Pyrido pyridazino triazines

The zwitterionic pyrido[l,2- ]pyridazino[3,4-e][l,2,4]triazines 794 and 795 were prepared from furopyridotriazinium salt 793 by reaction with... 

The reaction of the pyrido-fused pyridazino[3,4-< ][l,2,4]triazinium compound 105 with secondary amines results in the ring-opening of the pyridine moiety to yield dienyl-substituted pyridazino[3,4-< ][l,2,4]triazines of which the pyrrolidine compound 106, shown in Scheme 15, is typical <2003ARK62, 1998AC0285, 1995JOC4919>. Compound 106 has been the subject of a detailed study and has been shown to react as a diene in the presence of fumaronitrile and A -phenylmaleinimide to give the Diels-Alder adducts 107 and 108, respectively <2003ARK62>,... 

The arylfuro-fused pyrido[l,2-/ ][l,2,4]triazinium salt 179 undergoes reaction with methyl hydrazine to give the zwitterionic pyrido-fused pyridazino[3,4- ][l,2,4]triazine 31 as shown in Scheme 32 <2003ARK62, 1997CC757>. Nucleophilic attack of the hydrazine at the bridgehead results in the elimination of an enol that then allows ring closure via attack of the other hydrazine nitrogen onto the carbonyl of the keto tautomer. 

Methylation of the l,2-dihydro-2-hydroxy pseudo base (90) with trimethyloxonium hexa-fluorophosphate led to the formation of the 1-methyl derivative (91) (Equation (10)) <92T8459>. Compound (92) was cyclized on treatment with polyphosphoric acid to the furo[2,3-e]pyrido[l,2- ][1.2,4]triazinium salt (93) (Equation (11)) <89CB1935>, which proved to be a versatile starting material for 2-methoxy- and 2-pyrrolidinopyrido[l,2-h][l,2,4]triazines <89CB1935>, and also for pyridazino-fused derivatives <90CB1415>. 

---