Ring‐Opening of Pyridines with Acyl­acetylenes and Water: Straightforward Access to 5‐[(Z)‐Acylethenyl]amino‐2,4‐pentadienals

Pyridines undergo exceptionally facile ring opening under the action of acylacetylenes (e.g., benzoylphenylacetylene, furoylphenylacetylene, and thenoylphenylacetylene) and water (r.t., with or without KOH) to afford 5-[(Z)-acylethenyl]amino-2,4-pentadienals in 84–92 % yield. The reaction is stereoselective: only the Z configuration for ethenyl substituents and preferentially (85–95 %) the E,E configuration for the 2,4-pentadienal moiety were observed (the second configuration being E,Z). In the cases of 3-methylpyridine and benzoylphenylacetylene, two regioisomers, namely, the corresponding 2-methyl- and 4-methylaminopentadienals, were formed in a 7:3 ratio.