Versatile strategies for the solid phase synthesis of small heterocyclic scaffolds: [1,3,4]-thiadiazoles and [1,3,4]-oxadiazoles

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 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 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.

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

A 2-amino/amido-1,3,4-oxadiazole and 1,3,4-thiadiazole library has been constructed on solid-phase organic synthesis. The key step on this solid-phase synthesis involves the preparation of polymer-bound 2-amino-1,3,4-oxadiazole and 1,3,4-thiadiazole core skeleton resin by cyclization of thiosemicarbazide with EDC·HCl and p-TsCl, respectively. The resulting core skeleton undergoes functionalization reaction with various electrophiles such as alkyl halides, and acid chlorides to generate N-alkylamino and N-acylamino-1,3,4-oxadiazole, and 1,3,4-thiadiazole resin, respectively. Finally, the 2-amino and 2-amido-1,3,4-oxadiazole and 1,3,4-thiadiazole library was then generated in good yields and high purities by cleavage of the respective resin under trifluoroacetic acid(TFA) in dichloromethane(DCM). The constructed library shows reasonable, oral bioavailability drug properties as determine by using the Lipinski's Rule and similar parameters.

Synthesis of 5-substituted-3H-[1,3,4]-oxadiazol-2-one derivatives: a carbon dioxide route (CDR)

Access to a series of 5-substituted-3(H)-1,3,4-oxadiazole-2-one derivatives is described directly using the greenhouse gas CO₂.

A convenient four-component one-pot synthesis of 2-amino-1,3,4-thiadiazoles in water

A novel four-component one-pot approach for the synthesis of 2-amino-1,3,4-thiadiazoles from primary amines, carbon disulfide, hydrazine, and acyl chlorides has been developed. A series of 5-substituted-2-amino-1,3,4-thiadiazoles were synthesized in medium-to-good yields utilizing this newly developed method.

Regioselective synthesis of 2-amino-substituted 1,3,4-oxadiazole and 1,3,4-thiadiazole derivatives via reagent-based cyclization of thiosemicarbazide intermediate

A regioselective, reagent-based method for the cyclization reaction of 2-amino-1,3,4-oxadiazole and 2-amino-1,3,4-thiadiazole core skeletons is described. The thiosemicarbazide intermediate 3 was reacted with EDC·HCl in DMSO or p-TsCl, triethylamine in N-methyl-2-pyrrolidone to give the corresponding 2-amino-1,3,4-oxadiazoles 4 and 2-amino-1,3,4-thiadiazoles 5 through regioselcective cyclization processes. The regioselectivity was affected by both R(1) and R(2) in p-TsCl mediated cyclization. It is shown in select sets of thiosemicarbazide 3 with R(1)(benzyl) and R(2)(phenyl). 2-Amino-1,3,4-oxadiazole 4 was also shown in the reaction of p-TsCl mediated cyclization. The resulting 2-amino-1,3,4-oxadiazole and 2-amino-1,3,4-thiadiazole core skeleton are functionalized with various electrophiles such as alkyl halide, acid halides, and sulfornyl chloride in high yields.

Superior Reactivity of Thiosemicarbazides in the Synthesis of 2-Amino-1,3,4-oxadiazoles

A facile and general protocol for the preparation of 2-amino-1,3,4-oxadiazoles is reported. This method relies on a tosyl chloride/pyridine-mediated cyclization of a thiosemicarbazide, which is readily prepared by acylation of a given hydrazide with the appropriate isothiocyanate. Cyclization of the thiosemicarbazide consistently outperforms the analogous semicarbazide cyclization under these conditions, for 18 distinct examples. Utilizing this protocol, we have prepared 5-alkyl- and 5-aryl-2-amino-1,3,4-oxadiazoles in 78-99% yield.

Ureas with histamine H3-antagonist receptor activity—A new scaffold discovered by lead-hopping from cinnamic acid amides

A group of tri and tetrasubstituted urea derivatives have been found to be hH(3)-antagonists. The most potent compounds were found in the class of (piperazine-1-yl)-(piperidine-1-yl)-methanones which in addition showed negligible hERG inhibition.