Total Synthesis of Quinoxapeptin A-C: Establishment of Absolute Stereochemistry

Solid-Phase Total Synthesis of Sandramycin and Its Analogues

Solid-phase total synthesis of sandramycin (1), which is a C₂-symmetric cyclic decadepsipeptide natural product, and its analogues is described. On-resin ester formation and [5+5] peptide coupling allowed the preparation of a range of desymmetrized analogues. An amino acid residue that would not hamper the biological activity of 1 was successfully identified, and probe molecules and dimeric analogues were prepared on the basis of the result of the structure-activity relationship study.

Photoredox-neutral alkene aminoarylation for the synthesis of 1,4,5,6-tetrahydropyridazines

A mild and redox-neutral protocol is developed for the synthesis of 1,4,5,6-tetrahydropyridazines via photoredox catalysis.

Piperazic acid-containing natural products: structures and biosynthesis

Piperazic acid is a cyclic hydrazine and a non-proteinogenic amino acid found in diverse non-ribosomal peptide (NRP) and hybrid NRP–polyketide (PK) structures.

B(C6F5)3-catalyzed metal-free hydrogenation of 3,6-diarylpyridazines

Metal-free hydrogenation of 3,6-diarylpyridaxines was realized for the first time to furnish 1,4,5,6-tetrahydropyridazine derivatives in 85–95% yields.

Biosynthetic modularity rules in the bisintercalator family of antitumor compounds

Diverse actinomycetes produce a family of structurally and biosynthetically related non-ribosomal peptide compounds which belong to the chromodepsipeptide family. These compounds act as bisintercalators into the DNA helix. They give rise to antitumor, antiparasitic, antibacterial and antiviral bioactivities. These compounds show a high degree of conserved modularity (chromophores, number and type of amino acids). This modularity and their high sequence similarities at the genetic level imply a common biosynthetic origin for these pathways. Here, we describe insights about rules governing this modular biosynthesis, taking advantage of the fact that nowadays five of these gene clusters have been made public (thiocoraline, triostin, SW-163 and echinomycin/quinomycin). This modularity has potential application for designing and producing novel genetic engineered derivatives, as well as for developing new chemical synthesis strategies. These would facilitate their clinical development.

Total synthesis of sandramycin and its analogues via a multicomponent assemblage

The total synthesis of sandramycin has been accomplished by using a Staudinger/aza-Wittig/diastereoselective Ugi three-component reaction sequence as a key step to obtain a linear pentadepsipeptide. Subsequent [5 + 5] coupling of the penptapeptide, macrolactamization, and introduction of the quinaldin chromophores afforded sandramycin. Dihydroxy and diacetoxy analogues were also prepared, and the cytotoxic activity of these analogues against a range of human cancer cell lines was evaluated.

Photophysics of novel coumarin-labeled depsipeptides in solution: sensing interactions with SDS micelle via TICT model

N-Acylbenzotriazoles enable the synthesis (69-92% yield) of blue to green fluorescent coumarin-labeled depsidipeptides 8a-f (quantum yields 0.004-0.97) and depsitripeptides 12a-d (quantum yields 0.02-0.96). Detailed photophysical studies of fluorescent coumarin-labeled depsipeptides 8a-f and 12a-d are reported for both polar protic and polar aprotic solvents. 7-Methoxy and 7-diethylaminocoumarin-3-ylcarbonyl depsipeptides 8c,f and 12d are highly solvent sensitive. These highly fluorescent compounds could be useful for peptide assays. Further photophysical studies of 7-diethylaminocoumarin-labeled depsipeptides 8c,12d within the micellar microenvironment of SDS reflect their ability to bind with the biological membrane, suggesting potential applications in the fields of bio- and medicinal chemistry.

Density functional theory study of the platinum-catalyzed cyclopropanation reaction with olefin

A computational study of the platinum-catalyzed cyclopropanation reaction with olefin is presented. The model system is formed by an ethylene molecule and the active catalytic species, which forms from a CH2 fragment and the Cl2Pt(PH3)2 complex. The results show that the active catalytic species is not a metal-carbene of the type (PH3)2Cl2Pt=CH2 but two carbenoid complexes which can exist in almost two degenerate forms, namely (PH3)2Pt(CH2Cl)Cl (carbenoid A) and (PH3)Pt(CH2PH3)Cl2 (carbenoid B). The reaction proceeds through three pathways: methylene transfer, carbometalation for carbenoid A, and the reaction of a monophosphinic species for carbenoids (A and B). The most favored reaction channel is methylene transfer pathway for (PH3)Pt(CH2PH3)Cl2 (carbenoid B) species with a barrier of 31.32 kcal/mol in gas phase. The effects of dichloromethane, THF, and benzene solvent are investigated with PCM method. For carbenoid A, both methylene transfer and carbometalation pathway barriers to reaction become remarkably lower with the increasing polarity of solvent (from 43.25 and 52.50 kcal/mol for no solvent to 25.36 and 38.53 kcal/mol in the presence of the dichloromethane). In contrast, the reaction barriers for carbenoid B via the methylene transfer path hoist 6.30 kcal/mol, whereas the barriers do not change significantly for the reaction path of a monophosphinic species for carbenoids (A and B).

Bisintercalator natural products with potential therapeutic applications: isolation, structure determination, synthetic and biological studies

Echinomycin is the prototypical bisintercalator, a molecule that binds to DNA by inserting two planar chromophores between the base-pairs of duplex DNA, placing its cyclic depsipeptide backbone in the minor groove. As such, it has been the focus of an extensive number of investigations into its biological activity, nucleic acid binding and, to some extent, its structure-activity relationships. However, echinomycin is also the parent member of an extended family of natural products that interact with DNA by a similar mechanism of bisintercalation. The structural variety in these compounds leads to changes in sequence selectivity and and biological activity, particularly as anti-tumour and anti-viral agents. One of the more recently identified marine natural products that is moving close to clinical development is thiocoraline, and it therefore seems timely to review the various bisintercalator natural products.

Total Solid-Phase Synthesis of the Azathiocoraline Class of Symmetric Bicyclic Peptides

Thiocoraline is a potent antitumor agent isolated from the marine organism Micromonospora sp. This symmetric bicyclic depsipeptide binds the minor groove of DNA. Here we report two solid-phase strategies for the syntheses of azathiocoraline and its analogues. The thioester linkage was replaced by an amide bond to improve the compound's pharmacokinetic properties. The first strategy is based on a convergent (4+4) approach, whilst the second is a stepwise synthesis, cyclizations in both approaches occurring on the solid support. These two strategies were designed to overcome problems caused by the presence of consecutive noncommercial N-methyl amino acids, to avoid epimerization during cyclization and/or fragment condensation, and to form the disulfide bridge under solid-phase conditions. The heterocyclic moiety was added in the last step of the synthesis to assist the preparation of libraries of new compounds with potential therapeutic applications.