Synthesis of 2-Substituted Pyrazolo[1,5-a]pyridines through Cascade Direct Alkenylation/Cyclization Reactions

The synthesis of 2-substituted pyrazolo[1,5-a]pyridines from N-iminopyridinium ylides is described. These medicinally interesting compounds are formed through a cascade process involving a palladium-catalyzed direct alkenylation reaction followed by silver-mediated cyclization. The reaction can be performed with a wide range of electron-poor and electron-rich alkenyl iodides in good yields. This work represents perhaps the most direct route for the preparation of these compounds.