Pseudopericyclic Dearomative 1,6-Cyclization of 1-(2-Pyridyl)-2-azabuta-1,3-dienes: Synthesis and Ring–Chain Valence Equilibria of 4H-Pyrido[1,2-a]pyrazines

The 1,6‐electrocyclization of 1‐(2‐pyridyl)‐2‐azabuta‐1,3‐dienes obtained by Rh^II‐catalyzed reaction of pyridotriazoles with 2H‐azirines affords stable non‐aromatic 4H‐pyrido[1,2‐a]pyrazines despite the fact that the reaction proceeds with irreversible dearomatization of the pyridine aromatic system. This pseudopericyclic cyclization occurs when the 2‐azabutadiene contains H, alkyl, or Ph at the C4 atom and an electron‐withdrawing substituent at the C1 atom. A number of stable 4H‐pyrido[1,2‐a]pyrazines as well as 1H‐pyrazino[1,2‐a]quinoline and 4H‐benzo[4,5]oxazolo[3,2‐a]pyrazine derivatives were synthesized. Whereas 4‐alkyl‐substituted pyridopyrazines are stable at room temperature, 4‐phenyl‐substituted pyridopyrazines exist in ring–chain valence equilibrium with 1‐(2‐pyridyl)‐2‐azabutadienes. Thermodynamic stability of the 4H‐pyrido[1,2‐a]pyrazine decreases with increasing size of the substituent at the C6 atom.