1 -Substituted 1,2-dihydro-1,3,5-triazines formation

When a tautomeric equilibrium is possible, neutral dihydro-1,3,5-triazines exist predominantly or exclusively in their 1,2-dihydro forms. Thus, the formation of 1-substituted 1,2-dihydro-l,3,5-triazines 104 was reported on intramolecular cyclization of imidoylcarbodiimides (86CB3737). No corresponding 1,4-dihydro tautomers were observed by H, NMR, or IR spectroscopy in solution. 

Base-catalyzed Dimroth rearrangements of l-aryl-4,6-diamino-1,3,5-triazines or triflic acid-induced decomposition of 6-anilino-1,3,5-triazines led to the formation of the corresponding substituted 1,2-dihydro-l,3,5-triazines, whose structure was established on the basis of H NMR spectra (93JHC849). 

An interesting photochemical formation of different 1,4-dihydro-1,3,5-triazines (168-171) was demonstrated by Kobayashi et al.261 who irradiated substituted 1,3,5-triazines (167) of low aromaticity in pentane or cyclohexane, utilizing compounds with two different fluoroalkyl groups on the triazine ring. Two or three distinct products were isolated, the ratio of these dihydro compounds depending on the nature of both substituents and the solvent (which also serves as a reagent) [Eq. (82)].267... 

Interesting results were obtained by reaction of substituted 1,2-dihydro-1,3,5-triazines with esters of acetylenedicarboxylic acid (DMAD) and of propiolic acid.254 In the case of the reaction with DMAD, formation of the different products 175 and 176 has been reported,254 presumably due to differences in the nucleophilicity of the secondary nitrogen atom of the triazines. 

A special type of modification of substituents bound to the 1,2,4-triazine nucleus is the formation of condensed 1,2,4-triazines, such as formation of imidazo[4,5-c]-l,2,4-triazines 21 by reaction of 1,2,4-triazine-5,6-diamines with ortho esters 103-189 with glyoxal or substituted glyoxals dihydro- and tetrahydropyrazino[2,3-e]-l, 2,4-triazines 22 are obtained. Pyrazino[2,3-e]-1,2,4-triazine 23 is synthesized by dehydration of the tetrahydropyrazino[2,3-< ]-l,2,4-tri-azine.293-297-301... 

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