The chapter describes the functionalization of pyrimidine and purine nucleosides by direct C-H bond activation in nucleobases. The chapter is organized by the position of the C-H bond in the heterocyclic rings. Strategies for direct C-H bond functionalization of nucleobases are divided into several categories based on: position of functionalization (C8 versus C6 versus C2 in purines or C6 versus C5 in pyrimidines); type of substrates coupled to nucleobases (e.g. aryl-or alkenyl halides, arenes, alkenes, or amines); and kind of transition metal catalyst used.
