Diazadienes reactions with enamines

Thermolysis of 1,2,4-triazines with a side-chain terminal nitrile group leads to the formation of condensed pyrazines <89JOCl245>. This reaction involves cycloaddition of the nitrile dienophile across C-3 and C-6 of the 1,2,4-triazine followed by aromatization of the intermediate cycloadduct by extrusion of the N—N bridge. As a typical example, the 5,6-diphenyl-1,2,4-triazine (167) is fused under nitrogen at 225-235°C to form the condensed pyrazine (168) in 71% yield (Equation (21)). Intermolecular cyclization of oxadiazinones with enamines also yields condensed pyrazines. The bicyclic intermediates lose carbon dioxide to generate 1,4-diazadienes, which aromatize to the... 

Among the reactions of the 1,2,4-triazines, the hetero-DlELS-ALDER reactions with electron-rich alkenes and alkynes are of special importance in preparative chemistry [168]. The heterocyclic ring reacts with enamines, enol ethers and ketene acetals as an electron-deficient 2,3-diazadiene across the ring positions C-3 and C-6 ... 

Since no cycloaddition reactions between 1,3-diazadienes and electron-rich dienophiles, e.g., enamines, were previously recorded in the literature, we studied the inverse-electron demand [4 + 2] cycloaddition of 357 with acyclic and cyclic enamines derived from pyrrolidine or morpholine (89TL4573 90TH1) (Scheme 80). Thus, pyrimidinones and -thiones 359... 

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