1.2.3- Triazines monocyclic

Oxidation of 1 -(alkylamino)pyrazolones 1 allowed the preparation of monocyclic 1,2,3-triazin-4(3i/)-ones 2, which are a new class of heterocycles <06EJ03021>. 

Triazine has not yet been prepared, and until fairly recently there were no well-documented examples of monocyclic 1,2,3-triazine derivatives. Early estimates of the potential degree of stability of 1,2,3-triazine were made on the basis of the theoretical resonance energy the results obtained indicated a delocalization energy of about 25 kcal/mole, which implies that the ring system should be reasonably stable and certainly amenable to synthesis. Many other related molecular orbital (MO) calculations have been carried out on similar systems more recently. 

Thermolytic rearrangement of 1,2,3-trisubstituted cyclopropenyl azides constitutes the only proven method at present for the preparation of monocyclic 1,2,3-triazines, and both the generality and validity of the procedure have been established. Thus, a substantial number of derivatives of the type 2, R = alkyl, aryl, have now been prepared, and X-ray crystallographic analysis has confirmed the structures of the products (see Section II, D, 2). Thermolysis of triarylcyclopropenyl... 

Little is known as yet of the chemistry of the monocyclic 1,2,3-triazines. The only reactions that have been reported are hydrolysis to substituted butane-1,3-diones, photolytic decomposition to acetylenes, nitriles, and nitrogen, and high temperature (250°) thermolysis, which leads to loss of nitrogen and formation of the corresponding diarylindenone imines. The structures of the latter compounds were confirmed on the basis of their physical properties and by hydrolysis to the corresponding known diarylindenones. ... 

The general fragmentation pattern of monocyclic 1,2,3-triazines shows peaks for [M+-N2], for an acetylene, and for a nitrile, in accordance with the results of thermolysis and photolysis. The mass spectrum of the parent triazine showed peaks as follows 81 (M+, 47%), 53 (M+-N2, 69%), 27 (HCN,... 

Polyaza rings suffer complete hydrolytic ring cleavage. Monocyclic 1,2,3-triazines are hydrolyzed by acid to yield 1,3-dicarbonyl compounds (Scheme 15). 1,2,3-Benzotriazines are easily converted into derivatives of 2-aminobenzaldehyde. 1,2,4,5-Tetrazines (180) are hydrolyzed with a base to give aldehyde hydrazones, RCH = NNHCOR. 

The rearrangement of cyclopropenyl azides (396) is used for the synthesis of monocyclic 1,2,3-triazines (397) (73JOC3149, 79CB1514). However, the most general method is the oxidation of N-aminopyrazoles (398) with lead tetraacetate or variety of other oxidants (92AHC(53)85). 

Only a few papers dealing with monocyclic 1,2,3-triazines have been published, and the number of known compounds of this type is still low. Much more information is available on the 1,2,3-benzotriazines (2) and the first representative of this class was prepared in the last century (1887JPR(35)262>. The increasing interest in the chemistry of (2) is a result of the wide range of biological activity associated with many derivatives of 1,2,3-benzotriazin-4-(3//)-one (3 R=H), and especially of phosphoric and thiophosphoric acid derivatives of (3). 

Only a small number of monocyclic 1,2,3-triazines are known, so knowledge of the structure of these compounds is poor. The available data on the parent compound (1) and its methyl derivatives do not permit an evaluation of the degree of electron delocalization and aromaticity of (1) or its alkyl derivatives. Attempts at a crystallographic analysis of (1) were unsuccessful as the crystal was unstable under X-ray irradiation. 

The only monocyclic 1,2,3-triazine of which the structure has been determined by X-ray crystallographic analysis is 4,5,6-tris(4-methoxyphenyl)-l,2,3-triazine (72CB3704). The results show that the 1,2,3-triazine ring is planar, as one expects for a molecule with some degree of electron delocalization. The experimental bond distances and angles are given in Table 1. They are in reasonable agreement with calculated values. 

Nitrogen NMR spectra of monocyclic 1,2,3-triazines have not yet been published, but chemical shifts for 1,2,3-triazines (72BAP91) and 1,2,3-triazine 7V-oxides (76SA(A)345) have been calculated. 

Most known monocyclic 1,2,3-triazines and 1,2,3-benzotriazines are stable at room temperature. No detailed study of the stability of monocyclic 1,2,3-triazines towards water, aqueous acids or bases has been published, but one can assume from the reaction conditions used in the preparation of monocyclic 1,2,3-triazines, by oxidation of (V-aminopyrazoIes, that they are stable to water, aqueous acids or bases at room temperature, at least for a short time. Treatment of triaryl-1,2,3-triazines with aqueous hydrochloric acid at higher temperatures leads to hydrolysis of the ring and formation of 1,3-dicarbonyl compounds (Scheme 3) (60TL(13)19,76UP21800). 

Alkylation and acylation of 1,2,3-triazines have not yet been studied extensively. Protonation of monocyclic 1,2,3-triazines usually takes place at N-2. The tetrafluoroborate salt of 5-chloro-4,6-bis(dimethylamino)-l,2,3-triazine could be isolated. Similarly, reaction of... 

Oxidation of monocyclic 1,2,3-triazines with peracid affords mainly l(3)-oxides together with smaller amounts of the 2-oxides (80CCll82,73UP21800). The N-oxides can be reduced by trivalent phosphorus compounds (73UP21800). 

The [5 +1] atom fragment method has also been used indirectly for the synthesis of one monocyclic 1,2,3-triazine. The 5-aminoimidazo-4-carboxamidine (140) was cyclized with nitrous acid to give the 4-aminoimidazo[4,5-d][l,2,3]triazine (141) which gave 4,5-diamino-6-cyclopentylamino-l,2,3-triazine (142) together with other products on hydrolysis (72JMC182). 

Monocyclic 1,2,3-triazines react with ketene silyl acetal, or silyl enol ether, in the presence of 1-chloroethyl chloroformate to give 5-(substituted)-2-(l-chloroethoxycarbonyl)-2,5-dihydrotriazines which are readily oxidised, and hydrolysed, with ceric ammonium nitrate in aqueous acetonitrile to 5-(substituted)-triazines (Scheme 24) [95CPB881],... 

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