Divergent Synthesis of Isoxazolo[5,4-b]pyridines

Isoxazolopyridines are highly significant for the development of pharmaceuticals and new materials. In continuation of our ongoing research on transformations of 5-aminoisoxazoles, we report herein a divergent synthesis of two kinds of novel isoxazolo[5,4-b] pyridines through condensation of 5-aminoisoxazoles with beta,gamma-alkynyl-alpha-imino esters. By using two different silver salts and a phosphoric acid as catalysts, isoxazolo[5,4-b] pyridine-alpha-carboxylates and isoxazolo[5,4-b] pyridine-gamma-carboxylates were constructed as major isomers in moderate to good yields, respectively. The structures of the two kinds of the isoxazolo[5,4-b] pyridines were confirmed by X-ray crystal structural analysis. Furthermore, control experiments were conducted to elucidate the reaction mechanism.