What is reaction mechanism of beckmann rearrangement with POCl3 in the presence of pyridine?

The Beckmann rearrangement involves the conversion of oximes to amides, and when using phosphorus oxychloride (POCl3) in the presence of pyridine, the reaction mechanism begins with the activation of the oxime by POCl3, forming a chlorinated intermediate. Pyridine acts as a base, facilitating the protonation of the oxime nitrogen and promoting the migration of the acyl group. This results in the formation of a cyclic intermediate, which subsequently rearranges to yield the amide product, while regenerating pyridine and releasing byproducts. Overall, the presence of POCl3 and pyridine enhances the efficiency of the rearrangement by stabilizing intermediates and facilitating proton transfers.