INTRAMOLECULAR DIELS-ALDER REACTIONS OF QUATERNARY PYRAZINIUM SALTS AND PROTONATED PYRAZINIUM CATIONS. SYNTHESIS OF ANNELATED PYRIDINIUM SALTS AND ANNELATED PYRIDINES.

Article abstract of DOI:10.1016/S0040-4020(01)89528-4

Quaternization of alkynyl substituted pyrazines with triethyloxonium tetrafluoroborate in dichloromethane occurs exclusively at N-4 yielding 3-alkynyl-1-ethylpyrazinium salts, as shown by the 13C NMR data.Protonation of the same pyrazines with trifluoroacetic acid also occurs at N-4.The quaternary pyrazinium salts as well as the protonated pyrazines undergo an intramolecular Diels-Alder reaction under considerably milder conditions than the corresponding neutral pyrazines.The products of the reactions were -annelated quaternary pyridinium salts and -annelated protonated pyridinium cations, respectively.