Unexpected reactivity of pyridinium salts toward alkynyl Fischer complexes to produce oxo‐heterocycles

The unprecedented reaction of ketone‐containing aromatic pyridinium salts 3a‐e and alkynyl Fischer complexes 1a‐f proceeds via a mild domino process to provide 4,6‐disubstituted pyran‐2‐ones 5a‐k and 2,3,5‐trisubstituted furans 6a‐h (45‐97%). According of the results of isotopic labeling experiments, a mechanism involving an initial Michael addition appears to be the key step, obtaining a mesomeric structure responsible for the formation of both products. An unexpected reactivity of alkynyl Fischer carbenes and pyridinium salts were found. A plausible mechanism for the formation of the oxo‐heterocycles involves a domino process. Through this method, pyran‐2‐ones are obtained in a selective fashion.