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Keywords:

  • cycloaddition;
  • heterocycles;
  • metathesis;
  • ketenes;
  • pyrazines

Abstract

In order to extend the ring transformation reactions of pyrido[1,2-a]pyrazines 1 which contain a cyclic 2-aza-1,3-diene substructure acceptor-substituted heterocumulenes2 have been tested as dienophiles. In contrast to other reactions described to date, exclusively the exocyclic imino function was attacked. In the course of a hetero-metathesis the new aryl-(4-thiono-4H-pyrido[1,2-a]pyrazin-3-yl)amines 4Ra and aryl-(4-seleno-4H-pyrido[1,2-a]pyrazin-3-yl)amines4Rbwere formed. In the case of isocyanates of type 2e and 2f the preliminary [2+2]-cycloaddition reaction preferably takes place on the C–N-bond of the isocyanate group leading to acyl–aryl substituted pyridopyrazines 4Re and 4Rf. The reaction of 1 with in situ generated ketenes of type 6 gave the pyrrolo-condensed pyrido[1,2-a]pyrazine 9 which can further be transformed to the highly substituted pyrido-pyrrolinones 10. Whereas acetyl chloride only led to N-acylated pyrido[1,2-a]pyrazines 12, benzoyl chloride in addition to 12 astonishingly formed the double-acylated pyrido[1,2-a]pyrazines 13