The xanthate route to pyridines

Convergent routes to a variety of pyridines involving radical additions of xanthates are described. Emphasis is placed on reactions leading to tetrahydropyridines, which can be oxidized to pyridines, and to the formation of 1,5-diketones or 1,5-ketoaldehydes or their synthetic equivalents, which can then be condensed with ammonium acetate/-acetic acid under air in a variation of the classical synthesis of pyridines. The construction of pyridines fused to five-, six-, and seven-membered rings by intramolecular radical additions to the pyridine nucleus is also presented in detail. These include azaoxindoles, azaindolines, azaindoles, azaindanes, tetrahydronaphthyridines, and tetrahydropyridoazepines. Finally, the modification of pyridines by direct radical addition is discussed. (C) 2019 Elsevier Ltd. All rights reserved.