1.2.4- Triazines pyridine ring

Under Mannich-reaction conditions, pyrido-TA202 was converted into thiazinones 203 fused with triazine and pyridine rings (85FES65) (Scheme 72). 

The pyridine ring may contain additional heteroatoms, e.g. as diazines and triazines. These systems are subgrouped under the pyridine group according to the position of the... 

Heterocyclic azadienes like di- and triazines have been used in the synthesis of pyridine rings. In general terms the reaction involves a regiospecific inverse electron demand Diels-Alder cycloaddition between the heterocycle and the enamine 280 followed by elimination of HCN (diazines) or N2 (triazines) and an amine from the primary cycloadduct 281 or 283, respectively, to give pyridines 282 and 284 (equation 61). At least in one case the latter type of intermediate has been isolated and fully characterized148. 

Regiospecific inverse electron demand Diels-Alder reactions of enamines with 1,3-diazines or 1,2,3- and 1,2,4-triazines (see Section III.D.l), which on elimination of HCN or N2, respectively, produce a pyridine ring, can be used with 1,3,5-triazines and 1,2,4,5-tetrazines as a useful method for the synthesis of pyrimidines214-216 (1,3-diazines) and pyridazines217-219 (1,2-diazines). Examples of the use of this methodology are the preparation of the pyrimidine substituted benzomorphane 356 (equation 77)219 and the pyridazine 359 (equation 78), intermediate in the total synthesis of cis- and trans-trikentrin A216. 

Table 5. Calculated M-N and M-O distances (A). Ni corresponds to the central pyridine ring, two N2 atoms belong to terminal pyridine or triazine rings of Terpy or MeBTP, respectively).

Bipyridines have also been synthesized by a number of cycloaddition methods. For example, stannylated bipyridines, (12), which can serve as Stille coupling partners for the synthesis of terpyridines and higher oligopyridines, have been generated in 77% and 83% yield by a thermally induced [4 + 2], regioselective cycloaddition between 1,2,4-triazines (11) and tributyl(ethynyl)tin derivatives (Scheme 6).51 Because of the steric interaction of the bulky tributyl tin group with the pyridine ring, less than 5% of the final product was the 3-substituted isomer. 

Can the pyridine ring be changed to other aromatic heterocycles pyridazine (f), pyrimidine (g), pyrazine (h), triazines (i), thiazoles (j), etc ... 

Amino and nitro groups in different rings may be annulated by heating with aqueous potassium hydroxide in pyridine. The triazine oxide ring is also formed when an o-nitro-guanidine is heated with aqueous carbonate. 

The more common triazine to pyridine ring transformation is illustrated by the formation of pyridines by reaction of 2,5,6-triethoxycarbonyl-1,2, -triazine with both alkynes and enamines.30,31 The addition of vinyl isocyanate to 1-diethylamino-propyne gives mainly the pyridone (1 ) as a result of initial (2+2) cycloaddition followed by rearrangement the thiopyridone (15) is however formed from the corresponding reaction with vinyl isothiocyanate as a consequence of a (A+2)cycloaddition reaction.32... 

Only one synthetic approach has been reported in which a pyridine ring was elaborated onto a triazine. The reaction of (121) with dimethyl acetylenedicarboxylate gave several products, and the... 

A new approach to thiazolo[4,5- ]pyridines, based on the 1,2,4-triazine to pyridine ring-transformation reaction, has been reported recently <2005MC151>. The reaction of thiazolo[4,5-f][1.2.4]triazines with bicyclo[2.2.1]heptadiene was found to proceed under high-pressure conditions (15 kbar), thus yielding the corresponding thiazolo[4,5-, ]-pyridines (Scheme 95) <2005MC151>. 

In a similar way, azepino[2,3-. ]pyridines can be obtained through the intramolecular 1,2,4-triazine to pyridine ring-transformation reaction (Scheme 107) <1999JCD583>. 

H-Azepines. A suspension of trimethyl l,2,4-triazine-3,5,6-tricarboxylate in benzene allowed to react with cyclopropene 4H-azepine deriv. Y 60%. F. e., also pyridine ring, s. W. Dittmar, J. Sauer, and A. Steigel, Tetrah. Let. 1969, 5171. 

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