfoline

Herein, we report the first asymmetric total synthesis of aspidospermatan indole alkaloid (+)-epi-condyfoline (1) in 15 steps from commercially available 2-methylindole-3-carboxaldehyde. Key steps include (1) our domino Michael/Mannich annulation method of N-sulfinyl metallodienamines to set three contiguous stereocenters, (2) LiHMDS-mediated cyclization of an ω-tosyloxy N-sulfinamide to prepare the signature indole-fused 2-azabicyclo[3.3.1]nonane framework, and (3) DMTSF-promoted spirocyclization of a dithioacetal intermediate to access the final pyrrolidine ring. Functional group manipulations delivered the targeted alkaloid (+)-epi-condyfoline (1) in 13 steps and 1.25% overall yield from N-sulfinylimine (+)-8.

The assembly of (+)-vincadifformine- and (-)-tabersonine-derived monoterpenoid indole alkaloids in Catharanthus roseus involves separate branch pathways

The biological activity of monoterpenoid indole alkaloids (MIAs) has led to their use in cancer treatment and other medical applications. Their biosynthesis has involved the formation of reactive intermediates by responsible enzymes to elaborate several different chemical scaffolds. Modification of scaffolds through different substitution reactions has produced chemically diverse MIAs and related biological activities. The present study characterizes the three-step pathway involved in the formation of (+)-echitovenine, the major O-acetylated MIA of Catharanthus roseus roots, and differentiates it from a parallel pathway involved in the formation of hörhammericine. Separate hydrolases convert a common reactive MIA intermediate to aspidosperma skeletons of opposite specific rotations, that is (+)-vincadifformine and (-)-tabersonine, respectively. The formation of (+) minovincinine from (+) vincadifformine 19-hydroxylase (V19H) is catalyzed by a root-specific cytochrome P450 with high amino acid sequence similarity to the leaf-specific tabersonine-3-hydroxylase involved in vindoline biosynthesis. Similarly, O-acetylation of (+)-minovincinine to form (+) echitovenine involves minovincinine-O-acetytransferase. The substrate specificity of V19H and MAT for their respective (+)-enantiomers defines the separate enantiomer-specific pathway involved in (+)-echitovenine biosynthesis and differentiates it from a parallel (-)-enantiomer-specific pathway involved in the formation of hörhammericine from (-)-tabersonine.

Enantioselective Total Syntheses of (+)-Fendleridine and (+)-Acetylaspidoalbidine

Enantioselective syntheses of hexacyclic aspidoalbidine alkaloids (+)-fendleridine (2) and (+)-acetylaspidoalbidine (3) are described. These syntheses feature an asymmetric decarboxylative allylation and photocyclization of a highly substituted enaminone. Also, the synthesis highlights the formation of a C19-hemiaminal ether via a reduction/condensation/intramolecular cyclization cascade with the C21-alcohol. The present synthesis provides convenient access to the aspidoalbidine hexacyclic alkaloid family in an efficient manner.

Efficient Enantioselective Syntheses of Chiral Natural Products Facilitated by Ligand Design

The employment of enantioselective transition-metal-catalyzed transformations as key steps in asymmetric natural product syntheses have attracted considerable attention in recent years owing to their versatile synthetic utilities, mild conditions and high efficiency in chirality generation. The chiral catalysts or supporting ligands are believed to be crucial for the requisite reactivity and enantioselectivity. Therefore, the rational design of chiral ligands is at the heart of developing new asymmetric transition-metal catalyzed reactions and provides an avenue to the asymmetric synthesis of natural products. Our group has been engaged in the development of transition-metal-catalyzed enantioselective cross-coupling, cyclization and other related reactions and the application of these methodologies to natural product syntheses. In this account, we summarized our recent synthetic efforts towards the efficient total syntheses of several different types of natural products including terpenes, alkaloids and polyketides facilitated by the design of a series of versatile P-chiral phosphorous ligands.