Pyrido triazines formation

An unexpected ring formation involving a ring transformation occurred in the reaction of the pyrido[l,2- ]-[l,2,4]triazine derivative 101 with dimethyl acetylenedicarboxylate, and tricyclic compounds 102 were obtained in high yields (Equation 10) <2001JHC205>. 

The treatment of 1,4-di hydro-2 7/-pyrido[ 2,3-< [ 1,2,4]triazine-3-thione 82 with dimethyl acetylenedicarboxylate (DMAD) in methanol at room temperature leads to the formation of 5-oxo-8,8a,9,10-tetrahydro-5//-4,4b,9,10-tetra-azaphenanthrene-7-carboxylic acid methyl ester 83 <1998IJH303> (Equation 5). 

From liquid-phase hydrogenation studies56, we can derive the enthalpy of formation of the perhydro-tris[l,2-a 3,4-c 5,6-e]pyrrolo-l,3,5-triazine and of perhydro-tris[l,2-a 3,4-c 5,6-e]pyrido-l,3,5-triazine (35 with n = 5 and 6, respectively). Correcting for medium effects in directly measured enthalpies of solution, hydrogenolysis reaction 43... 

Catalytic hydrogenation of 3-phenylpyrido[3,4-e]-l,2,4-triazine (45b) over a palladium catalyst gave the 1,4-dihydro compound (29) in a 90% yield this is an unusual result as the position of the pyridine nitrogen atom does not seem to affect the formation of 1,4-dihydro derivatives (see also Section 7.17.5.4). Electrochemical reduction of the pyrido-l,2,4-triazine (45b) in both protic and... 

There are many possibilities for mesomeric betaine formation in the 1,2,4-triazine series, illustrated by the A-methyl-oxo compounds in structures (495)-(498), and in the benzotriazine series by (499). These compounds seem not to have been investigated, however, and the closest example we have found is the pyrido-fused system (500) <77JOC443>. See also Section 6.11.9.1.4. 

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>. 

---