Stereocontrolled Synthesis of (-)-Hapalindole G

Asymmetric 1,2-Migration at Vicinal Tetrasubstituted Stereocenters Constructed from α-Keto Imines

A carbonyl-assisted asymmetric 1,2-migratory allylation through in situ generation of vicinal tetrasubstituted stereocenters is reported to access enantiopure α-amino ketones and amino alcohols with excellent yields and diastereoselectivities. In a remarkable divergence, despite higher steric hindrance, the allylation exclusively occurs on ketones over imines in the first step, followed by a face-selective 1,2-allyl transfer, thus highlighting an exciting interplay between two distinct electrophiles. The methodology distinguishes itself through its adaptability to gram-scale synthesis, showcasing broad functional-group tolerance and stereodivergence. Density functional theory (DFT) analysis led to a deeper understanding of its selectivity and mechanistic framework. Highlighting its transformative potential, the method was applied to the total synthesis of hapalindole alkaloids.

Total Synthesis of the Chlorinated Pentacyclic Indole Alkaloid (+)-Ambiguine G

Reported herein is the total synthesis of (+)-ambiguine G, the first member of the chlorinated pentacyclic ambiguines to yield to chemical synthesis. The synthesis is accomplished through a convergent strategy that proceeds in 10 steps from (S)-carvone oxide. Pivotal to the concise route is the successful realization of a [4+3] cycloaddition that conjoins two easily synthesized components of the carbon framework of the natural product. Also featured in the synthesis is the efficient, diastereoselective construction of a key vinylated chloro ketone and the unprecedented, one-pot reduction-elimination-oxidation sequence that transforms an enone to an advanced hydroxylated-diene intermediate.

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.

Fragment Coupling Reactions in Total Synthesis That Form Carbon-Carbon Bonds via Carbanionic or Free Radical Intermediates

Fragment coupling reactions that form carbon-carbon bonds are valuable transformations in synthetic design. Advances in metal-catalyzed cross-coupling reactions in the early 2000s brought a high level of predictability and reliability to carbon-carbon bond constructions involving the union of unsaturated fragments. By comparison, recent years have witnessed an increase in fragment couplings proceeding via carbanionic and open-shell (free radical) intermediates. The latter has been driven by advances in methods to generate and utilize carbon-centered radicals under mild conditions. In this Review, we survey a selection of recent syntheses that have implemented carbanion- or radical-based fragment couplings to form carbon-carbon bonds. We aim to highlight the strategic value of these disconnections in their respective settings and to identify extensible lessons from each example that might be instructive to students.

Meglumine as a green, efficient and reusable catalyst for synthesis and molecular docking studies of bis(indolyl)methanes as antioxidant agents

An efficient and convenient Meglumine catalyzed procedure for the synthesis of bis(indolyl) methanes at ambient temperature under aqueous conditions in high yields. The catalytic reaction proceeds very smoothly. Clean reaction, ease of product isolation/purification, easily available reactants, metal free and environmentally friendly reaction conditions are the notable advantages of the present methodology. All the entitled compounds were characterized by IR, ¹H, ¹³C NMR, mass spectra and evaluated for their antioxidant (DPPH, H₂O₂ and NO scavenging methods). They exhibited potent in vitro antioxidant activity dose-dependently. The binding interactions and molecular docking studies for entitled compounds were studied against 3MNG protein. 4d exhibited marked binding affinity with excellent docking score of -7.6 K.cal/mol and emerged as a lead compound.

Total Synthesis of Pentacyclic (-)-Ambiguine P Using Sequential Indole Functionalizations

The first synthesis of a pentacyclic ambiguine (ambiguine P) is reported. The synthesis takes advantage of sequential alkylations of an indole core to rapidly construct the pentacyclic framework of the natural product. Key to the success of the synthesis was the use of a Nicholas reaction to alkylate at C2, crafting a fused seven-membered ring that is characteristic of the pentacyclic ambiguines, as well as the use of an amide-directed functionalization at C12 to set a requisite quaternary center. A versatile late-stage intermediate was prepared that may be applicable to the synthesis of the other pentacyclic ambiguines.

Biomimetic Enantioselective Total Synthesis of (-)-Petromindole

The first enantioselective total synthesis of (-)-petromindole, an architecturally distinct congener of indole diterpene family, has been achieved. Key features of this synthetic route include the scalable and concise synthesis of tricyclic allylic alcohol from enantiopure Wieland-Mischer ketone derivative, and TMSOTf-mediated, highly efficient biomimetic C-4 cyclization of indole derivative for the rapid construction of a hexacyclic skeleton of petromindole.

Synthesis of bis-indolylmethanes as new potential inhibitors of β-glucuronidase and their molecular docking studies

Thirty-two (32) bis-indolylmethane-hydrazone hybrids 1-32 were synthesized and characterized by ¹HNMR, ¹³CNNMR and HREI-MS. All compounds were evaluated in vitro for β-glucuronidase inhibitory potential. All analogs showed varying degree of β-glucuronidase inhibitory potential ranging from 0.10 ± 0.01 to 48.50 ± 1.10 μM when compared with the standard drug d-saccharic acid-1,4-lactone (IC₅₀ value 48.30 ± 1.20 μM). Derivatives 1-32 showed the highest β-glucuronidase inhibitory potentials which is many folds better than the standard drug d-saccharic acid-1,4-lactone. Further molecular docking study validated the experimental results. It was proposed that bis-indolylmethane may interact with some amino acid residues located within the active site of β-glucuronidase enzyme. This study has culminated in the identification of a new class of potent β-glucuronidase inhibitors.

Divergent enantioselective synthesis of hapalindole-type alkaloids using catalytic asymmetric hydrogenation of a ketone to construct the chiral core structure

A divergent enantioselective approach to hapalindole-type alkaloids is described. The route features a ruthenium-catalyzed asymmetric hydrogenation of a ketone via DKR to construct the chiral trans-1-indolyl-2-isopropenylcyclohexane skeleton and a switchable sequence of methylation and acetylation/aldol reaction to access a chiral quaternary stereocenter. (+)-Hapalindole Q (1, 13 steps, 5.9% overall yield), (-)-12-epi-hapalindole Q isonitrile (2, 15 steps, 5.5% overall yield), (-)-hapalindole D (3, 14 steps, 2.3% overall yield), and (+)-12-epi-fischerindole U isothiocyanate (4, 14 steps, 3.0% overall yield) were synthesized in 13-15 steps from a commercially available material to demonstrate the application of this approach.

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.

A concise total synthesis of sespenine, a structurally unusual indole terpenoid from Streptomyces

A ten-step (the longest linear sequence) total synthesis of sespenine was accomplished.

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.

Synthesis of a chiral fluorescence active probe and its application as an efficient catalyst in the asymmetric Friedel–Crafts alkylation of indole derivatives with nitroalkenes

A green protocol for the asymmetric Friedel–Crafts alkylation of indole derivatives with nitrostyrene using chiral Zn(ii) catalyst has been developed.

Use of a palladium(II)-catalyzed oxidative kinetic resolution in synthetic efforts toward bielschowskysin

Progress toward the cyclobutane core of bielshowskysin is reported. The core was thought to arise from a cyclopropane intermediate via a furan-mediated cyclopropane fragmentation, followed by a 1,4-Michael addition. The synthesis of the cyclopropane intermediate utilizes a Suzuki coupling reaction, an esterification with 2-diazoacetoacetic acid, and a copper catalyzed cyclopropanation. An alcohol intermediate within the synthetic route was obtained in high enantiopurity via a highly selective palladium(II)-catalyzed oxidative kinetic resolution (OKR).