A Highly Efficient Diversification of 2-Amino/Amido-1,3,4-oxadiazole and 1,3,4-Thiadiazole Derivatives via Reagent-Based Cyclization of Thiosemicarbazide Intermediate on Solid-Phase

Substrate-based synthetic strategies and biological activities of 1,3,4-oxadiazole: A review

The five-membered 1,3,4-oxadiazole heterocyclic ring has received considerable attention because of its unique bio-isosteric properties and an unusually wide spectrum of biological activities. After a century since 1,3,4-oxadiazole was discovered, its uncommon potential attracted medicinal chemist's attention, leading to the discovery of a few presently accessible drugs containing 1,3,4-oxadiazole units, and a large number of patents have been granted on research related to 1,3,4-oxadiazole. It is worth noting that interest in 1,3,4-oxadiazoles' biological applications has doubled in the last few years. Herein, this review presents a comprehensive overview of the recent achievements in the synthesis of 1,3,4-oxadiazole-based compounds and highlights the major advances in their biological applications in the last 10 years, as well as brief remarks on prospects for further development. We hope that researchers across the scientific streams will benefit from the presented review articles for designing their work related to 1,3,4-oxadiazoles.

Regioselective Synthesis of 2,5-Disubstituted-1,3,4-thiadiazoles and 1,3,4-Oxadiazoles via Alkyl 2-(Methylthio)-2-thioxoacetates and Alkyl 2-Amino-2-thioxoacetates

An acid-catalyzed regioselective cyclization reaction of 2,5-disubstituted-1,3,4-thiadiazoles and 1,3,4-oxadiazoles has been developed. The synthetic precursors alkyl 2-(methylthio)-2-thioxoacetates/alkyl 2-amino-2-thioxoacetates react efficiently with acyl hydrazides, which transformed into a series of dehydrative and desulfurative products with employment of p-TSA and AcOH through a regioselective cyclization process. The alkyl 2-amino-2-thioxoacetate pathway generates excellent yield among the mentioned procedures. The reported methods are operationally simplistic and highly efficient with metal-free conditions and demonstrate significant functional group compatibility. Regioselective cyclized products were confirmed by single-crystal X-ray diffraction studies.

Comparative Study of the Synthetic Approaches and Biological Activities of the Bioisosteres of 1,3,4-Oxadiazoles and 1,3,4-Thiadiazoles over the Past Decade

The bioisosteres of 1,3,4-oxadiazoles and 1,3,4-thiadiazoles are well-known pharmacophores for many medicinally important drugs. Throughout the past 10 years, 1,3,4-oxa-/thiadiazole nuclei have been very attractive to researchers for drug design, synthesis, and the study of their potential activity towards a variety of diseases, including microbial and viral infections, cancer, diabetes, pain, and inflammation. This work is an up-to-date comparative study that identifies the differences between 1,3,4-thiadiazoles and 1,3,4-oxadiazoles concerning their methods of synthesis from different classes of starting compounds under various reaction conditions, as well as their biological activities and structure–activity relationship.

trans-Translation inhibitors bind to a novel site on the ribosome and clear Neisseria gonorrhoeae in vivo

Bacterial ribosome rescue pathways that remove ribosomes stalled on mRNAs during translation have been proposed as novel antibiotic targets because they are essential in bacteria and are not conserved in humans. We previously reported the discovery of a family of acylaminooxadiazoles that selectively inhibit trans-translation, the main ribosome rescue pathway in bacteria. Here, we report optimization of the pharmacokinetic and antibiotic properties of the acylaminooxadiazoles, producing MBX-4132, which clears multiple-drug resistant Neisseria gonorrhoeae infection in mice after a single oral dose. Single particle cryogenic-EM studies of non-stop ribosomes show that acylaminooxadiazoles bind to a unique site near the peptidyl-transfer center and significantly alter the conformation of ribosomal protein bL27, suggesting a novel mechanism for specific inhibition of trans-translation by these molecules. These results show that trans-translation is a viable therapeutic target and reveal a new conformation within the bacterial ribosome that may be critical for ribosome rescue pathways.

Traceless Solid-Phase Organic Synthesis

Traceless solid-phase synthesis represents an ultimate sophisticated synthetic strategy on insoluble supports. Compounds synthesized on solid supports can be released without a trace of the linker that was used to tether the intermediates during the synthesis. Thus, the target products are composed only of the components (atoms, functional groups) inherent to the target core structure. A wide variety of synthetic strategies have been developed to prepare products in a traceless manner, and this review is dedicated to all aspects of traceless solid-phase organic synthesis. Importantly, the synthesis does not need to be carried out on a linker designed for traceless synthesis; most of the synthetic approaches described herein were developed using standard, commercially available linkers (originally devised for solid-phase peptide synthesis). The type of structure prepared in a traceless fashion is not restricted. The individual synthetic approaches are divided into eight sections, each devoted to a different methodology for traceless synthesis. Each section consists of a brief outline of the synthetic strategy followed by a description of individual reported syntheses.

Synthesis of 2-Amino-5-Carboxamide Thiazole Derivatives via Dehydrative Cyclization of Thiourea Intermediate Resin on Solid Phase

In this study, we synthesized 2-amino-5-carboxamide thiazole derivatives on solid phase. The synthesis of the library starts with the reductive amination of the 4-formyl-3-methoxy phenoxy resin to prevent isomer formation. The dehydrative cyclization of thiourea intermediate resin, which is the key step in the synthetic process, was successfully synthesized using α-bromoketone in the presence of the DMF so as to afford 2-amino-5-carboxylate thiazole resin. The resulting resin is coupled with various amines. Finally, the 2-amino-5-carboxamide thiazole resin was cleaved from the polymer support using a TFA and DCM cocktail. The physicochemical properties of the proposed 2-amino-5-carboxamide thiazole derivatives were calculated and showed potential to be an reasonable oral bioavailability drug properties as determined by Lipinski's Rule.

A Useful Synthesis of 2-Acylamino-1,3,4-oxadiazoles from Acylthiosemicarbazides Using Potassium Iodate and the Discovery of New Antibacterial Compounds

A useful method for the synthesis of 2-acylamino-1,3,4-oxadiazoles was developed. By using potassium iodate as an oxidant in water at 60 °C, a wide range of 2-acylamino-1,3,4-oxadiazoles were afforded in moderate to excellent yields within two hours. This method could provide a facile shortcut to generate a series of 2-acylamino-1,3,4-oxadiazoles in medicinal chemistry. Interestingly, some highly potent antibiotic compounds were found through this synthetic method, and some of them displayed a significant improvement in activity compared with the corresponding 1,4-diacylthiosemicarbazides. Compound 2n was the most active against Staphylococcus aureus with MIC (minimum inhibitory concentration) of 1.56 mg/mL, and compounds 2m and 2q were the most active against Bacillus subtilis with MIC of 0.78 mg/mL. The preliminary cytotoxic activities of the most potent compounds 2m, 2n, and 2q against the androgen-independent (PC-3) prostate cancer cell line were more than 30 μM (IC₅₀ > 30 μM).

Current Parallel Solid-Phase Synthesis of Drug-like Oxadiazole and Thiadiazole Derivatives for Combinatorial Chemistry

Solid-phase organic synthesis is a powerful tool in the synthesis of small organic molecules and building of libraries of compounds for drug discovery. Heterocyclic compounds are important components of the drug discovery field as well and serve as a core for hundreds of marketed drugs. In particular, oxadiazole and thiadiazole cores are compounds of great interest due to their comprehensive biological activities and structural features. Therefore, a plethora of oxadiazole and thiadiazole synthesis methodologies have been reported to date, including solution and solid-phase synthesis methodologies. In this review, we concentrate on and summarize solid-phase synthetic approaches of the oxadiazole and thiadiazole derivatives.

Construction of Druglike 2-Amido Benzo[ d]imidazole Analogues via Desulfurative Cyclization of Thiourea Intermediate Resin on Solid-Phase

A 2-amido benzo[ d]imidazole library has been constructed by solid-phase synthesis. The key step of this solid-phase synthesis involves the preparation of polymer-bound 2-amino benzo[ d]imidazole resin through desulfurative cyclization of thiourea resin using 2-chloro-1,3-dimethylimidazolinium chloride and N, N-diisopropylethylamine in dichloromethane (DCM), and the resin is then functionalized by acylation at the 2-amine position to afford 2-amidobenzo[ d]imidazole resin. In the case of 2-amidobenzo[ d]imidazole resin having a p-I or m-NO₂, the resin was further functionalized by Suzuki/Sonogashira-coupling ( p-I) and reduction to the primary amine ( m-NO₂) followed by acylation. Finally, the functionalized 2-amido-benzo[ d]imidazole resin was cleaved from the polymer support by treatment with a cocktail of trifluoroacetic acid and DCM. As a result, we obtained 2-amidobenzo[ d]imidazole analogues in high yield and good purities.

Construction of 1,3,4-Oxadiazole and 1,3,4-Thiadiazole Library with a High Level of Skeletal Diversity Based on Branching Diversity-Oriented Synthesis on Solid-Phase Supports

An efficient solid-phase synthetic route for the construction of 1,3,4-oxadiazole and 1,3,4-thiadiazole libraries based on branching diversity-oriented synthesis (DOS) has been developed in this study. The core skeleton resins, 1,3,4-oxadiazole and 1,3,4-thiadiazole, were obtained through desulfurative and dehydrative cyclizations of thiosemicarbazide resin, respectively. Various functional groups have been introduced to the core skeleton resins, such as aryl, amine, amide, urea, thiourea, and an amino acid. Most of the libraries were purified by simple trituration without extraction or column chromatography after cleavage of the products from the solid-supported resin. As a result, we obtained high yields of pure 1,3,4-oxadiazole and 1,3,4-thiadiazole derivatives (total numbers = 128). Finally, we confirmed the drug-like properties of our library by calculation of physicochemical properties, displays of the skeletal diversities of the library in 3D-space, and occupation of a broad range of areas by their functional groups.

Facile one-pot synthesis of 2-amino-1,3,4-oxadiazole tethered peptidomimetics by molecular-iodine-mediated cyclodeselenization

Mild and highly efficient I₂ and Et₃N aided cyclodeselenization of in situ generated selenosemicarbazide is described to obtain 2-amino-1,3,4-oxadiazole peptidomimetics.

Solid-Phase Parallel Synthesis of N-Substituted-2-aminothiazolo[4,5-b]pyrazine Derivatives via Tandem Reaction of Isothiocyanate Terminated Resin with o-Bromo-2-Aminopyrazines

A novel solid-phase synthesis methodology of N-substituted-2-aminothiazolo[4,5-b]pyrazine derivatives was developed. The key step in this synthesis strategy is the tandem reaction of isothiocyanate terminated resin 2 with o-bromo-2-aminopyrazine, affording cyclized 2-aminothiazolo[4,5-b]pyrazine resin 4. To increase the diversity of our library, Suzuki coupling reaction was performed at the position C6. Further functionalization of 2-aminothiazolo[4,5-b]pyrazine core skeleton with various electrophiles such as alkyl halides, acyl chlorides, and sulfonyl chlorides and cleavage from the resin with TFA in DCM generated N-alkyl-, N-acyl-, and N-sulfonyl-2-aminothiazolo[4,5-b]pyrazine derivatives. The physicochemical properties and the polar surface areas of synthesized compounds were evaluated.

Solid-Phase Synthesis of 1,3,4-Thiadiazole Derivatives via Desulfurative Cyclization of Thiosemicarbazide Intermediate Resin

A 1,3,4-thiadiazole library was constructed by solid-phase organic synthesis. The key step of this solid-phase synthesis involves the preparation of polymer-bound 2-amido-5-amino-1,3,4-thiadiazole resin by the cyclization of thiosemicarbazide resin using p-TsCl as the desulfurative agent, followed by the functionalization of the resin by alkylation, acylation, alkylation/acylation, and Suzuki coupling reactions. Both the alkylation and acylation reactions chemoselectively occurred at the 2-amide position of 2-amido-5-amino-1,3,4-thiadiazole resin and the 5-amine position of 2-amido-5-amino-1,3,4-thiadiazole resin, respectively. Finally, these functionalized 1,3,4-thiadiazole resins were treated with trifluoroacetic acid in dichloromethane, affording diverse 1,3,4-thiadiazole analogs in high yields and purities. The 1,3,4-thiadiazole analogs show a different distribution of physicochemical and biological properties compared with our previously constructed 1,3,4-oxadiazole and 1,3,4-thiadiazole libraries in a range of orally available drug properties.