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Nickel-Catalyzed Synthesis of Pyrazolo[3,4-c]pyridines via Alkyne Annulation
S. T. Heller*, S. R. Natarajan
Merck Research Laboratories, Rahway, USA
23 January 2008 (online)
Reported is a reasonably efficient Ni-catalyzed synthesis of polysubstituted pyrazolo[3,4-c]pyridines by the annulation of alkynes with iodopyrazolocarboximines. The starting pyrazoles are readily prepared in two steps from protected 4-iodopyrazoles in 50-60% overall yield. Annulations using internal alkynes (R3 and R4 ∂ H) always proceed in higher yields but with lower regioselectivity (when R3 and R4 are different) than reactions using terminal alkynes. The tolerance of functional R1 and R2 substitutents was inadequately investigated. In one case, a Pd catalyst, PdCl2(PhCN)2, was successfully tested for the annulation.