Directed oxidative cyclizations to C2- or C4-positions of indole: efficient construction of the bicyclo[4.3.1]decane core of welwitindolinones

Recent Advances in Manganese(III)-Assisted Radical Cyclization for the Synthesis of Natural Products: A Comprehensive Review

Natural products play an important part in synthetic chemistry since they have many pharmacological properties and are used as active drug compounds in pharmaceutical chemistry. However, synthesis of these complex molecules is difficult due to the requirement of various synthetic steps, which include highly regio- and stereoselectivity. Therefore, oxidative radical cyclization assisted by manganese(III) acetate serves as an important step in obtaining spiro-, tricyclic, tetracyclic, and polycyclic derivatives of these compounds. Manganese(III)-based reactions offer a single-step regio- and stereoselective cyclizations and α-acetoxidations, reducing the number of synthetic steps. Also, the manganese(III)-mediated oxidative free radical cyclization method has been successfully applied for the synthesis of cyclic structures found in many natural products. This article presents a broad overview of manganese(III)-based radical reactions of natural products as a key step in overall synthesis. The authors have classified natural product synthesis processes assisted by manganese(III) acetate as intermolecular, intramolecular, oxidation, acetoxidation, aromatization, and polymerization reactions, respectively.

Copper-Mediated Decarboxylative Coupling between Arylacetic Acids and 1,3-Dicarbonyl Compounds

A copper-mediated decarboxylative coupling reaction between arylacetic acids and 1,3-dicarbonyl compounds was described. Significantly, methanocycloocta[b]indoles were also obtained by sequential intramolecular dehydrocyclization process in some cases. This protocol featured a broad substrate scope, simple operations, and good yields. Moreover, the products exhibited potent antiproliferative activity against the human cancer cell lines by a MTT assay.

Recent advances in hapalindole-type cyanobacterial alkaloids: biosynthesis, synthesis, and biological activity

Covering: 1984 up to the end of 2020Hapalindoles, fischerindoles, ambiguines and welwitindolinones are all members of a class of indole alkaloid natural products that have been isolated from the Stigonematales order of cyanobacteria. These compounds possess a polycyclic ring system, unique functional groups and various stereo- and regiochemical isomers. Since their initial isolation in 1984, they have been explored as potential therapeutics due to their wide variety of biological activities. Although numerous groups have pursued total syntheses of these densely functionalized structures, hapalindole biosynthesis has only recently been unveiled. Several groups have uncovered a wide range of novel enzymes that catalyze formation and tailoring of the hapalindole-type metabolites. In this article, we provide an overview of these natural products, their biological activities, highlight general synthetic routes, and provide an extensive review on the surprising biosynthetic processes leading to these structurally diverse metabolites.

Structural diversification of hapalindole and fischerindole natural products via cascade biocatalysis

Hapalindoles and related compounds (ambiguines, fischerindoles, welwitindolinones) are a diverse class of indole alkaloid natural products. They are typically isolated from the Stigonemataceae order of cyanobacteria and possess a broad scope of biological activities. Recently the biosynthetic pathway for assembly of these metabolites has been elucidated. In order to generate the core ring system, L-tryptophan is converted into the cis-indole isonitrile subunit before being prenylated with geranyl pyrophosphate at the C-3 position. A class of cyclases (Stig) catalyzes a three-step process including a Cope rearrangement, 6-exo-trig cyclization and electrophilic aromatic substitution to create a polycyclic core. Formation of the initial alkaloid is followed by diverse late-stage tailoring reactions mediated by additional biosynthetic enzymes to give rise to the wide array of structural variations observed in this compound class. Herein, we demonstrate the versatility and utility of the Fam prenyltransferase and Stig cyclases toward core structural diversification of this family of indole alkaloids. Through synthesis of cis-indole isonitrile subunit derivatives, and aided by protein engineering and computational analysis, we have employed cascade biocatalysis to generate a range of derivatives, and gained insights into the basis for substrate flexibility in this system.

C4-H indole functionalisation: precedent and prospects

C4-decorated indoles feature in a plethora of bioactive and functional compounds of importance to natural product synthesis, material sciences, as well as crop protection and pharmaceutical industries. Traditionally, their syntheses largely involved harsh stoichiometric metalations and radical reactions. However, transition metal catalysed C-H activation has recently evolved into a powerful strategy for the late-stage diversification of indoles at the C4-H position. Modern photoredox, enzymatic and precious transition metal catalysis represent the key stimuli for developing challenging C-C and C-Het bond forming transformations under mild reaction conditions. Herein, we discuss the evolution and application of these methods for the step-economical transformations of otherwise inert C4-H bonds up to December 2017.

Oxidative coupling strategies for the synthesis of indole alkaloids

Direct functionalization of indole through oxidative coupling reactions with enolates or phenols provides a powerful tool for assembling indole alkaloids.

An Approach to the Welwistatin Core via a Diazoketone Rearrangement-Ring Expansion Strategy

The rhodium-catalyzed decomposition of fused bicyclic α-diazo-β-hydroxyketone 16 and rearrangement to 17 is featured in an approach to the bridged bicyclic core of welwistatin. The bicyclic [4.3.1] core of 25 is furnished from a subsequent cyclopropanation to generate 23, followed by its ring expansion.

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.

Manganese(iii) acetate catalyzed oxidative amination of benzylic C(sp3)-H bonds with nitriles

Mn-Catalyzed oxidative amination of benzylic C(sp³)-H bonds with nitriles is disclosed, which enables the synthesis of a broad range of secondary amides in moderate to excellent yields under mild conditions. The interaction between Mn(iii) and DDQ facilitates the oxidation and makes it highly efficient and selective.

Iridium(III) Photocatalysis: A Visible-Light-Induced Dearomatizative Tandem [4+2] Cyclization to Furnish Benzindolizidines

A photocatalytic dearomatizative tandem [4+2] cyclization between N-(2-iodoethyl)indoles and a variety of alkenes leads to tri- and tetracyclic benzindolizidines with high diastereoselectivity and yield. The intermolecular annulation reaction is performed under visible-light irradiation and employs [Ir(ppy)3] or [Ir(dtbbpy)(ppy)2] PF6 as photocatalysts, in combination with tertiary amines as electron and hydrogen atom donors.

Total synthesis of (-)-N-methylwelwitindolinone B isothiocyanate via a chlorinative oxabicycle ring-opening strategy

The first total synthesis of N-methylwelwitindolinone B isothiocyanate is reported. The route features several key steps, including a regio- and diastereoselective chlorinative oxabicycle ring-opening reaction to introduce the challenging alkyl chloride motif.

Progress toward the total synthesis of N-methylwelwitindolinone B isothiocyanate

Progress toward the welwitindolinone alkaloid N-methylwelwitindolinone B isothiocyanate is reported. A key reaction to synthesize the [4.3.1] bicycle embedded in the core of the molecule is a furan type 2 intramolecular Diels-Alder reaction with a tetrasubstituted dienophile, which sets the two vicinal quaternary centers present in the natural product. The sterically encumbered cycloaddition precursor was synthesized using a Horner-Wadsworth-Emmons reaction followed by a Suzuki cross-coupling reaction. Finally, introduction of the secondary alkyl chloride was achieved by a regio- and diastereoselective opening of a [2.2.1] oxobicycloheptane functionality.

Manganese-catalyzed intermolecular C–H/C–H coupling of carbonyls and heteroarenes

Manganese-catalyzed intermolecular C–H/C–H coupling of carbonyls and heteroarenes has been developed.

Efficient catalytic-free method to produce α-aryl cycloalkanones through highly chemoselective coupling of aryl compounds with oxyallyl cations

Efficient catalytic-free method to produce α-aryl cycloalkanones through highly chemoselective coupling of aryl compounds with oxyallyl cations at room temperature.

Total synthesis of (-)-ophiodilactone A and (-)-ophiodilactone B

The first asymmetric total synthesis of (-)-ophiodilactone A and (-)-ophiodilactone B, isolated from the ophiuroid (Ophiocoma scolopendrina), is reported. The key features of the synthesis include the highly stereocontrolled construction of the structurally congested γ-lactone/δ-lactone skeleton through an asymmetric epoxidation, diastereoselective iodolactonization, and intramolecular epoxide-opening with a carboxylic acid, and biomimetic radical cyclization of ophiodilactone A to ophiodilactone B.

Enantiospecific total synthesis of N-methylwelwitindolinone D isonitrile

The total synthesis of N-methylwelwitindolinone D isonitrile (1) has been achieved in 17 steps from a readily available carvone derivative. The route features a double C-H functionalization event involving a keto oxindole substrate to introduce the tetrahydrofuran ring of the natural product.

Studies toward welwitindolinones: formal syntheses of N-methylwelwitindolinone C isothiocyanate and related natural products

The formal syntheses of N-methylwelwitindolinone C isothiocyanate (4) and several other welwitindolinones 5-8 were achieved by the independent synthesis of 79. The synthesis featured a Lewis acid-mediated coupling between a heteroaryl carbinol and bis-TMS enol ether, an intramolecular enolate arylation, and an unprecedented intramolecular allylic alkylation of a γ-acyloxyenone.

Formal syntheses of naturally occurring welwitindolinones

The formal syntheses of N-methylwelwitindolinone C isothiocyanate, N-methylwelwitindolinone C isonitrile, N-methylwelwitindolinone D isonitrile, 3-hydroxy-N-methylwelwitindolinone C isothiocyanate, and 3-hydroxy-N-methylwelwitindolinone C isonitrile are reported. The synthesis features several novel processes, including a Lewis acid mediated coupling between a benzylic-type heteroaromatic alcohol and a highly functionalized silyl ketene acetal, an intramolecular enolate arylation, and a regioselective, Pd(0)-catalyzed π-allylic cyclization of a γ-benzoyloxy enone moiety that is revealed by unmasking a furan ring.

Synthesis of functionalized cyclohexenone core of welwitindolinones via rhodium-catalyzed [5 + 1] cycloaddition

The cyclohexenone core of welwitindolinones was synthesized by a Rh(I)-catalyzed [5 + 1]-cycloaddition of an allenylcyclopropane with CO. A pentasubstituted cyclopropane was prepared successfully by a Rh(II)-catalyzed intramolecular cyclopropanation of alkenes with chlorodiazoacetates.

Total synthesis of oxidized welwitindolinones and (-)-N-methylwelwitindolinone C isonitrile

We report the total synthesis of (-)-N-methylwelwitindolinone C isonitrile, in addition to the total syntheses of the 3-hydroxylated welwitindolinones. Our routes to these elusive natural products feature the strategic use of a deuterium kinetic isotope effect to improve the efficiency of a late-stage nitrene insertion reaction. We also provide a computational prediction for the stereochemical configuration at C3 of the hydroxylated welwitindolinones, which was confirmed by experimental studies.

A unified route to the welwitindolinone alkaloids: total syntheses of (-)-N-methylwelwitindolinone C isothiocyanate, (-)-N-methylwelwitindolinone C isonitrile, and (-)-3-hydroxy-N-methylwelwitindolinone C isothiocyanate

As part of a comprehensive strategy to the welwitindolinone alkaloids possessing a bicyclo[4.3.1]decane core, we report herein concise asymmetric total syntheses of (-)-N-methylwelwitindolinone C isothiocyanate (2a), (-)-N-methylwelwitindolinone C isonitrile (2b), and (-)-3-hydroxy-N-methylwelwitindolinone C isothiocyanate (3a) from a common tetracyclic intermediate. The crucial vinyl chloride moiety was installed through electrophilic chlorination of a hydrazone, but only after adjustment of reactivity to circumvent a facile skeletal rearrangement. Selective desulfurization and oxidation of 2a provided access to 2b and 3a, respectively. Notably, this work provides corrected (1)H and (13)C NMR spectral data for 3a.

Lessons Learned while Traversing the Welwitindolinone Alkaloids Obstacle Course

We recount several unexpected results observed in the course of our work toward the synthesis of welwitindolinone alkaloids. The surprising results provide an opportunity to refine one's understanding of the interplay between chemical structure and reactivity.