Synthesis of 2,6-disubstituted pyrido[2,3-b][1,4]oxazines

Palladium-Catalyzed anti-Michael-Type Hydroamination of N-(Quinolin-8-yl)acrylamide with 2-Pyridones

Here we report a palladium-catalyzed anti-Michael-type hydroamination of N-(quinolin-8-yl)acrylamide with 2-pyridones. The use of a palladium catalyst enables the α-addition of 2-pyridones, resulting in the formation of a range of N-substituted 2-pyridone carboxamides with yields ranging from 10% to 80%. Derivatization of the products highlights the utility of this transformation. Preliminary mechanistic investigations suggest that the reaction proceeds through the direct addition of a nitrogen atom of 2-pyridones, ruling out other pathways such as O-to-N-alkyl migration.

Cs2CO3-Promoted Alkylation of 3-Cyano-2(1H)-Pyridones: Anticancer Evaluation and Molecular Docking

Herein, a Cs2CO3-promoted N-alkylation of 3-cyano-2(1H)-pyridones containing alkyl groups with diverse alkyl halides to synthesize N-alkyl-2-pyridones over O-alkylpyridines is reported. The use of alkyl dihalides resulted in complex mixtures of N- and O-alkylated products. The primary factor influencing regioselectivity in these reactions is the electronic effects of substituents on the 2(1H)-pyridone ring, as evidenced by the preferential formation of O-alkylpyridines upon the introduction of aryl groups. Remarkably, we efficiently employed CuAAC and Ti(Oi-Pr)4-catalyzed amidation reactions to functionalize N-alkyl-2-pyridones containing propargyl and ester groups, leading to the synthesis of 1,2,3-triazoles and amides, respectively. Moreover, O-alkylpyridines 10 b and 10 d displayed remarkable selectivity toward the A-498 renal cancer cell line with growth inhibition percentages (%GI) of 54.75 and 67.64, respectively. The binding modes of compounds 10 b and 10 d to the PIM-1 kinase enzyme were determined through molecular docking studies.

Coordination effect enabled palladium-catalyzed regioselective O-alkylation of 2-pyridones

A novel tactic for the regioselective O-alkylation of 2-pyridones has been realized through palladium catalysis in moderate to high yields. The coordination effect between palladium and nitrogen on the pyridine ring plays a versatile role.

Competition between N and O: use of diazine N-oxides as a test case for the Marcus theory rationale for ambident reactivity

The preferred site of alkylation of diazine N-oxides by representative hard and soft alkylating agents was established conclusively using the 1H-15N HMBC NMR technique in combination with other NMR spectroscopic methods. Alkylation of pyrazine N-oxides (1 and 2) occurs preferentially on nitrogen regardless of the alkylating agent employed, while O-methylation of pyrimidine N-oxide (3) is favoured in its reaction with MeOTf. As these outcomes cannot be explained in the context of the hard/soft acid/base (HSAB) principle, we have instead turned to Marcus theory to rationalise these results. Marcus intrinsic barriers (ΔG ‡ 0) and Δr G° values were calculated at the DLPNO-CCSD(T)/def2-TZVPPD/SMD//M06-2X-D3/6-311+G(d,p)/SMD level of theory for methylation reactions of 1 and 3 by MeI and MeOTf, and used to derive Gibbs energies of activation (ΔG ‡) for the processes of N- and O-methylation, respectively. These values, as well as those derived directly from the DFT calculations, closely reproduce the observed experimental N- vs. O-alkylation selectivities for methylation reactions of 1 and 3, indicating that Marcus theory can be used in a semi-quantitative manner to understand how the activation barriers for these reactions are constructed. It was found that N-alkylation of 1 is favoured due to the dominant contribution of Δr G° to the activation barrier in this case, while O-alkylation of 3 is favoured due to the dominant contribution of the intrinsic barrier (ΔG ‡ 0) for this process. These results are of profound significance in understanding the outcomes of reactions of ambident reactants in general.

Design, synthesis, and herbicidal activity of novel 2-(arylamino)-5-methyl-4-methylene-7-(methylthio)-4H-pyrido[4,3-d][1,3]oxazine-8-carbonitrile derivatives

A series of 5-methyl-4-methylene-7-methylthio-2-arylmino-4 H-pyrido[4,3- d][1,3]oxazine-8-carbonitrile derivatives were synthesized via tandem aza-Wittig and annulation reactions with { N-[3-acetyl-5-cyano-2-methyl-6-(methylthio)pyridin-4-yl]imino}triphenylphosphorane and aryl isocyanate in dry dichloromethane. Their structures were clearly confirmed by infrared, 1H NMR, 13C NMR, high-resolution mass spectrometry, and X-ray single-crystal diffraction. All newly synthesized compounds were screened for herbicidal activities against monocotyledonous and dicotyledonous plants. The results indicated that the target compound 2-[(4-methoxyphenyl)amino]-5-methyl-4-methylene-7-(methylthio)-4 H-pyrido[4,3- d][1,3]oxazine-8-carbonitrile showed 100% inhibition rate to grain sorghum (monocotyledonous) at the concentration of 100 mg L−1.

Specific N-Alkylation of Hydroxypyridines Achieved by a Catalyst- and Base-Free Reaction with Organohalides

A specific N-alkylation of 2-hydroxypyridines is achieved by reacting with organohalides under catalyst- and base-free conditions. The observed HX-facilitated conversion of pyridyl ether intermediates to 2-pyridone products may account for the success and specific N-alkylation of the reaction under the unexpectedly simple conditions. This new reaction may provide a useful alternative for the synthesis of 2-pyridones and analogous structures because of its >99% N-selectivity, relatively broad scopes of both substrates, and no mandatory use of catalysts and bases.