Total Synthesis of Plusbacin A₃ and Its Dideoxy Derivative Using a Solvent-Dependent Diastereodivergent Joullié–Ugi Three-Component Reaction

Full details of our synthetic studies toward plusbacin A₃ (<b>1</b>), which is a depsipeptide with antibacterial activity, and its dideoxy derivative are described. To establish an efficient synthetic route of <b>1</b>, a solvent-dependent diastereodivergent Joullié–Ugi three-component reaction (JU-3CR) was used to construct <i>trans</i>-Pro­(3-OH) in a small number of steps. Two strategies were investigated toward the total synthesis. In the first synthetic strategy, the key steps were the <i>trans-</i>selective JU-3CR and a macrolactonization at the final stage of the synthesis. The JU-3CR using alkyl isocyanides in 1,1,1,3,3,3-hexa­fluoro­isopropanol provided the <i>trans</i> products, and the coupling of the fragments to prepare the macrocyclization precursor proceeded smoothly. However, attempts toward the macrolactonization did not provide the desired product. Then, the second strategy that included esterification in an initial stage was investigated. Methods for constructing <i>trans</i>-Pro­(3-OH) were examined using a convertible isocyanide, which could be converted to a carboxylic acid required for the following amidation. Ester bond formation was achieved through an intermolecular coupling using a hydroxyl-Asp derivative and the corresponding alcohol, and the amidation afforded a linear depsipeptide. The macrolactamization of the linear peptide gave the cyclic depsipeptide, and then the global deprotection accomplished the total synthesis of <b>1</b> and its dideoxy derivative

Total Synthesis of Tunicamycin V

The total synthesis of tunicamycin V is described. This strategy is based on the initial construction of tunicaminyluracil, which is regarded to play an important role in the observed biological activities. The key to the synthesis was a Mukaiyama aldol reaction followed by a furan-oxidation to construct the undecose skeleton, a [3,3] sigmatropic rearrangement of a cyanate, and a highly selective trehalose-type glycosylation

Solid-Phase Synthesis of Nannocystin Ax and Its Analogues

Solid-phase total synthesis of nannocystin Ax (1) was disclosed. A coupling reaction between a peptide and a polyketide moiety was conducted on a solid support, and macrocyclization was achieved by Mitsunobu cyclization. The established synthetic route was efficient to prepare its analogues, which contain different types of peptide moieties

Novel Hybrid-Type Antimicrobial Agents Targeting the Switch Region of Bacterial RNA Polymerase

The bacterial RNA polymerase (RNAP) is an ideal target for the development of antimicrobial agents against drug-resistant bacteria. Especially, the switch region within RNAP has been considered as an attractive binding site for drug discovery. Here, we designed and synthesized a series of novel hybrid-type inhibitors of bacterial RNAP. The antimicrobial activities were evaluated using a paper disk diffusion assay, and selected derivatives were tested to determine their MIC values. The hybrid-type antimicrobial agent <b>29</b> showed inhibitory activity against <i>Escherichia coli</i> RNAP. The molecular docking study suggested that the RNAP switch region would be the binding site of <b>29</b>

Total Synthesis and Antibacterial Investigation of Plusbacin A₃

The total synthesis of plusbacin A₃ (<b>1</b>) has been accomplished using a solvent-dependent diastereodivergent Joullié–Ugi three-component reaction (JU-3CR) as a key step. Two <i>trans</i>-3-hydroxy-l-proline residues were constructed by combining the JU-3CR with a convertible isocyanide strategy. Subsequent peptide coupling and macrolactamization afforded plusbacin A₃. Investigating the antibacterial activity of <b>1</b> compared with that of its dideoxy analogue revealed that the <i>threo</i>-β-hydroxyaspartic acid residues are essential for antibacterial activity. Notably, there is a low potential for the development of resistance in <i>S</i>. <i>aureus</i> against plusbacin A₃

Discovery of Potent Hexapeptide Agonists to Human Neuromedin U Receptor 1 and Identification of Their Serum Metabolites

Neuromedin U (NMU) and S (NMS) display various physiological activities, including an anorexigenic effect, and share a common C-terminal heptapeptide-amide sequence that is necessary to activate two NMU receptors (NMUR1 and NMUR2). On the basis of this knowledge, we recently developed hexapeptide agonists <b>2</b> and <b>3</b>, which are highly selective to human NMUR1 and NMUR2, respectively. However, the agonists are still less potent than the endogenous ligand, hNMU. Therefore, we performed an additional structure–activity relationship study, which led to the identification of the more potent hexapeptide <b>5d</b> that exhibits similar NMUR1-agonistic activity as compared to hNMU. Additionally, we studied the stability of synthesized agonists, including <b>5d</b>, in rat serum, and identified two major biodegradation sites: Phe²-Arg³ and Arg⁵-Asn⁶. The latter was more predominantly cleaved than the former. Moreover, substitution with 4-fluorophenylalanine, as in <b>5d</b>, enhanced the metabolic stability at Phe²-Arg³. These results provide important information to guide the development of practical hNMU agonists

Development of a New Benzophenone–Diketopiperazine-Type Potent Antimicrotubule Agent Possessing a 2‑Pyridine Structure

A new benzophenone–diketopiperazine-type potent antimicrotubule agent was developed by modifying the structure of the clinical candidate plinabulin (<b>1</b>). Although the right-hand imidazole ring with a branched alkyl chain at the 5-position in <b>1</b> was critical for the potency of the antimicrotubule activity, we successfully substituted this moiety with a simpler 2-pyridyl structure by converting the left-hand ring from a phenyl to a benzophenone structure without decreasing the potency. The resultant compound <b>6b</b> (KPU-300) exhibited a potent cytotoxicity, with an IC₅₀ value of 7.0 nM against HT-29 cells, by strongly binding to tubulin (<i>K</i>_d = 1.3 μM) and inducing microtubule depolymerization

Negamycin Analogue with Readthrough-Promoting Activity as a Potential Drug Candidate for Duchenne Muscular Dystrophy

A series of (+)-negamycin <b>1</b> analogues were synthesized, and their readthrough-promoting activity was evaluated for nonsense mutations in Duchenne muscular dystrophy (DMD). A structure–activity relationship study indicated that <b>11b</b> was the most potent drug candidate. Immunohistochemical analyses suggested that treatment with <b>11b</b> restored dystrophin expression in <i>mdx</i> mice, a DMD mouse model. Furthermore, <b>11b</b> decreased serum creatine kinase (CK) levels, an indicator of muscle fiber destruction. Most importantly, <b>11b</b> demonstrated lower toxicity than <b>1</b>, and thus, it could be a useful candidate for long-term treatment of DMD

Novel Hybrid Compound of a Plinabulin Prodrug with an IgG Binding Peptide for Generating a Tumor Selective Noncovalent-Type Antibody–Drug Conjugate

Although several approaches for making antibody–drug conjugates (ADC) have been developed, it has yet to be reported that an antibody binding peptide such as Z33 from protein A is utilized as the pivotal unit to generate the noncovalent-type ADC (NC-ADC). Herein we aim to establish a novel probe for NC-ADC by synthesizing the Z33-conjugated antitumor agent, plinabulin. Due to the different solubility of two components, including hydrophobic plinabulin and hydrophilic Z33, an innovative method with a solid-supported disulfide coupling reagent is required for the synthesis of the target compounds with prominent efficiency (29% isolated yield). We demonstrate that the synthesized hybrid exhibits a binding affinity against the anti-HER2 antibody (Herceptin) and the anti-CD71 antibody (6E1) (<i>K</i>_d = 46.6 ± 0.5 nM and 4.5 ± 0.56 μM, respectively) in the surface plasmon resonance (SPR) assay. In the cell-based assays, the hybrid provides a significant cytotoxicity in the presence of Herceptin against HER2 overexpressing SKBR-3 cells, but not against HER2 low-expressing MCF-7 cells. Further, it is noteworthy that the hybrid in combination with Herceptin induces cytotoxicity against Herceptin-resistant SKBR-3 (SKBR-3HR) cells. Similar results are obtained with the 6E1 antibody, suggesting that the synthesized hybrid can be widely applicable for NC-ADC using the antibody of interest. In summary, a series of evidence presented here strongly indicate that NC-ADCs have high potential for the next generation of antitumor agents

Synthesis and Structure–Activity Relationship Study of Antimicrotubule Agents Phenylahistin Derivatives with a Didehydropiperazine-2,5-dione Structure

Plinabulin (<b>11</b>, NPI-2358) is a potent microtubule-targeting agent derived from the natural diketopiperazine “phenylahistin” (<b>1</b>) with a colchicine-like tubulin depolymerization activity. Compound <b>11</b> was recently developed as VDA and is now under phase II clinical trials as an anticancer drug. To develop more potent antimicrotubule and cytotoxic derivatives based on the didehydro-DKP skeleton, we performed further modification on the <i>tert</i>-butyl or phenyl groups of <b>11</b>, and evaluated their cytotoxic and tubulin-binding activities. In the SAR study, we developed more potent derivatives <b>33</b> with 2,5-difluorophenyl and <b>50</b> with a benzophenone in place of the phenyl group. The anti-HuVEC activity of <b>33</b> and <b>50</b> exhibited a lowest effective concentration of 2 and 1 nM for microtubule depolymerization, respectively. The values of <b>33</b> and <b>50</b> were 5 and 10 times more potent than that of CA-4, respectively. These derivatives could be a valuable second-generation derivative with both vascular disrupting and cytotoxic activities