Suzuki Coupling of Oxazoles

Activation of enamine by photoexcited organocatalyst assisted singlet oxygen: synthesis of oxazoles and quinoxalines

Herein, a novel transition-metal-free thiol-based donor-acceptor organophotocatalyst-assisted, singlet-oxygen-mediated tandem oxidative cyclization for the synthesis of substituted oxazoles in moderate-to-good yields is described. The developed method demonstrates applicability for the synthesis of various substituted quinoxalines in good-to-excellent yields. The metal-free methodology shows a practical route for the synthesis of oxazole and quinoxaline derivatives, which are privileged moieties prevalent in various biologically active compounds and natural products. To the best of our knowledge, both the thiol photocatalyst and synthesis of oxazoles by visible-light irradiation are reported for the first time.

Total Synthesis of (-)-Levesquamide

The total synthesis of levesquamide, a natural product with an unprecedented pentasubstituted pyridine-isothiazolinone skeleton, has been accomplished from kojic acid for the first time. The key features of the synthesis include a Suzuki coupling reaction between bromopyranone and oxazolyl borate fragments, a copper-mediated introduction of a thioether, a mild hydrolysis of a pyridine 2-N-methoxyamide, and a Pummerer-type cyclization of a tert-butyl sulfoxide to form the key pyridine-isothiazolinone unit of the natural product.

Synthesis of Trisubstituted Oxazoles via Aryne Induced [2,3] Sigmatropic Rearrangement-Annulation Cascade

A transition-metal-free, [2,3] sigmatropic rearrangement-annulation cascade of 2-substituted thio/amino acetonitriles with arynes allowing the synthesis of 2,4,5-trisubstituted oxazoles under mild conditions has been demonstrated. The key sulfur/nitrogen ylides were generated by the initial S/N arylation followed by proton transfer, which was followed by the selective [2,3] sigmatropic rearrangement involving the -CN moiety and a subsequent annulation to afford the desired products in reasonable yields.

Silver Benzoate Facilitates the Copper-Catalyzed C-N Coupling of Iodoazoles with Aromatic Nitrogen Heterocycles

In the literature, C-N coupling methods for the reaction of iodo-oxazole with 2-pyridinone were found to be low yielding. C-N coupling using silver benzoate additives with CuI catalysts and 4,7-dimethoxy-1,10-phenanthroline ligands has been developed to afford synthetically useful yields of the desired heterobicycle product. The reaction conditions are applied to the coupling of a range of iodo-heterocycles with 2-pyridinone. The coupling of a variety of NH-containing heterocycles with 4-iodo-oxazole is also demonstrated. The use of 2-, 4-, or 5-iodo-oxazole allows for the coupling of pyridinone to each oxazole position.

Copper-Catalyzed Oxygenation Approach to Oxazoles from Amines, Alkynes, and Molecular Oxygen

A novel and efficient oxygenation approach to trisubstituted oxazoles via a copper-catalyzed aerobic oxidative dehydrogenative annulation of amines, alkynes, and O₂ has been developed. This transformation combines dioxygen activation and oxidative C-H bond functionalization and provides a practical protocol for the preparation of oxazole derivatives, which are privileged units found in various bioactive compounds or other natural products. ¹⁸O-labeling experiments were conducted to reveal that oxygenation was involved in this chemistry.

A Heterogeneous Gold(I)-Catalyzed [2 + 2 + 1] Annulation of Terminal Alkynes, Nitriles, and Oxygen Atoms Leading to 2,5-Disubstituted Oxazoles

The first heterogeneous gold(I)-catalyzed [2 + 2 + 1] annulation of terminal alkynes, nitriles, and oxygen atoms has been achieved by using an MCM-41-immobilized phosphine-gold(I) complex as catalyst and 8-methylquinoline N-oxide as oxidant under mild conditions, yielding a variety of 2,5-disubstituted oxazoles in good to excellent yields with broad substrate scope. The new heterogeneous gold(I) catalyst can easily be recovered by simple filtration of the reaction solution and recycled for at least eight times without significant loss of activity.

A facile synthesis of 2,5-disubstituted oxazoles via a copper-catalyzed cascade reaction of alkenes with azides

A novel approach to the synthesis of 2,5-disubstituted oxazoles is developed via a 1,3-dipolar cycloaddition/ring cleavage/1,2-H migration/denitrogenation/copper-catalyzed aerobic oxidative dehydrogenative cyclization cascade.

Synthesis of 4-substituted oxazolo[4,5-c]quinolines by direct reaction at the C-4 position of oxazoles

Cu(TFA)₂ catalysed synthesis of 4-arylsubstituted oxazolo[4,5-c]quinolines/[1,8] naphthyridines has been described via a modified Pictet–Spengler method, without prefunctionalization of the unreactive 4^th position of oxazoles.

Discovery of MK-4409, a Novel Oxazole FAAH Inhibitor for the Treatment of Inflammatory and Neuropathic Pain

We report herein the identification of MK-4409, a potent and selective fatty acid amide hydrolase (FAAH) inhibitor. Starting from a high throughput screening (HTS) hit, medicinal chemistry efforts focused on optimizing of FAAH inhibition in vitro potency, improving the pharmacokinetic (PK) profile, and increasing in vivo efficacy in rodent inflammatory and neuropathic pain assays.

Electrochemically promoted synthesis of polysubstituted oxazoles from β-diketone derivatives and benzylamines under mild conditions

An efficient electrochemical route has been developed to synthesize polysubstituted oxazoles, without the use of hazardous oxidants and transition metal catalysts.

Molecular iodine mediated synthesis of polysubstituted oxazoles by oxidative domino cyclization in water

A simple and efficient iodine-mediated domino protocol has been developed to construct polysubstituted oxazoles through the C-heteroatom bond formation in water from readily available β-ketoesters and primary amines in high yields.

Metal-free synthesis of polysubstituted oxazoles via a decarboxylative cyclization from primary α-amino acids

Background

The ubiquitous oxazoles have attracted more and more attention in both industrial and academic fields for decades. This interest arises from the fact that a variety of natural and synthetic compounds which contain the oxazole substructure exhibit significant biological activities and antiviral properties. Although various synthetic methodologies for synthesis of oxazols have been reported, the development of milder and more general procedure to access oxazoles is still desirable.

Results

In this manuscript, a novel method for synthesis of polysubstituted oxazoles was developed from metal-free decarboxylative cyclization of easily available primary α-amino acids with 2-bromoacetophenones.

Conclusions

The method was simple, and this reaction could be carried out smoothly under mild and metal-free conditions. By virtue of this method, various polysubstituted oxazoles were obtained from the primary α-amino acids with moderate yields.

Mechanistic origin of antagonist effects of usual anionic bases (OH-, CO3(2-)) as modulated by their countercations (Na+, Cs+, K+) in palladium-catalyzed Suzuki-Miyaura reactions

The mechanism of the reaction of trans-ArPdBrL(2) (Ar=p-Z-C(6)H(4), Z=CN, H; L=PPh(3)) with Ar'B(OH)(2) (Ar'=p-Z'-C(6)H(4), Z'=H, CN, MeO), which is a key step in the Suzuki-Miyaura process, has been established in N,N-dimethylformamide (DMF) with two bases, acetate (nBu(4)NOAc) or carbonate (Cs(2)CO(3)) and compared with that of hydroxide (nBu(4)NOH), reported in our previous work. As anionic bases are inevitably introduced with a countercation M(+) (e.g., M(+)OH(-)), the role of cations in the transmetalation/reductive elimination has been first investigated. Cations M(+) (Na(+), Cs(+), K(+)) are not innocent since they induce an unexpected decelerating effect in the transmetalation via their complexation to the OH ligand in the reactive ArPd(OH)L(2), partly inhibiting its transmetalation with Ar'B(OH)(2). A decreasing reactivity order is observed when M(+) is associated with OH(-): nBu(4)N(+) > K(+) > Cs(+) > Na(+). Acetates lead to the formation of trans-ArPd(OAc)L(2), which does not undergo transmetalation with Ar'B(OH)(2). This explains why acetates are not used as bases in Suzuki-Miyaura reactions that involve Ar'B(OH)(2). Carbonates (Cs(2)CO(3)) give rise to slower reactions than those performed from nBu(4)NOH at the same concentration, even if the reactions are accelerated in the presence of water due to the generation of OH(-). The mechanism of the reaction with carbonates is then similar to that established for nBu(4)NOH, involving ArPd(OH)L(2) in the transmetalation with Ar'B(OH)(2). Due to the low concentration of OH(-) generated from CO(3)(2-) in water, both transmetalation and reductive elimination result slower than those performed from nBu(4)NOH at equal concentrations as Cs(2)CO(3). Therefore, the overall reactivity is finely tuned by the concentration of the common base OH(-) and the ratio [OH(-)]/[Ar'B(OH)(2)]. Hence, the anionic base (pure OH(-) or OH(-) generated from CO(3)(2-)) associated with its countercation (Na(+), Cs(+), K(+)) plays four antagonist kinetic roles: acceleration of the transmetalation by formation of the reactive ArPd(OH)L(2), acceleration of the reductive elimination, deceleration of the transmetalation by formation of unreactive Ar'B(OH)(3)(-) and by complexation of ArPd(OH)L(2) by M(+).

Aldehyde- and ketone-induced tandem decarboxylation-coupling (Csp(3)-Csp) of natural alpha-amino acids and alkynes

An interesting aldehyde- and ketone-induced intermolecular tandem decarboxylation-coupling (Csp(3)-Csp) catalyzed by copper with use of natural alpha-amino acids as starting materials is developed under neutral conditions with the production of CO(2) and H(2)O as the only byproducts. Various functionalized nitrogen-containing compounds were obtained by this method. In these processes, interesting regioselectivites of the alkylation were observed, which has been rationalized by the relative stability of proposed resonance structures based on computation methods.

Condensation reactions to form oxazoline-substituted potassium organotrifluoroborates

A library of oxazoline-substituted potassium organotrifluoroborates was prepared via the condensation of various potassium formyl-substituted aryl- and heteroaryltrifluoroborates with tosylmethyl isocyanide under basic conditions. The efficient Suzuki-Miyaura cross-coupling of products thus formed to various aryl bromides was achieved in good yields. The method allows the facile preparation of oxazole-containing triaromatic products in two steps from simple potassium formyl-substituted aryl- or heteroaryltrifluoroborates.