Unexpected Behavior of the Reaction between 1,2-Diaza-1,3-Butadienes and 3-Dimethylaminopropenoates: A Useful Entry to New Pyrrolines, Pyrroles, and Oxazolines

We observed a nucleophilic attack by the ene-amino carbon of 3-dimethylaminopropenoates at the
terminal carbon of the azo-ene system of 1,2-diaza-1,3-butadienes. In tetrahydrofuran at 65 °C,
this attack produced 1-aminopyrrolines with a high degree of cis-stereoselectivity by means of an
unusual zwitterionic adduct intermediate followed by intramolecular ring closure. In toluene under
reflux, 1-aminopyrrolines produced oxazoline-fused 1-aminopyrrolines. Oxazoline-fused 1-aminopyrrolines were directly obtained by reaction of 1,2-diaza-1,3-butadienes with 3-dimethylaminopropenoates in toluene under reflux. The ring opening of oxazoline-fused 1-aminopyrrolines in acidic
or basic media provides highly substituted 1-aminopyrroles. 5-Unsubstituted 1-aminopyrrole
derivatives were obtained from 1-aminopyrrolines under basic conditions by loss of dimethylamino
and ester groups. We discuss the plausible mechanisms of the ring closure and opening.