The 1,6-electrocyclization of 1-(2-pyridyl)-2-azabuta-1,3-dienes obtained by RhII-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.
Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry