Regioselective C-H Functionalization of the Six-Membered Ring of the 6,5-Fused Heterocyclic Systems: An Overview.

The regioselective C-H functionalization of the five-membered ring of the 6,5-fused heterocyclic systems is nowadays well documented due to its high reactivity compared to the six-membered ring. So, developing new procedures of C-H functionalization of the six-membered ring "by thinking out of the box" is extremely challenging, which explains the limited number of reports published to date. This review paper aims to highlight advances achieved in this emerging chemistry research and discusses recently reported methods.

Palladium-Catalyzed C3-Arylations of 1 H- and 2 H-Pyrazolo[4,3- b]pyridines on Water.

Direct C3-arylation of 1 H-pyrazolo[4,3- b]pyridines and direct C3-arylation of 2 H-pyrazolo[4,3- b]pyridines in water has been developed. A new protocol for a sequential C3-arylation procedure on a mixture of 1 H- and 2 H-pyrazolo[4,3- b]pyridines followed by in situ PMB cleavage has also been achieved. This procedure led to unprotected ( NH) C3-arylated 1 H-pyrazolo[4,3- b]pyridines in good yields.

"On Water" Direct C-3 Arylation of 2H-Pyrazolo[3,4-b]pyridines.

An "on water" palladium-catalyzed direct (hetero)arylation of 2H-pyrazolo[3,4-b]pyridines has been developed. The reactions proceeds smoothly with at low catalytic loading at low temperature providing the C3 (hetero)arylated products in good to excellent isolated yields. Free NH 3-arylated 7-azaindazoles were also prepared by simple cleavage of the N-protected groups.