Aryl Coupling Reactions of Pyrazolopyrimidin-4-yl Radicals

Article abstract of DOI:10.1021/jo00172a033

4-Arylpyrazolo<3,4-d>pyrimidines (4) were the subjects of a synthetic investigation in order to evaluate their biological activity.Attempts to prepare 4 from 4-aminopyrazolo<3,4-d>pyrimidines (5) via classical Gomberg-Bachmann-Hey aryl coupling conditions failed.Conversion of 5 to 4 was accomplished by diazotiazation of 5 using alkyl nitrites with an acid catalyst in aromatic solvents.Isomer distribution of the aryl-coupled products 4 was that predicted for a radical intermediate (ortho > meta ca. para); isomer structures were assigned by a detailed ¹H NMR analysis.Unusual fragmentation products 17 and 18 were isolated during the course of investigations.These oxadiazoles probably arise from collapse of intermediate pyrazolo<3,4-d>pyrimidin-4-yl radicals 15.