Total Synthesis of NW-G01, a Cyclic Hexapeptide Antibiotic, and 34-epi-NW-G01

Asymmetric Synthesis with Organoselenium Compounds - The Past Twelve Years

The discovery and synthetic applications of novel organoselenium compounds and their reactions proceeded rapidly during the past fifty years and such processes are now carried out routinely in many laboratories. At the same time, the growing demand for new enantioselective processes provided new challenges. The convergence of selenium chemistry and asymmetric synthesis led to key developments in the 1970s, although the majority of early work was based on stoichiometric processes. More recently, greater emphasis has been placed on greener catalytic variations, along with the discovery of novel reactions and a deeper understanding of their mechanisms. The present review covers the literature in this field from 2010 to early 2023 and encompasses asymmetric reactions mediated by chiral selenium-based reagents, auxiliaries, and especially, catalysts. Protocols based on achiral selenium compounds in conjunction with other species of chiral catalysts, as well as reactions that are controlled by chiral substrates, are also included.

Concise Total Synthesis and Biological Evaluation of Pargamicin A and its Diastereomer, Piperazic Acid-containing Cyclopeptides

We have accomplished the total synthesis, structure determination, and biological evaluation of pargamicin A and one of its diastereomers. Two key tripeptide segments were synthesized using a linear peptide elongation process that includes the direct coupling of a poorly nucleophilic piperazic acid derivative. The resulting tripeptides were coupled using triphosgene/collidine at ambient temperature leading to a precursor for the final cyclization step. T3P-promoted macrolactamization under high-dilution conditions, followed by the removal of the benzyl protecting group was used to furnish two putative structures of pargamicin A. Comparison of the 1 H and 13 C NMR spectra and the antibacterial activity of the natural and synthetic products successfully revealed that the absolute configuration of the N-hydroxy-Ile residue of pargamicin A is 2S,3S. A biological evaluation of synthetically obtained pargamicin A and its diastereomer suggested that the stereostructure of the cyclic peptide scaffold of the natural product plays a crucial role in determining the strength of its antibacterial activity.

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.

Total Synthesis of Melicoptelines C-E: Antiviral Cyclopeptides Containing a Hexahydropyrrolo[2,3-b]indole Moiety

Melicoptelines, natural cyclopeptides containing a 3a-hydroxy hexahydropyrrolo[2,3-b]indole (HPI) moiety, exhibit anti-influenza activity. Herein, we report the first total synthesis of melicoptelines C-E (1-3, respectively). The core 3a-hydroxy HPIs were synthesized in a diastereoselective manner from l-tryptophan using dimethyldioxirane-mediated oxidation. Subsequently, sequential peptide couplings and cyclization completed the synthesis of melicoptelines C-E and unnatural melicopteline 4. The synthesized melicoptelines were evaluated for their anti-influenza activity, and melicopteline E showed the most potent inhibition of cytopathic effects.

Free Piperazic Acid as a Precursor to Nonribosomal Peptides

Piperazic acid (Piz) is a nonproteinogenic amino acid possessing a rare nitrogen-nitrogen bond. However, little is known about how Piz is incorporated into nonribosomal peptides, including whether adenylation domains specific to Piz exist. In this study, we show that free piperazic acid is directly adenylated and then incorporated into the incarnatapeptin nonribosomal peptides through isotopic incorporation studies. We also use in vitro reconstitution to demonstrate adenylation of free piperazic acid with a three-domain nonribosomal peptide synthetase from the incarnatapeptin gene cluster. We furthermore use bioinformatics and site-directed mutagenesis to outline consensus sequences for the adenylation of piperazic acid, which can now be used for the prediction of gene clusters linked to piperazic-acid-containing peptides. Finally, we discover a fusion protein of a piperazate synthase and an adenylation domain, highlighting the close biosynthetic relationship of piperazic acid formation and its adenylation. Altogether, our work demonstrates the evolution of biosynthetic systems for the activation of free piperazic acid through adenylation, a pathway we suggest is likely to be employed in the majority of pathways to piperazic-acid-containing peptides.

Synthetic and biosynthetic routes to nitrogen-nitrogen bonds

The nitrogen-nitrogen bond is a core feature of diverse functional groups like hydrazines, nitrosamines, diazos, and pyrazoles. Such functional groups are found in >300 known natural products. Such N-N bond-containing functional groups are also found in significant percentage of clinical drugs. Therefore, there is wide interest in synthetic and enzymatic methods to form nitrogen-nitrogen bonds. In this review, we summarize synthetic and biosynthetic approaches to diverse nitrogen-nitrogen-bond-containing functional groups, with a focus on biosynthetic pathways and enzymes.

Selenocyclization by formation of carbon-nitrogen bonds

Selenium promoted cyclization of unsaturated substrates containing internal nitrogen nucleophiles, such as different amines and amides, including the examples of its application in the synthesis of more complex polycyclic compounds is reviewed. Selenocyclization reactions of some more specific polyfunctional substrates, like Biginelli hybrids and hydantoins, are also covered.

A Reliable Enantioselective Route to Mono-Protected N1-Cbz Piperazic Acid Building Block

The chiral N1-Cbz, N2-H derivative of the piperazic acid monomer is a valuable building block in the total synthesis of natural products, comprising this nonproteinogenic amino acid. In that context, we wish to report an improved synthetic protocol for the synthesis of both (3R)- and (3S)-piperazic acids bearing the carboxybenzyl protecting group (Cbz) selectively at the N1 position. Our method builds on previously reported protocols, circumventing their potential shortcomings, and optimizing the ultimate selective deprotection at the N2 position, thus, offering an efficient and reproducible pathway to suitably modified piperazates in high optical purity.

Procedure-Economical, Enantioselective Total Syntheses of Polycyclic Natural Products and Analogues Containing a 3a-Hydroxyhexahydropyrrolo[2,3-b]indole-2-carboxylic Acid Residue

The 3a-hydroxyhexahydropyrrolo[2,3-b]indole-2-carboxylic acid (HPIC) residue and its aza-analogue are found in many bioactive natural products. In this account, short divergent total syntheses of several such natural products, diastereomers and analogues are described. It is demonstrated that by appropriate combination of different efficient tactics such as biomimetic/bio-inspired synthesis, chemo/regioselective reactions, umpolung of regioselectivity and/or reactivity, and tandem reactions, the enantioselective syntheses of polycyclic molecules such as (+)-asperlicin E and (–)-robustanoids A and B can be achieved in a protecting-group-free and redox-economical manner, in only three to four steps starting from l-tryptophan.

1 Introduction

2 Strategic Considerations

2.1 Occurrence of HO-HPIC and HO-aza-HPIC Residues in Natural Products

2.2 Biosyntheses of HO-HPIC and HO-aza-HPIC Residues

2.3 Chemical Syntheses of HO-HPIC and HO-aza-HPIC Residues

3 Procedure-Economical Syntheses of HO-HPIC-Containing Natural Products

3.1 Protecting-Group-Free Syntheses of Asperlicin E, Its Diastereomer, and an Analogue

3.2 Divergent Syntheses of (–)-Robustanoids A and B, a Diastereomer, and Analogues

4 Conclusion and Future Perspectives

Synthesis of Inducamides A and B

The inducamides are a family of chlorinated alkaloids featuring an amide arising from union of an l-tryptophan to a rare chlorosalicylic acid unit, the production of which is linked to a chemically induced mutation in the RNA polymerase of Streptomyces sp. (SNC-109-M3). The synthesis of inducamides A and B has been accomplished by the coupling of 6-hydroxy-3-chloro-2-methylbenzoic acid with l-6-chlorotryptophan and l-tryptophan, respectively, followed by ester hydrolysis. The spectroscopic data and optical rotation for each synthetic sample confirm the structures of these silent secondary metabolites and their biosynthesis from l-tryptophan.

Identification of pyrroloindoline-containing cyclic hexapeptides in the metabolites of Streptomyces alboflavus 313 by HPLC-DAD-ESI-MS/MS

To investigate the chemical biodiversity of biologically active cyclic hexapeptides in the metabolites from microorganisms, the fermentation broth of Streptomyces alboflavus 313 was analyzed using HPLC, equipped with a diode array detector (DAD), coupled with ESI tandem MS (HPLC-DAD-ESI-MS/MS). In the mass spectra of cyclic hexapeptides, predominant ions [M+H](+), as well as [M-18+H](+), [M-28+H](+) and [M+Na](+), were observed and used to determine the molecular masses, while fragmentation reactions of [M+H](+) were recorded to provide information on the contents of amino acids and their linkage sequence. Based on the fragmentation patterns and comparison with standards, 15 pyrroloindoline-containing natural cyclic hexapeptides, cp01-15, were identified from the microorganism and six of these are reported for the first time.