Selective Synthesis of the Aminobutadiene Intermediate and Mechanistic Analysis of 1,4-Dihydropyridine Formation Reaction in Water.

We previously isolated an aminobutadiene derivative as a by-product in the synthesis of a 1,4-dihydropyridine (1,4-DHP) derivative by the reaction of methyl propiolate with excess ammonium acetate in water, and we proposed that it is an intermediate in the formation of 1,4-DHP. Here, to test this idea and to investigate the reaction mechanism, we selectively synthesized the aminobutadiene derivative in EtOH and examined its reactivity. The yield of the aminobutadiene derivative was increased in the presence of excess ammonium salt. X-Ray crystal structure analysis indicated the presence of an intramolecular hydrogen bond between the terminal amine and ester carbonyl oxygen, together with a short C-N bond length consistent with enamine-imine equilibrium. Direct cyclization of the aminobutadiene derivative with methyl propiolate to afford the 1,4-DHP derivative did not proceed well, but the yield was increased in the presence of morpholine salt as an additive. These results suggest that the predominant reaction pathway from the intermediate to 1,4-DHP in water involves Michael addition of a second amine molecule and reaction with methyl propiolate, followed by intramolecular cyclization and elimination of amine.