Enantioselective Construction of Tetrahydroindole Skeletons by Rh-Catalyzed [2+2+2] Cycloaddition of Homopropargyl Enamides with Alkynes

We have developed the Rh-catalyzed enantioselective [2+2+2] cycloaddition of homopropargyl enamides (tosylamide-tethered 1,6-enynes) with alkynes to construct tetrahydroindole skeletons found in natural alkaloids and pharmaceuticals. This cycloaddition proceeds at room temperature in high yields and regio- and enantioselectivity with a broad substrate scope. The preparative scale reaction followed by substituent conversion on the nitrogen atom and the diastereoselective [4+2] cycloaddition with singlet O2 affords hexahydroindole-diols bearing three stereogenic centers and variable substituents on the nitrogen. Mechanistic studies have revealed that the substituents of the enynes change the ratio of intramolecular and intermolecular rhodacycle formation when using terminal alkynes, varying the ee values of the cycloadducts.

Diastereoselective synthesis of functionalized spiroindolines via intramolecular ipso-iodocyclization/nucleophile addition cascade reactions of indole-tethered ynones

Herein, we describe a highly diastereoselective approach for synthesizing polyfunctionalized spiroindolines from indolyl-ynones involving an ipso-iodocyclization/nucleophile addition cascade. The developed strategy allows the formation of a spirocyclic core and the installation of two functional groups in a single operation. Also this strategy is accompanied by the generation of two C-C and one C-I bonds and two contiguous stereocenters.

Ag(I)-catalyzed dearomatizing spirocyclization/nucleophile addition cascade reactions of indole-tethered ynones

Ag(I)-catalyzed highly diastereoselective construction of divergent spiroindolines is disclosed herein. The approach proceeds via dearomatizing spirocyclization of indole-tethered ynones followed by C-nucleophile or hydride trapping. The established strategy is accompanied by the generation of two new C-C bonds and two contiguous stereocenters. This strategy features a broad range of (hetero)arenes as C-nucleophiles and excellent diastereoselectivity.

A bridged backbone strategy enables collective synthesis of strychnan alkaloids

Bridged frameworks are of high chemical and biological significance, being ubiquitous in pharmaceutical molecules and natural products. Specific structures are usually preformed to build these rigid segments at the middle or late stage in the synthesis of polycyclic molecules, resulting in decreased synthetic efficiency and target-specific syntheses. As a logically distinct synthetic strategy, we constructed an allene/ketone-equipped morphan core at the outset through an enantioselective α-allenylation of ketones. Experimental and theoretical results revealed that the high reactivity and enantioselectivity of this reaction are attributed to the cooperative effects of the organocatalyst and metal catalyst. The bridged backbone generated was employed as a structural platform to guide and facilitate the assembly of up to five fusing rings, and the allene and ketone groups thereon were used to precisely install various functionalities at C16 and C20 at the late stage, leading to a concise, collective total synthesis of nine strychnan alkaloids.

Enantioselective Total Synthesis of (-)-Himalensine A via a Palladium and 4-Hydroxyproline Co-catalyzed Desymmetrization of Vinyl-bromide-tethered Cyclohexanones

Herein, we describe the convergent enantioselective total synthesis of himalensine A in 18 steps, enabled by a highly enantio- and diastereoselective construction of the morphan core via a palladium/hydroxy proline co-catalyzed desymmetrization of vinyl-bromide-tethered cyclohexanones. The reaction pathway was illuminated by density functional theory calculations, which support an intramolecular Heck reaction of an in situ-generated enamine intermediate, where exquisite enantioselectivity arises from intramolecular carboxylate coordination to the vinyl palladium species in the rate- and enantio-determining carbopalladation steps. The reaction tolerates diverse N-derivatives, all-carbon quaternary centers, and trisubstituted olefins, providing access to molecular scaffolds found in a range of complex natural products. Following large-scale preparation of a key substrate and installation of a β-substituted enone moiety, the rapid construction of himalensine A was achieved using a highly convergent strategy based on an amide coupling/Michael addition/allylation/ring-closing metathesis sequence which allowed the introduction of three of the five rings in only three synthetic steps (after telescoping). Moreover, our strategy provides a new enantioselective access to a known tetracyclic late-stage intermediate that has been used previously in the synthesis of many Daphniphyllum alkaloids.

Strychnine and its mono- and dimeric analogues: a pharmaco-chemical perspective

Covering: up to November 2021Since its isolation in 1818, strychnine has attracted the attention of a plethora of chemists and pharmacologists who have established its structure, developed total syntheses, and examined its complex pharmacology. While numerous reviews on structure elucidation and total synthesis of strychnine are available, reports on structure-activity relationships (SARs) of this fascinating alkaloid are rare. In this review, we present and discuss structures, synthetic approaches, metabolic transformations, and the diverse pharmacological actions of strychnine and its mono- and dimeric analogues. Particular attention is given to its SARs at glycine receptors (GlyRs) in light of recently published high-resolution structures of strychnine-GlyR complexes. Other pharmacological actions of strychnine and its derivatives, such as their antagonistic properties at nicotinic acetylcholine receptors (nAChRs), allosteric modulation of muscarinic acetylcholine receptors as well as anti-cancer and anti-plasmodial effects are also critically reviewed, and possible future developments in the field are discussed.

Asymmetric Total Syntheses of Strychnos Alkaloids via Selective Fischer Indolization

The complex structures and important biological functions of Strychnos alkaloids have attracted a great deal of attention from synthetic chemists. Herein, we describe the concise asymmetric total syntheses of the Strychnos alkaloids, (-)-dehydrotubifoline, (-)-tubifoline, and (-)-tubifolidine, as well as the formal total synthesis of (-)-strychnine. Our strategy features the construction of the common tetracyclic pyrrolo[2,3-d]carbazole structure using regioselective Fischer indolization on unsymmetrical cyclic ketones and late-stage functionalization for divergent synthesis. We developed a stepwise Fischer indolization featuring selective formation of enol triflate to solve the challenging regioselectivity problem, leading to the common tetracyclic ring skeleton in these Strychnos alkaloids. The regioselectivity of Fischer indolization on unsymmetrical cyclic ketones was studied on the basis of different types of ring systems and supported by density functional theory calculations. Overall, our success in the construction of this tetracyclic ring secured the syntheses of Strychnos alkaloids and may provide a general method for the total syntheses of various alkaloids containing this skeleton.

Asymmetric Bromoaminocyclization and Desymmetrization of Cyclohexa-1,4-dienes through Anion Phase-Transfer Catalysis

The catalytic enantioselective desymmetrizing bromoaminocyclization of prochiral cyclohexa-1,4-dienes has been achieved by using chiral anion phase-transfer catalysis, providing a range of enantioenriched cis-3a-arylhydroindoles bearing an all-carbon quaternary stereocenter in good yields (up to 78%) and excellent enantioselectivities (up to 97% ee). Furthermore, the potential application of this methodology to natural product total synthesis was demonstrated by the asymmetric synthesis of (+)-Mesembrane.

Divergent Synthesis of Skeletally Distinct Arboridinine and Arborisidine

A divergent synthesis of skeletally distinct arboridinine and arborisidine was achieved. The central divergent strategy was inspired by the divergent biosynthetic cyclization mode of arboridinine and arborisidine and their hidden topological connection. The branch point was reached through a Michael and Mannich cascade process. A site-selective intramolecular Mannich reaction was developed to construct the tetracyclic core of arboridinine, while a site-selective intramolecular α-amination of ketone was used to access the tetracyclic core of arborisidine. A strategic Peterson olefination through intramolecular nucleophile delivery was able to set up the exocyclic olefin of arboridinine.

Efficient synthesis of polyfunctionalized carbazoles and pyrrolo[3,4-c]carbazoles via domino Diels-Alder reaction

The p-TsOH-catalyzed Diels–Alder reaction of 3-(indol-3-yl)maleimides with chalcone in toluene at 60 °C afforded two diastereoisomers of tetrahydropyrrolo[3,4-c]carbazoles, which can be dehydrogenated by DDQ oxidation in acetonitrile at room temperature to give the aromatized pyrrolo[3,4-c]carbazoles in high yields. On the other hand, the one-pot reaction of 3-(indol-3-yl)-1,3-diphenylpropan-1-ones with chalcones or benzylideneacetone in acetonitrile in the presence of p-TsOH and DDQ resulted in polyfunctionalized carbazoles in satisfactory yields. The reaction mechanism included the DDQ oxidative dehydrogenation of 3-(indol-3-yl)-1,3-diphenylpropan-1-ones to the corresponding 3-vinylindoles, their acid-catalyzed Diels–Alder reaction and sequential aromatization process.

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.

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.

An Asymmetric Synthesis of (+)-Isostrychnine Based on Catalytic Asymmetric Tandem Double Michael Addition

Herein, a concise asymmetric synthesis of (+)-isostrychnine is achieved in nine longest-linear steps with a 16% overall yield. The key features of this synthesis include the catalytic asymmetric tandem double Michael addition of a tryptamine-derived oxindole to an alkynone to facilely forge the A/B/C ring framework, a one-pot intramolecular dehydrative condensation/lactamization reaction to efficiently establish the E/G ring system, and an allylic diazene rearrangement to introduce the pivotal olefin for the subsequent intramolecular Heck reaction.

Total Synthesis of (-)-Strictosidine and Interception of Aryne Natural Product Derivatives "Strictosidyne" and "Strictosamidyne"

Monoterpene indole alkaloids are a large class of natural products derived from a single biosynthetic precursor, strictosidine. We describe a synthetic approach to strictosidine that relies on a key facially selective Diels-Alder reaction between a glucosyl-modified alkene and an enal to set the C15-C20-C21 stereotriad. DFT calculations were used to examine the origin of stereoselectivity in this key step, wherein two of 16 possible isomers are predominantly formed. These calculations suggest the presence of a glucosyl unit, also inherent in the strictosidine structure, guides diastereoselectivity, with the reactive conformation of the vinyl glycoside dienophile being controlled by an exo-anomeric effect. (-)-Strictosidine was subsequently accessed using late-stage synthetic manipulations and an enzymatic Pictet-Spengler reaction. Several new natural product analogs were also accessed, including precursors to two unusual aryne natural product derivatives termed "strictosidyne" and "strictosamidyne". These studies provide a strategy for accessing glycosylic natural products and a new platform to access monoterpene indole alkaloids and their derivatives.

Nonbiomimetic total synthesis of indole alkaloids using alkyne-based strategies

Biomimetic natural product synthesis is generally straightforward and efficient because of its established feasibility in nature and utility in comprehensive synthesis, and the cost-effectiveness of naturally derived starting materials. On the other hand, nonbiomimetic strategies can be an important option in natural product synthesis since (1) nonbiomimetic synthesis offers more flexibility and can demonstrate the originality of chemists, and (2) the structures of derivatives accessible by nonbiomimetic synthesis can be considerably different from those that are synthesised in nature. This review summarises nonbiomimetic total syntheses of indole alkaloids using alkyne chemistry for constructing core structures, including ergot alkaloids, monoterpene indole alkaloids (mainly corynanthe, aspidosperma, strychnos, and akuammiline), and pyrroloindole and related alkaloids. To clarify the differences between alkyne-based strategies and biosynthesis, the alkynes in nature and the biosyntheses of indole alkaloids are also outlined.

Potential anti-neuroinflammatory compounds from Australian plants - A review

Neuroinflammation is a complex response to brain injury involving the activation of glia, release of inflammatory mediators, such as cytokines and chemokines, and generation of reactive oxygen and nitrogen species. Even though it is considered an event secondary to neuronal death or dysfunction, neuro-inflammation comprises a majority of the non-neuronal contributors to the cause and progression of neurodegenerative diseases like Alzheimer's Disease (AD), Parkinson's Disease (PD), Multiple Sclerosis (MS), Chronic Traumatic Encephalopathy (CTE) and others. As a result of the lack of effectiveness of current treatments for neurodegenerative diseases, neuroinflammation has become a legitimate therapeutic target for drug discovery, leading to the study of various in vivo and in vitro models of neuroinflammation. Several molecules sourced from plants have displayed anti-inflammatory properties in the study of neurodegenerative diseases. A group of these anti-inflammatory compounds has been classified as cytokine-suppressive anti-inflammatory drugs (CSAIDs), which target the pro-inflammatory AP1 and nuclear factor-κB signaling pathways and inhibit the expression of many pro-inflammatory cytokines, such as interleukin IL-1, IL-6, TNF-α, or nitric oxide. Australian plants, thriving amid the driest inhabited continent of the world, are an untapped source of chemical diversity in the form of secondary metabolites. These compounds are produced in response to biotic and abiotic stresses that the plants are exposed to in the highly biodiverse environment. This review is an attempt to highlight anti-inflammatory compounds isolated from Australian plants.

Dual catalytic enantioselective desymmetrization of allene-tethered cyclohexanones

The construction of enantioenriched azabicyclo[3.3.1]nonan-6-one heterocycles via an enantioselective desymmetrization of allene-linked cyclohexanones, enabled through a dual catalytic system, that provides synchronous activation of the cyclohexanone with a chiral prolinamide and the allene with a copper(i) co-catalyst to deliver the stereodefined bicyclic core, is described. Successful application to oxygen analogues was also achieved, thereby providing a new enantioselective synthetic entry to architecturally complex bicyclic ethereal frameworks. The mechanistic pathway and the origin of enantio- and diastereoselectivities has been uncovered using density functional theory (DFT) calculations.