TY - JOUR

T1 - Pseudopericyclic Dearomative 1,6-Cyclization of 1-(2-Pyridyl)-2-azabuta-1,3-dienes

T2 - Synthesis and Ring–Chain Valence Equilibria of 4H-Pyrido[1,2-a]pyrazines

AU - Filippov, Ilya P.

AU - Novikov, Mikhail S.

AU - Khlebnikov, Alexander F.

AU - Rostovskii, Nikolai V.

PY - 2020/5/22

Y1 - 2020/5/22

N2 - 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.

AB - 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.

KW - Electrocyclic reactions

KW - Fused-ring systems

KW - Nitrogen heterocycles

KW - Rhodium

KW - Strained molecules

UR - http://www.scopus.com/inward/record.url?scp=85085151031&partnerID=8YFLogxK

U2 - 10.1002/ejoc.202000210

DO - 10.1002/ejoc.202000210

M3 - Article

AN - SCOPUS:85085151031

VL - 2020

SP - 2904

EP - 2913

JO - European Journal of Organic Chemistry

JF - European Journal of Organic Chemistry

SN - 1434-193X

IS - 19

ER -