Functions, Structures, and Engineering of the Teleocidin Biosynthetic Enzymes

Teleocidins are natural products belonging to the indole alkaloid family and show potent protein kinase C activation activity. The structural feature of teleocidins is an indole-fused nine-membered lactam ring structure. Due to their unique structures and strong biological activities, many total synthesis and biosynthetic studies of teleocidins have been performed. Teleocidin biosynthesis involves interesting enzymatic reactions that are challenging in organic synthesis, including oxidative intramolecular C-N bond-forming reactions, regio- and stereo-selective reverse prenylation reactions, and methylation-triggered terpene cyclization. This review summarizes the recent research on functional and structural analyses, as well as enzyme engineering, of teleocidin biosynthetic enzymes.

Rapid and efficient syntheses of tryptophans using a continuous-flow quaternization-substitution reaction of gramines with a chiral nucleophilic glycine equivalent

A continuous-flow quaternization reaction of gramines with MeI (<1 min) followed by a substitution reaction with a chiral nucleophilic glycine-derived Ni-complex (S)-2 (<1 min) has successfully been developed to afford the corresponding alkylated Ni-complexes 3 in good yields with excellent diastereoselectivity, based on the results of a one-pot quaternization-substitution reaction of gramines with (S)-2 in a batch process. The continuous-flow process allowed the safe and efficient scale-up synthesis of 3j (84% yield, 99% de, 540 g h^-1) to give 7-azatryptophan derivative (S)-4j readily by an acid-catalyzed hydrolysis reaction followed by protection with an Fmoc group. The present method for the rapid and efficient syntheses of enantiopure unnatural tryptophan derivatives from various gramines and (S)-2 will be useful to further promote peptide and protein drug discovery and development research.

Synthesis of N-indolated amino acids or peptides from 2-alkynylanilines via a dearomatization process

A process for the rapid synthesis of N-indolated amino acids or peptides from readily available 2-alkynylanilines via dearomatization was reported.

Palladium-Catalyzed [2 + 2 + 1] Annulation of Alkyne-Tethered Aryl Iodides with Diaziridinone: Synthesis of 3,4-Fused Tricyclic Indoles

A novel palladium-catalyzed [2 + 2 + 1] annulation of alkyne-tethered aryl iodides with diaziridinone was developed, leading to the formation of 3,4-fused tricyclic indoles. From a mechanistic standpoint, the formation of fused tricyclic indole scaffolds involved C,C-palladacycles, which were synthesized through the intramolecular reaction of aryl halides and alkynes. The cascade reaction described herein could be carried out with a broad range of substrates and provided various 3,4-fused tricyclic indoles with yields up to 98%.

Direct Aromatic Nitration System for Synthesis of Nitrotryptophans in Escherichia coli

Nitrotryptophan and its analogues are useful building blocks for synthesizing bioactive and biotechnologically relevant chemicals, materials, and proteins. However, synthetic routes to enantiopure nitro-containing tryptophan derivatives are either complex and polluting or even unestablished yet. Herein, we describe microbial production of 4-NO₂-l-tryptophan (Nitrotrp) and its analogues by designing and expressing the biosynthetic pathway in Escherichia coli. The biosynthetic pathway comprised one engineered self-sufficient P450 TB14 of Streptomyces origin for direct nitration of the C-4 of l-Trp indole and one nitric oxide synthase from Bacillus subtilis (BsNOS) for the production of nitric oxide (NO) from l-Arg to support the direct aromatic nitration. As both TB14 and BsNOS require reducing agent NADPH for their reactions, we also included one glucose dehydrogenase (GDH) from B. subtilis for in situ NADPH regeneration. The initially designed pathway led to 16.2 ± 2.3 mg/L of Nitrotrp by the engineered E. coli fermented in the M9 minimal medium for 3 days. A combination of the design and screening of three additional pathways, fermentation optimization and the knockout of competitive metabolic pathways together improved the Nitrotrp titer to around 192 mg/L within 20 h. Finally, the whole-cell biotransformation system produced eight Nitrotrp analogues with their titers varying from 2.5 to 61.5 mg/L. This work provides the first microbial direct aromatic nitration processes and sets the stage for the development of biocatalytic routes to other useful nitroaromatics in the future.

11-Step Total Synthesis of Teleocidins B-1-B-4

A unified and modular approach to the teleocidin B family of natural products is presented that proceeds in 11 steps and features an array of interesting strategies and methods. Indolactam V, the known biosynthetic precursor to this family, was accessed through electrochemical amination, Cu-mediated aziridine opening, and a remarkable base-induced macrolactamization. Guided by a desire to minimize concession steps, the tactical combination of C-H borylation and a Sigman-Heck transform enabled the convergent, stereocontrolled synthesis of the teleocidins.

High-Yield Production of Herbicidal Thaxtomins and Thaxtomin Analogs in a Nonpathogenic Streptomyces Strain

Thaxtomins are virulence factors of most plant-pathogenic Streptomyces strains. Due to their potent herbicidal activity, attractive environmental compatibility, and inherent biodegradability, thaxtomins are key active ingredients of bioherbicides approved by the U.S. Environmental Protection Agency. However, the low yield of thaxtomins in native Streptomyces producers limits their wide agricultural applications. Here, we describe the high-yield production of thaxtomins in a heterologous host. The thaxtomin gene cluster from S. scabiei 87.22 was cloned and expressed in S. albus J1074 after chromosomal integration. The production of thaxtomins and nitrotryptophan analogs was observed using liquid chromatography-mass spectrometry (LC-MS) analysis. When the engineered S. albus J1074 was cultured in the minimal medium Thx defined medium supplemented with 1% cellobiose (TDMc), the yield of the most abundant and herbicidal analog, thaxtomin A, was 10 times higher than that in S. scabiei 87.22, and optimization of the medium resulted in the highest yield of thaxtomin analogs at about 222 mg/liter. Further engineering of the thaxtomin biosynthetic gene cluster through gene deletion led to the production of multiple biosynthetic intermediates important to the chemical synthesis of new analogs. Additionally, the versatility of the thaxtomin biosynthetic system in S. albus J1074 was capitalized on to produce one unnatural fluorinated analog, 5-fluoro-thaxtomin A (5-F-thaxtomin A), whose structure was elucidated by a combination of MS and one-dimensional (1D) and 2D nuclear magnetic resonance (NMR) analyses. Natural and unnatural thaxtomins demonstrated potent herbicidal activity in radish seedling assays. These results indicated that S. albus J1074 has the potential to produce thaxtomins and analogs thereof with high yield, fostering their agricultural applications.IMPORTANCE Thaxtomins are agriculturally valuable herbicidal natural products, but the productivity of native producers is limiting. Heterologous expression of the thaxtomin gene cluster in S. albus J1074 resulted in the highest yield of thaxtomins ever reported, representing a significant leap forward in its wide agricultural use. Furthermore, current synthetic routes to thaxtomins and analogs are lengthy, and two thaxtomin biosynthetic intermediates produced at high yields in this work can provide precursors and building blocks to advanced synthetic routes. Importantly, the production of 5-F-thaxtomin A in engineered S. albus J1074 demonstrated a viable alternative to chemical methods in the synthesis of new thaxtomin analogs. Moreover, our work presents an attractive synthetic biology strategy to improve the supply of herbicidal thaxtomins, likely finding general applications in the discovery and production of many other bioactive natural products.

Synthetic Indolactam V Analogues as Inhibitors of PAR2-Induced Calcium Mobilization in Triple-Negative Breast Cancer Cells

Human proteinase-activated receptor 2 (PAR2), a transmembrane G-protein-coupled receptor (GPCR), is an attractive target for a novel anticancer therapy, as it plays a critical role in cell migration and invasion. Selective PAR2 inhibitors therefore have potential as anti-metastatic drugs. Knowing that the natural product teleocidin A2 is able to inhibit PAR2 in tumor cells, the goal of the present study was to elaborate structure-activity relationships and to identify potent PAR2 inhibitors with lower activity against the adverse target, protein kinase C (PKC). For this purpose, an efficient gram-scale total synthesis of indolactam V (i.e., the parent structure of all teleocidins) was developed, and a library of derivatives was prepared. Some compounds were indeed found to exhibit high potency as PAR2 inhibitors at low nanomolar concentrations with improved selectivity (relative to teleocidin A2). The pseudopeptidic fragment bridging the C3 and C4 positions of the indole core proved to be essential for target binding, whereas activity and target selectivity depends on the substituents at N1 or C7. This study revealed novel derivatives that show high efficacy in PAR2 antagonism combined with increased selectivity.

Total synthesis of natural productsviairidium catalysis

An overview of the highlights in total synthesis of natural products using iridium as a catalyst is given.

Tryptophan-Containing Non-Cationizable Opioid Peptides - a new chemotype with unusual structure and in vivo activity

Recently, a new family of opioid peptides containing tryptophan came to the spotlight for the absence of the fundamental protonable tyramine 'message' pharmacophore. Structure-activity relationship investigations led to diverse compounds, characterized by different selectivity profiles and agonist or antagonist effects. Substitution at the indole of Trp clearly impacted peripheral/central antinociceptivity. These peculiarities prompted to gather all the compounds in a new class, and to coin the definition 'Tryptophan-Containing Non-Cationizable Opioid Peptides', in short 'TryCoNCOPs'. Molecular docking analysis suggested that the TryCoNCOPs can still interact with the receptors in an agonist-like fashion. However, most TryCoNCOPs showed significant differences between the in vitro and in vivo activities, suggesting that opioid activity may be elicited also via alternative mechanisms.

Unlocking Reactivity of TrpB: A General Biocatalytic Platform for Synthesis of Tryptophan Analogues

Derivatives of the amino acid tryptophan (Trp) serve as precursors for the chemical and biological synthesis of complex molecules with a wide range of biological properties. Trp analogues are also valuable as building blocks for medicinal chemistry and as tools for chemical biology. While the enantioselective synthesis of Trp analogues is often lengthy and requires the use of protecting groups, enzymes have the potential to synthesize such products in fewer steps and with the pristine chemo- and stereoselectivity that is a hallmark of biocatalysis. The enzyme TrpB is especially attractive because it can form Trp analogues directly from serine (Ser) and the corresponding indole analogue. However, many potentially useful substrates, including bulky or electron-deficient indoles, are poorly accepted. We have applied directed evolution to TrpB from Pyrococcus furiosus and Thermotoga maritima to generate a suite of catalysts for the synthesis of previously intractable Trp analogues. For the most challenging substrates, such as nitroindoles, the key to improving activity lay in the mutation of a universally conserved and mechanistically important residue, E104. The new catalysts express at high levels (>200 mg/L of Escherichia coli culture) and can be purified by heat treatment; they can operate up to 75 °C (where solubility is enhanced) and can synthesize enantiopure Trp analogues substituted at the 4-, 5-, 6-, and 7-positions, using Ser and readily available indole analogues as starting materials. Spectroscopic analysis shows that many of the activating mutations suppress the decomposition of the active electrophilic intermediate, an amino-acrylate, which aids in unlocking the synthetic potential of TrpB.

Directing Stem Cell Fate: The Synthetic Natural Product Connection

Stem cells possess remarkable potential for the treatment of a broad array of diseases including many that lack therapeutic options. However, the use of cell-based products derived from stem cells as therapeutics has limitations including rejection, sufficient availability, and lack of appropriate engraftment. Chemical control of stem cells provides potential solutions for overcoming many of the current limitations in cell-based therapeutics. The development of exogenous molecules to control stem cell self-renewal or differentiation has arrived at natural product-based agents as an important class of modulators. The ex vivo production of cryopreserved cellular products for use in tissue repair is a relatively new area of medicine in which the conventional hurdles to implementing chemicals to effect human health are changed. Translational challenges centered on chemistry, such as pharmacokinetics, are reduced. Importantly, in many cases the desired human tissues can be evaluated against new chemicals, and approaches to cellular regulation can be validated in the clinically applicable system. As a result linking new and existing laboratory syntheses of natural products with findings of the compounds' unique abilities to regulate stem cell fate provides opportunities for developing improved methods for tissue manufacture, accessing probe compounds, and generating new leads that yield manufactured cells with improved properties. This review provides a summary of natural products that have shown promise in controlling stem cell fate and which have also been fully synthesized thereby providing chemistry platforms for further development.

Unlocking Reactivity of TrpB: A General Biocatalytic Platform for Synthesis of Tryptophan Analogues

Derivatives of the amino acid tryptophan (Trp) serve as precursors for the chemical and biological synthesis of complex molecules with a wide range of biological properties. Trp analogues are also valuable as building blocks for medicinal chemistry and as tools for chemical biology. While the enantioselective synthesis of Trp analogues is often lengthy and requires the use of protecting groups, enzymes have the potential to synthesize such products in fewer steps and with the pristine chemo- and stereoselectivity that is a hallmark of biocatalysis. The enzyme TrpB is especially attractive because it can form Trp analogues directly from serine (Ser) and the corresponding indole analogue. However, many potentially useful substrates, including bulky or electron-deficient indoles, are poorly accepted. We have applied directed evolution to TrpB from Pyrococcus furiosus and Thermotoga maritima to generate a suite of catalysts for the synthesis of previously intractable Trp analogues. For the most challenging substrates, such as nitroindoles, the key to improving activity lay in the mutation of a universally conserved and mechanistically important residue, E104. The new catalysts express at high levels (>200 mg/L of Escherichia coli culture) and can be purified by heat treatment; they can operate up to 75 °C (where solubility is enhanced) and can synthesize enantiopure Trp analogues substituted at the 4-, 5-, 6-, and 7-positions, using Ser and readily available indole analogues as starting materials. Spectroscopic analysis shows that many of the activating mutations suppress the decomposition of the active electrophilic intermediate, an amino-acrylate, which aids in unlocking the synthetic potential of TrpB.

Total Syntheses of Festuclavine, Pyroclavine, Costaclavine, epi-Costaclavine, Pibocin A, 9-Deacetoxyfumigaclavine C, Fumigaclavine G, and Dihydrosetoclavine

A new approach for the divergent total synthesis of eight ergot alkaloids is reported. The approach allows the first total syntheses of pyroclavine, pibocin A, 9-deacetoxyfumigaclavine C, and fumigaclavine G and also enables the efficient synthesis of festuclavine, costaclavine, epi-costaclavine, and dihydrosetoclavine. The main feature of the synthesis is the use of an unprecedented Pd-catalyzed intramolecular Larock indole annulation/Tsuji-Trost allylation cascade to assemble the tetracyclic core in one step.

Modular Total Synthesis of Protein Kinase C Activator (-)-Indolactam V

A concise, eight-step total synthesis of (-)-indolactam V, a nanomolar agonist of protein kinase C, is reported. The synthesis relies upon an efficient copper-catalyzed amino acid arylation to establish the indole C4-nitrogen bond. This cross-coupling method is applicable to a range of hydrophobic amino acids, providing a platform for further diversification of indolactam alkaloid scaffolds and studies on their potent biological activity.

Total synthesis of haliclamide

A stereoselective synthesis of haliclamide has been developed. The synthesis includes MacMillan cross aldol, Mitsunobu inversion, Yamaguchi–Hirao alkylation, Steglich esterification and macrolactamization reactions and the Corey–Fuchs protocol as the key steps.

A concise gram-scale synthesis of ht-13-A via a rhodium-catalyzed intramolecular C–H activation reaction

The enantioselective total synthesis of the 3,4-fused indole alkaloid ht-13-A has been achieved.

Rhodium-catalyzed intramolecular annulation via C–H activation leading to fused tricyclic indole scaffolds

A new method for the synthesis of fused tricyclic indoles via a rhodium catalyzed intramolecular C–H activation reaction is described.

Total syntheses of indolactam alkaloids (-)-indolactam V, (-)-pendolmycin, (-)-lyngbyatoxin A, and (-)-teleocidin A-2

We report the total syntheses of (-)-indolactam V and the C7-substituted indolactam alkaloids (-)-pendolmycin, (-)-lyngbyatoxin A, and (-)-teleocidin A-2. The strategy for preparing indolactam V relies on a distortion-controlled indolyne functionalization reaction to establish the C4-N linkage, in addition to an intramolecular conjugate addition to build the conformationally-flexible nine-membered ring. The total synthesis of indolactam V then sets the stage for the divergent synthesis of the other targeted alkaloids. Specifically, late-stage sp²-sp³ cross-couplings on an indolactam V derivative are used to introduce the key C7 substituents and the necessary quaternary carbons. These challenging couplings, in addition to other delicate manipulations, all proceed in the presence of a basic tertiary amine, an unprotected secondary amide, and an unprotected indole. Thus, our approach not only enables the enantiospecific total syntheses of four indolactam alkaloids, but also serves as a platform for probing complexity-generating and chemoselective transformations in the context of alkaloid total synthesis.

Rhodium(iii)-catalyzed intramolecular amidoarylation and hydroarylation of alkyne via C–H activation: switchable synthesis of 3,4-fused tricyclic indoles and chromans

An intramolecular amidoarylation and hydroarylation of alkyne via rhodium(iii)-catalyzed C–H activation was developed for the switchable synthesis of 3,4-fused indoles and chromans.

Total synthesis of rugulovasine A

A concise total synthesis of rugulovasine A is achieved by using Uhle's ketone derivative as the key intermediate, which was synthesized by intramolecular cyclization via metal-halogen exchange. Two different routes to construct a spirocyclic butyrolactone subunit involving a Ru-catalyzed cyclocarbonylation and a special Ru-catalyzed double bond rearrangement were studied.

A new approach toward the total synthesis of (+)-batzellaside B

A new synthetic approach to (+)-batzellaside B from naturally abundant L-pyroglutamic acid is presented in this article. The key synthetic step involves Sharpless asymmetric dihydroxylation of an olefinic substrate functionalized with an acetoxy group to introduce two chiral centres diastereoselectively into the structure. Heterocyclic hemiaminal 4, which could be converted from the resulting product, was found to provide stereospecific access to enantiomerically enriched allylated intermediate, offering better prospects for the total synthesis of this natural product.

Enantioselective synthesis of tryptophan derivatives by a tandem Friedel-Crafts conjugate addition/asymmetric protonation reaction

The tandem Friedel-Crafts conjugate addition/asymmetric protonation reaction between 2-substituted indoles and methyl 2-acetamidoacrylate is reported. The reaction is catalyzed by (R)-3,3'-dibromo-BINOL in the presence of stoichiometric SnCl(4), and is the first example of a tandem conjugate addition/asymmetric protonation reaction using a BINOL·SnCl(4) complex as the catalyst. A range of indoles furnished synthetic tryptophan derivatives in good yields and high levels of enantioselectivity, even on a preparative scale. The convergent nature of this transformation should lend itself to the preparation of unnatural tryptophan derivatives for use in a broad array of synthetic and biological applications.