Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

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.

Synthetic applications of type II intramolecular cycloadditions

Type II intramolecular cycloadditions ([4+2], [4+3], [4+4] and [5+2]) have emerged recently as an efficient and powerful strategy for the construction of bridged ring systems. In general, type II cycloadditions provide access to a wide range of bridged bicyclo[m.n.1] ring systems with high regio- and diastereoselectivity in an easy and straightforward manner. In each section of this review, an overview of the corresponding type II cycloadditions is presented, which is followed by highlights of method development and synthetic applications in natural product synthesis. The goal of this review is to provide a survey of recent advances in the field covering literature up to 2020. The review will serve as a useful reference for organic chemists engaged in the total synthesis of natural products containing bridged bicyclo[m.n.1] ring systems and provide strong stimulus for invention and further advances in this exciting research field.

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.

Stereoselective Halogenation in Natural Product Synthesis

At last count, nearly 5000 halogenated natural products have been discovered. In approximately half of these compounds, the carbon atom to which the halogen is bound is sp(3) -hybridized; therefore, there are an enormous number of natural products for which stereocontrolled halogenation must be a critical component of any synthesis strategy. In this Review, we critically discuss the methods and strategies used for stereoselective introduction of halogen atoms in the context of natural product synthesis. Using the successes of the past, we also attempt to identify gaps in our synthesis technology that would aid the synthesis of halogenated natural products, as well as existing methods that have not yet seen application in complex molecule synthesis. The chemistry described herein demonstrates yet again how natural products continue to provide the inspiration for critical advances in chemical synthesis.

Reductive Chlorination and Bromination of Ketones via Trityl Hydrazones

A method is presented for the direct transformation of a ketone to the corresponding reduced alkyl chloride or bromide. The process involves the reaction of a ketone trityl hydrazone with tBuOCl to give a diazene which readily collapses to the α-chlorocarbinyl radical, reduction of which by a hydrogen atom source gives the alkyl chloride product. The use of N-bromosuccinimide provides the corresponding alkyl bromide. This unique transformation provides a reductive halogenation that complements Barton's redox-neutral vinyl halide synthesis.

Total Synthesis of (-)-N-Methylwelwitindolinone C Isothiocyanate Based on a Pd-Catalyzed Tandem Enolate Coupling Strategy

The highly stereocontrolled total synthesis of (-)-N-methylwelwitindolinone C isothiocyanate is described, which features the expeditious construction of a bicyclo[4.3.1]decane ring system by a palladium-catalyzed tandem enolate allylation/arylation reaction.

Scalable Total Syntheses of (-)-Hapalindole U and (+)-Ambiguine H

The Stigonemataceae family of cyanobacteria produces a class of biogenetically related indole natural products that include hapalindoles and ambiguines. In this full account, a practical route to the tetracyclic hapalindole family is presented by way of an eight-step, enantiospecific, protecting-group-free total synthesis of (-)-hapalindole U that features an oxidative indole-enolate coupling. With gram-scale access to hapalindole U, the first total synthesis of an ambiguine alkaloid, (+)-ambiguine H, was completed via an isonitrile-assisted prenylation of an indole followed by a photofragmentation cascade.

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.

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.