Total syntheses and biological investigations of tamandarins A and B and tamandarin A analogs

Novel 1,4-Dihydropyridine Derivatives as Mineralocorticoid Receptor Antagonists

The mineralocorticoid receptor (MR) belongs to the steroid receptor subfamily of nuclear receptors. MR is a transcription factor key in regulating blood pressure and mineral homeostasis. In addition, it plays an important role in a broad range of biological and pathological conditions, greatly expanding its interest as a pharmacological target. Non-steroidal MR antagonists (MRAs) are of particular interest to avoid side effects and achieve tissue-specific modulation of the receptor. The 1,4-dihydropyridine (1,4-DHP) ring has been identified as an appropriate scaffold to develop non-steroidal MRAs. We report the identification of a novel series of 1,4-DHP that has been guided by structure-based drug design, focusing on the less explored DHP position 2. Interestingly, substituents at this position might interfere with MR helix H12 disposition, which is essential for the recruitment of co-regulators. Several of the newly synthesized 1,4-DHPs show interesting properties as MRAs and have a good selectivity profile. These 1,4-DHPs promote MR nuclear translocation with less efficiency than the natural agonist aldosterone, which explains, at least in part, its antagonist character. Molecular dynamic studies are suggestive of several derivatives interfering with the disposition of H12 in the agonist-associated conformation, and thus, they might stabilize an MR conformation unable to recruit co-activators.

Therapeutic Potential of Marine Peptides in Prostate Cancer: Mechanistic Insights

Prostate cancer (PCa) is the leading cause of cancer death in men, and its treatment is commonly associated with severe adverse effects. Thus, new treatment modalities are required. In this context, natural compounds have been widely explored for their anti-PCa properties. Aquatic organisms contain numerous potential medications. Anticancer peptides are less toxic to normal cells and provide an efficacious treatment approach via multiple mechanisms, including altered cell viability, apoptosis, cell migration/invasion, suppression of angiogenesis and microtubule balance disturbances. This review sheds light on marine peptides as efficacious and safe therapeutic agents for PCa.

Natural Cyclopeptides as Anticancer Agents in the Last 20 Years

Cyclopeptides or cyclic peptides are polypeptides formed by ring closing of terminal amino acids. A large number of natural cyclopeptides have been reported to be highly effective against different cancer cells, some of which are renowned for their clinical uses. Compared to linear peptides, cyclopeptides have absolute advantages of structural rigidity, biochemical stability, binding affinity as well as membrane permeability, which contribute greatly to their anticancer potency. Therefore, the discovery and development of natural cyclopeptides as anticancer agents remains attractive to academic researchers and pharmaceutical companies. Herein, we provide an overview of anticancer cyclopeptides that were discovered in the past 20 years. The present review mainly focuses on the anticancer efficacies, mechanisms of action and chemical structures of cyclopeptides with natural origins. Additionally, studies of the structure-activity relationship, total synthetic strategies as well as bioactivities of natural cyclopeptides are also included in this article. In conclusion, due to their characteristic structural features, natural cyclopeptides have great potential to be developed as anticancer agents. Indeed, they can also serve as excellent scaffolds for the synthesis of novel derivatives for combating cancerous pathologies.

Recent Advances in Small Peptides of Marine Origin in Cancer Therapy

Cancer is one of the leading causes of death in the world, and antineoplastic drug research continues to be a major field in medicine development. The marine milieu has thousands of biological species that are a valuable source of novel functional proteins and peptides, which have been used in the treatment of many diseases, including cancer. In contrast with proteins and polypeptides, small peptides (with a molecular weight of less than 1000 Da) have overwhelming advantages, such as preferential and fast absorption, which can decrease the burden on human gastrointestinal function. Besides, these peptides are only connected by a few peptide bonds, and their small molecular weight makes it easy to modify and synthesize them. Specifically, small peptides can deliver nutrients and drugs to cells and tissues in the body. These characteristics make them stand out in relation to targeted drug therapy. Nowadays, the anticancer mechanisms of the small marine peptides are still largely not well understood; however, several marine peptides have been applied in preclinical treatment. This paper highlights the anticancer linear and cyclic small peptides in marine resources and presents a review of peptides and the derivatives and their mechanisms.

A nickel-mediated oxidative α-C(sp3)–H functionalization of amides with allylic alcohols terminated by radical 1,2-aryl migration

A Ni-mediated oxidative C(sp³)–H functionalization of N,N-substituted amides with α,α-diaryl allylic alcohols through a radical 1,2-aryl migration process has been developed. This process features a broad substrate scope and excellent functional group tolerance.

"Head-to-side-chain" cyclodepsipeptides of marine origin

Since the late 1980s, a large number of depsipeptides that contain a new topography, referred to as "head-to-side-chain" cyclodepsipeptides, have been isolated and characterized. These peptides present a unique structural arrangement that comprises a macrocyclic region closed through an ester bond between the C-terminus and a β-hydroxyl group, and terminated with a polyketide moiety or a more simple branched aliphatic acid. This structural pattern, the presence of unique and complex residues, and relevant bioactivity are the main features shared by all the members of this new class of depsipeptides, which are reviewed herein.

Didemnins, tamandarins and related natural products

Since the discovery and isolation of the didemnin family of marine depsipeptides in 1981, the synthesis and biological activity of its congeners have been of great interest to the scientific community. The didemnins have demonstrated antitumor, antiviral, and immunosuppressive activity at low nano- and femtomolar levels. Of the congeners, didemnin B was the first marine natural product to reach phase II clinical trials in the United States, stimulating many analogue syntheses to date. About two decades later, tamandarins A and B were isolated, and were found to possess very similar structure and biological activity to that of the didemnin B. These compounds have shown impressive biological activity and some progress has been made in establishing structure-activity relationships. However, their molecular mechanism of action still remains unclear. This review highlights the long-standing study of didemnins and its critical application towards the understanding of the molecular mechanism of action of tamandarins and their potential use as therapeutic agents.

Total synthesis of Lys(3) tamandarin M: a potential affinity ligand

The synthesis of Lys(3) tamandarin M is described. This analogue can be used as a protein affinity ligand to probe the mechanism of action of this unique class of molecules.

Synthetic studies of tamandarin B side chain analogues

The syntheses of three tamandarin B analogues are described. The goal of these studies was to prepare material to determine their relative therapeutic index and to gain an oversight as to their potential for clinical applications.

An efficient synthesis of the tamandarin B macrocycle

A reliable, high yielding cyclization protocol for the macrocycle of tamandarin B is presented. This strategy will facilitate the synthesis of side chain analogues.

A simple microscale method for determining the relative stereochemistry of statine units

A simple method to determine the relative stereochemistry of statine amino acids (gamma-amino-beta-hydroxyacids) by using (1)H NMR spectroscopy is described. Configurational assignment of statine units within complex natural products is possible without degradation or derivatization as the syn and anti diastereomers can be distinguished by using a combination of chemical shift and coupling constant information derived from the alpha-methylene ABX system. Seventy-three examples are provided, demonstrating the scope and limitations of the methodology. These examples range in complexity from simple statine units to cyclic depsipeptides, such as tamandarin B.

Synthesis and biological evaluation of tamandarin B analogues

[structure: see text]. The synthesis of two tamandarin B analogues in which the N,O-Me2Tyr5 unit was replaced by N-Me-phenylalanine (N-MePhe5) and (S)-2-(methylamino)-3-(naphthalen-2-yl)propanoic acid (N-MeNaphth5) is described. The choice of the macrocyclization site was crucial to achieve satisfactory macrolactamization. Coupling between norstatine (Nst1) and threonine (Thr6) afforded only a 15% yield, while lactamization between proline (Pro4) and the aromatic moiety could be achieved in 65% yield.

Direct Synthesis of Esters and Amides from Unprotected Hydroxyaromatic and -aliphatic Carboxylic Acids

A facile method for the activation of hydroxy-substituted carboxylic acids using benzotriazole chemistry without prior protection of the hydroxy substituents is presented. The N-acylbenzotriazole intermediates 2a-g, 6a-d, and 9a-c have been used for high-yielding synthesis of both aliphatic (3a-l) and aromatic (7a-h, 10a-f) hydroxy carboxamides. High yields of aromatic hydroxy esters 12a-h and 13a-i were obtained using either neat alcohols in neutral microwave conditions or nucleophilic alkoxides and the intermediate N-(arylacyl)benzotriazoles. Moderate yields were obtained in the case of aliphatic hydroxy esters 11a,b and thiolesters 11e-g from the intermediates 2a-c.

The cyclization of peptides and depsipeptides

Constricting the peptide backbone into a more defined conformational form through cyclization is an activity evolved in nature and in synthetic work, the latter straddling only the most recent decades. The resulting conformational constraints increase the probability of an optimum response with bio-receptors. The purpose of this review is to highlight developments that have proved to be reasonably efficient in the macrocyclization of linear precursors into cyclic peptides and depsipeptides.

Natural products as probes of cell biology: 20 years of didemnin research

The discovery of the didemnin family of marine depsipeptides launched an exciting and intriguing chapter in natural product chemistry. The unusual structure of the didemnin congeners has led to several total syntheses by research groups from around the world. The impressive in vitro and in vivo biological activities of the didemnins resulted in the first human clinical trials in the U.S. of a marine natural product against cancer, and additional clinical trials of a second-generation didemnin, dehydrodidemnin B (aplidine), are underway. As we mark the 20-year anniversary of the discovery of the didemnins, this class of natural products continues to stimulate active research in fields ranging from synthetic and medicinal chemistry to clinical oncology and cell biology. While some progress was made in dissecting the molecular mechanism of action and in establishing structure-activity relationships, there are still more questions than answers. This review covers the recent didemnin literature, highlighting the work directed towards understanding how this group of natural products interact with fundamental processes such as cell proliferation, protein biosynthesis, and apoptosis. The didemnin field illustrates how natural product chemistry may be used as a critical tool for the study of cell biology.