Synthesis of pyridines via [4 + 2] cycloadditions of vinylallenes with azadienophiles

Abstract

Pyridines are an important class of heterocycle with widespread applications. However, the efficient preparation of substituted pyridines remains a challenge in organic synthesis. This thesis describes new annulation strategies for the synthesis of highly substituted pyridines that involve [4 + 2] cycloadditions of vinylallenes with azadienophiles. Part II of this thesis describes a unimolecular, formal [2 + 2 + 2] cycloaddition strategy for the synthesis of tricyclic pyridines based on intramolecular propargylic ene/imino Diels-Alder reaction cascades. In this variant, unactivated oximino ethers and NN-dialkylhydrazones function as dienophiles in thermal Diels-Alder reactions. Part III of this thesis describes strategies for the synthesis of bicyclic and monocyclic 2-sulfonylpyrdines based on [4 + 2] cycloadditions of vinylallenes with tosyl cyanide. In addition, Part III discusses the synthetic elaboration of these 2- sulfonylpyridines via nucleophilic aromatic substitution reactions. Overall, annulation strategies based on [4 + 2] cycloadditions of vinylallenes with azadienophiles have been developed for the preparation of highly substituted pyridines in an efficient, convergent, and regioselective fashion.