Pyrido triazinium salts

It was shown that ring-opening reaction of the pyrido[l,2-A [l,2,4]triazinium salt 97 may easily occur with pyrrolidine to afford cis-trans and/or trans-trans triazinyl dienes 98 and 99 98 could be isomerized to -trans-i-trans isomer 99 (Equations 7 and 8) <1996JOC4423, 2001CPH(266)77>. 

Pyrido[l,2-3][l,2,4]triazinium salts 132 could be obtained from 131 with benzil either in the presence of TEA <2001CPH(266)77> or sulfuric acid <2003ARK155> (for example, diaminopyridines 131 are generally available from 2-aminopyridines 120 via N-amination with tosyl hydroxylamine (Scheme 14) <2003ARK155>). 

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. 

The tricyclic furo-fused 1,2,4-triazinium salt (74 X= O) reacts with nucleophiles such as sodium hydrogen sulfide, sodium diethyl malonate, or sodium ethyl cyanoacetate to yield the new ring systems thieno[2,3-e pyrido[l,2-Z>]-l,2,4-triazinium perchlorate (74 X= S) and cyclopenta(e]pyrido[ 1,2-b -1,2,4-triazines (75) [94CB1799]. 

Expansion of the furan ring is achieved by reacting compound (15) with hydrazine derivatives to give pyrido[l,2-6]pyridazino[3,4-c]-fl5-triazinium salts (20) (Scheme 4) <90CB1415>. 

Pyrido[l,2-7][l,2,4]triazinium salts 113 can be obtained from 112 with benzil in the presence of either base <2001CPH77> or sulfuric acid (Scheme 73) <2003ARK(xiv)155>. In an analogous reaction, 1,2-diaminopyridines 114 are transformed into 2,3,6-trioxopyridotriazines 115 by treatment with oxalyl chloride in the presence of triethylamine (Scheme 74) <1998SC3331, CHEC-III(12.03.2A4)237>. 

Tricyclic furo[2,3-e]pyrido[l,2-fr][l,2,4]triazinium salt 612 underwent ring transformation by reaction with carbon nucleophiles. Thus, the sodium salt of 1 generated from reaction of 1 and sodium hydride in acetonitrile, was treated with 612 to give 7-benzoyl-8,9,10,ll-tetrahydro-9,9-dimethylindenoll,2-eJpyrido[l,2-frJl,2,4-triazin-ll-one (94CB1799), which because of the enhanced delocalization of the n electronic system, it existed in two equilibrium forms 613 and 614 (Scheme 132). 

Diaminopyridinium iodide undergoes reaction with ethyl aceto-acetate and acetylacetone to form somewhat unexpected products, the pyrido[l,2-a]pyrimidine derivatives 33 and 34, respectively.154 The initial step has been shown to be decomposition of 1,2-diaminopyridinium iodide to 2-aminopyridinium iodide which undergoes condensation with [i-dicarbonyl compounds. The reaction of 1,2-diaminopyridinium salt with 1,2-dicarbonyl compounds gives pyrido[ 1,2-A] [1,2,4]-triazinium salts 35.161... 

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>. 

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