Synthesis of α-carbolines and β-carbolinones via intramolecular Diels- Alder reactions of 2(1H)-pyrazinones

Article abstract of DOI:10.1016/S0040-4020(98)00802-3

2(1H)-Pyrazinones bearing a X-(o-C₆H₄)-C≡C-R moiety (X=NH, NAc) are shown to undergo an intramolecular cycloaddition-elimination reaction on thermolysis in refluxing bromobenzene yielding α-carbolines or β- carbolinones. The product distribution depends strongly on the substitution pattern of the pyrazinone precursors and the solvent used for thermolysis. A high yield and selective formation of β-carbolinones is possible when heating in tetrahydronaphthalene at reflux. Use of acetic anhydride as solvent facilitated the reaction and made it possible to realise a carbolin(on)e unaccessible by thermolysis in the previously mentioned solvents.