Pyridines—continued from pyrroles

Nevertheless, this collection of heterocycles does share certain characteristics. The trend we have seen of decreasing tendency towards electrophilic substitution on going from furan, pyrrole, and thiophene to the azoles is continued into these series. The presence of additional pyridine-like nitrogen atoms renders these systems particularly electron-deficient, and electrophilic substitution is of little importance. 

Hydrogen borrowing and dehydrogenative condensations provide new opportunities for the preparation of both saturated and aromatic heterocycles respectively. The ability to directly access azacycles from stable species such as alcohols and amines allows chemists to circumvent the preparation and use of relatively unstable carbonyls and alkyl halides that conventional methods require. Pyridines, pyrazines, pyrroles, as well as fused bicyclic heteroaromatics, may all be prepared by dehydrogenative condensation this reactivity will likely be extended to pyrimidines, imidazoles, pyrazoles, and triazoles in the near future. Continuous advances in scope and scalability will expand the role of hydrogen transfer in the discovery and production of small molecule therapeutics. 

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