Lewis Acid-Promoted Three-Component Reactions of Propargylic Alcohols with 2-Butynedioates and Secondary Amines

We report herein a three-component reaction of propargylic
alcohols with 2-butynedioates and secondary amines, which furnished
functionalized dihydroazepines. In the cases where benzylmethylamine
and benzyl-i-propylamine were used as the secondary
amine, the reaction afforded 2,5-dihydro-1H-pyrroles
and 2,3-dihydro-1H-pyrroles, respectively, as the
major product along with the desired dihydroazepines. The reaction
mode depends on the electronic and steric effect of the substitutents
on the secondary amines used. A tentative mechanism for this cascade
process is postulated. The key intermediate is ascribed to 1,3,4-pentatrien-1-amine,
which is formed by trapping the in situ generating allenic carbocation
with enamine. Because of the reactivity of 1,3,4-pentatrien-1-amine
formed, different products were thus formed.