Scalable Sonochemical Synthetic Strategy for Pyrazolo[1,5-a]pyridine Derivatives: First Catalyst-Free Concerted [3 + 2] Cycloaddition of Alkyne and Alkene Derivatives to 2-Imino-1H-pyridin-1-amines

A highly efficient and convenient one-pot sonochemical synthetic strategy has been sophisticated for synthesizing a novel class of polysubstituted pyrazolo[1,5-a]pyridines via [3 + 2] cycloaddition of dialkyl acetylenedicarboxylates, ethyl propiolate, and alkenes to 2-imino-1H-pyridin-1-amines under catalyst-free conditions. A series of uniquely substituted pyrazolo[1,5-a]pyridines has been synthesized with a very good to excellent yield, and the mechanistic pathway that involves a [3 + 2] annulation process was also proposed. In this study, several spectroscopic tools of analyses were employed for structure elucidation, and the X-ray single-crystal technique was utilized to confirm the proposed mechanism and the regioselectivity.