Aza Diels–Alder Reactions of Nitriles, N,N-Dimethylhydrazones, and Oximino Ethers. Application in Formal [2 + 2 + 2] Cycloadditions for the Synthesis of Pyridines

Recent Advances in Pyridine Scaffold: Focus on Chemistry, Synthesis, and Antibacterial Activities

Multidrug-resistant (MDR) pathogens have created a fatal problem for human health and antimicrobial treatment. Among the currently available antibiotics, many are inactive against MDR pathogens. In this context, heterocyclic compounds/drugs play a vital role. Thus, it is very much essential to explore new research to combat the issue. Of the available nitrogen-bearing heterocyclic compounds/drugs, pyridine derivatives are of special interest due to their solubility. Encouragingly, some of the newly synthesized pyridine compounds/drugs are found to inhibit multidrug-resistant S. aureus (MRSA). Pyridine scaffold bearing poor basicity generally improves water solubility in pharmaceutically potential molecules and has led to the discovery of numerous broad-spectrum therapeutic agents. Keeping these in mind, we have reviewed the chemistry, recent synthetic techniques, and bacterial preventative activity of pyridine derivatives since 2015. This will facilitate the development of pyridine-based novel antibiotic/drug design in the near future as a versatile scaffold with limited side effects for the next-generation therapeutics.

First Diels–Alder reaction of a N-nitrogen-substituted iminium ion as dienophile furnishing N-amino-substituted 1,2,5,6-tetrahydropyridines

The exploratory and preliminary work on the reaction of 1-(methoxydiphenylmethyl)-2-methyldiazene with 1,3-dienes reveals a remarkable dichotomy furnishing different heterocyclic Diels–Alder products depending on the presence or absence of water. In contrast to the acid-induced hydrolysis of the N,O-ketal starting material with 1,3-dienes furnishing 1-methyl-1,2,3,6-tetrahydropyridazines (preceding communication), the same acid-promoted reaction albeit under exclusion of water affords N-(methyleneamino)-1,2,5,6-tetrahydropyridines. The serendipitous discovery of the novel 1,2,5,6-tetrahydropyridines was corroborated by an alternative and independent synthesis providing the in situ generated formaldehyde benzophenone azine and its conjugate acid, 2-(diphenylmethylene)-1-methylenehydrazin-1-ium ion. The protonated azine serving as the putative dienophile resembles the first example of an N-nitrogen-substituted iminium ion employed as a dienophile in a bona fide normal electron-demand Diels–Alder reaction.

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Formation of 6-Azaindoles by Intramolecular Diels-Alder Reaction of Oxazoles and Total Synthesis of Marinoquinoline A

A new variant of the intramolecular Diels-Alder oxazole (IMDAO) cycloaddition that provides direct access to 6-azaindoles was developed. The IMDAO reaction was applied in a total synthesis of the aminophenylpyrrole-derived alkaloid marinoquinoline A, also featuring the use of a Curtius reaction for preparation of a 5-aminooxazole, a propargylic C,H-bond insertion, an in situ alkyne-allene isomerization, and a ruthenium-catalyzed cycloisomerization for benzene ring annulation to the 6-azaindole.

Synthesis of Highly Substituted Pyridines via [4 + 2] Cycloadditions of Vinylallenes and Sulfonyl Cyanides

A convergent strategy for the synthesis of multisubstituted pyridines is described. Vinylallenes combine with commercially available arylsulfonyl cyanides in Diels-Alder cycloadditions to generate isopyridine cycloadducts that are converted to pyridines upon further heating or addition of a base. The 2-sulfonylpyridine products undergo nucleophilic aromatic substitution reactions with oxygen and carbon nucleophiles to provide access to a variety of highly substituted pyridines.

Formal Bimolecular [2 + 2 + 2] Cycloaddition Strategy for the Synthesis of Pyridines: Intramolecular Propargylic Ene Reaction/Aza Diels-Alder Reaction Cascades

Two methods for the synthesis of multisubstituted pyridines are described. In each strategy, a highly reactive vinylallene is generated via an intramolecular propargylic ene reaction in the presence of an azadienophile. Reactions employing ethyl N-(tosyl)iminoacetate furnish an intermediate that undergoes elimination and isomerization upon the addition of DBU. The reaction of the intermediate vinylallene with TsCN leads to the isolation of a 2-sulfonylpyridine that serves as a versatile intermediate undergoing substitution reactions with oxygen and carbon nucleophiles.

Domino Reaction Sequence for the Synthesis of [2.2.2]Diazabicycloalkenes and Base-Promoted Cycloreversion to 2-Pyridone Alkaloids

A new domino reaction sequence for the construction of 2-pyridone structures is reported. The reaction sequence begins with diacetyldiketopiperazine and proceeds via aldol condensation, alkene isomerization, and intramolecular Diels-Alder cycloaddition. The intermediate [2.2.2]diazabicycloalkene cycloadducts can be isolated or can engage in a base-accelerated extrusion of one lactam bridge to provide the 2-pyridone cycloreversion products. The operation leading to pyridone products can occur in one reaction vessel and proceeds at convenient temperatures.