Regioselective functionalization of trisubstituted pyridines using a bromine-magnesium exchange

A tosyloxy substituent in position 2 allows a highly regioselective Br/Mg exchange reaction on 3,5-dibromopyridine derivatives using iso-(PrMgClLiCl)-Li-.. The resulting functionalized pyridylmagnesium reagents react with various electrophiles in position 3. Acylated pyridines of this type can be readily converted into pyrazolo [3,4-b] pyridines.