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

Abstract

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-[a] I. 2904 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. Scheme 1. 1,6-Electrocyclizations of aza-and diazahexa-1,3,5-trienes. 2905 Scheme 2. The reaction of pyridotriazole 1a with azirine 2a.Scheme 4. The reaction of 3-benzoylpyridotriazole 1g with azirine 2b.sium hydroxide (4 equiv.) in methanol at room temperature for 24 h followed by chromatographic purification on silica gel.Compound 3d: (mixture of Z and E isomers in 1.9:1 ratio). Unstable yellow oil which is rapidly transformed to 5d. 1 H NMR (400 MHz, CDCl 3 ) δ: 1.72 (d, J = 7.1 Hz, 5.7H, Z), 1.79 (d, J = 7.2 Hz, 3H, E), 3.73 (s, 3.7H, Z), 3.81 (s, 3H, E), 3.91 (s, 3H, E), 3.92 (s, 5.7H, Z), 5.42 (q, J = 7.2 Hz, 1H, E), 5.52 (q, J = 7.0 Hz, 1.9H, Z), 6.82 (d, J = 8.1 Hz, 1H, E), 6.89 (d, J = 8.2 Hz, 1.9H, Z), 7.31-7.46 (m, 16H), 7.63-7.67 (m, 1H, E), 7.71-7.77 (m, 3H), 7.93 (d, J = 7.3 Hz, 1H, Z). Compound 5d: Orange solid, m.p. 175-177°C. 1 H NMR (400 MHz, CDCl 3 ) δ: 1.20 (d, J = 6.7 Hz, 3H), 3.89 (s, 3H), 3.96 (s, 3H), 5.78 (dd, J = 7.5, 0.9 Hz, 1H), 6.23 (q, J = 6.7 Hz, 1H), 7.11 (dd, J = 9.3, 7.5 Hz, 1H), 7.29-7.34 (m, 1H), 7.37-7.42 (m, 2H), 7.88-7.91 (m, 2H), 8.09 (dd, J = 9.3, 0.8 Hz, 1H). 13 Methyl 4-{[1-(6-Chloropyridin-2-yl)-2-methoxy-2-oxoethylidene]amino}but-3-enoate (3e) and Methyl 6-Chloro-4-(2-methoxy-2-oxoethyl)-3-phenyl-4H-pyrido[1,2-a]pyrazine-1-carboxylate (5e): Azadiene 3e (114 mg, yield 63 %) and pyridopyrazine 5e (28 mg, yield 15 %) were obtained according to the general procedure from pyridotriazole 1b (116 mg, 0.55 mmol) and azirine 2c (89 mg, 0.5 mmol) [110°C, 15 min, Rh 2 (OAc) 4 (2 mol-%, 4.4 mg), eluent for chromatography EtOAc/hexane, 1:5]. Compound 3e: (mixture of Z and E isomers in 1:1.6 ratio). Unstable orange oil which is rapidly transformed to 5e. 1 H NMR (400 MHz, CDCl 3 ) δ: 3.16-3.20 (m, 5.2H), 3.71-3.73 (m, 11H), 3.92 (s, 5H, E), 5.41 (t, Compound 5e: Red solid, m.p. 132-134°C. 1 H NMR (400 MHz, CDCl 3 ) δ: 2.33 (dd, J = 15.6, 4.0 Hz, 1H), 2.83 (dd, J = 15.6, 9.7 Hz, 1H), 3.60 (s, 3H), 3.93 (s, 3H), 6.55 (dd, J = 6.9, 1.1 Hz, 1H), 6.73 (dd, J = 9.7, 4.0 Hz, 1H), 6.99 (dd, J = 9.4, 7.0 Hz, 1H), 7. 1H), 7.46 (t, J = 7.4 Hz, 2H), 2H), 1H). 13 C C...

Electrocyclizations of Conjugated Azapolyenes Produced in Reactions of Azaheterocycles with Metal Carbenes

“…The rhodium-catalyzed reactions of azirines 120 with ortho-fused triazoles, [1][2][3]triazolo[1,5-a]pyridines 118, have been studied in [54] (Scheme 23). The final products of the reaction were previously unknown 4H-pyrido[1,2-a]pyrazine derivatives 122, which resulted from 1,6-electrocyclization of 1-(pyridin-2-yl)-2-azabuta-1,3-dienes 121 (1,4-diazahexa-1,3,5trienes with the terminal C=N bond as part of the pyridine system).…”

“…This process is quite remarkable because it belongs to a rare type of electrocyclization accompanied by the dearomatization of a pyridine ring. It was found that such 1,6-electrocyclization has limitations with respect to the substituents at the 2-azabutadiene [54]. The cyclization occurs for azadienes 121 bearing an electron-withdrawing substituent at C1 and a hydrogen atom, an alkyl, or aryl group at C4.…”

Denitrogenative Transformations of Pyridotriazoles and Related Compounds: Synthesis of N-Containing Heterocyclic Compounds and Beyond

“…Very recently, the Rostovskii group disclosed an interesting rhodium-catalyzed reaction of pyridotriazoles 147 with azirines 148 to afford 1-(2-pyridyl)-2-azabuta-1,3-dienes 150 (Scheme ). First, the thermally generated rhodium carbene reacts with azirine 148 to form diazatriene 149 , which upon reversible 1,6-electrocyclization delivers non-aromatic pyridopyrazine 150 . This protocol was found to be efficient for synthesis of 1 H -pyrazino­[1,2- a ]­quinoline and 4 H -benzo­[4,5]­oxazolo­[3,2- a ]­pyrazine 150b , c .…”

Recent Advances in Denitrogenative Reactions of Pyridotriazoles

“…Recently, the Rh(II)-catalyzed denitrogenative reaction of pyridotriazoles 17 with 2H-azirines 18 was studied by our research group (Scheme 5). 12 The 1,6-electrocyclization of 1-(2-pyridyl)-2-azabuta-1, 3…”