Imidazole 1,2,4-triazine ring

The number of possible triazinobenzimidazoles is less than that for imidazolotriazines when fusion of the triazine ring takes place on the imidazole ring. It should be noted that z in imidazotriazines indicates the edge of the triazine ring, whereas in the triazinobenzimidazoles it indicates the edge of the benzimidazoles. 

All synthetic approaches to imidazo[2,l-f][l,2,4]triazines published during recent years utilize the ring closure of the six-membered triazine ring by conversion of a properly functionalized imidazole compound. 

Reduction of 2,4,6-triphenyltriazine gives 2,4,5-triphenylimida-zole,140 while the 1,2,4-triazine ring also contracts to an imidazole141 with zinc dust and acetic acid. Treatment of 2,5-diaminothiazoles with alkali leads to imidazole-2-thiones.142,143... 

The more general syntheses of this ring system have all involved annulation of the triazine ring onto a preformed imidazole ring. A series of variations within this theme has been developed. 

Under reducing regimes some 1,3,5- and 1,2,4-triazines ring contract to imidazoles [84-86], but there is no obvious synthetic importance. When a-halogenooximes react with amidines, they appear to form AH-... 

In very strongly alkaline media (cqh- > 0. the rapid reactions are followed by a much slower reaction. The triazine ring is split open and BBU (29) is formed. The opening of the triazine ring is caused by nucleophilic attack of the OH ions on the carbonyl group next to the N, atom of the imidazole ring. 

Imidazole from 1,2,4-triazine ring Purines from 7-azapteridines... 

Many chemical compounds have been described in the Hterature as fluorescent, and since the 1950s intensive research has yielded many fluorescent compounds that provide a suitable whitening effect however, only a small number of these compounds have found practical uses. Collectively these materials are aromatic or heterocycHc compounds many of them contain condensed ring systems. An important feature of these compounds is the presence of an unintermpted chain of conjugated double bonds, the number of which is dependent on substituents as well as the planarity of the fluorescent part of the molecule. Almost all of these compounds ate derivatives of stilbene [588-59-0] or 4,4 -diaminostilbene biphenyl 5-membeted heterocycles such as triazoles, oxazoles, imidazoles, etc or 6-membeted heterocycles, eg, coumarins, naphthaUmide, t-triazine, etc. 

Construction of the imidazole ring onto a triazine has been also used for the synthesis of this group of compounds. The reaction of 3-aminobenzofl, 2,4]triazine with a-bromoacetaldehyde acetal gave the [1,2-b] linear isomer. On the other hand, the similar reaction on 1-oxide 461... 

Investigation of l,2,3,7-tetrahydroimida/o[ l, 2-3][l,2,4]triazin-7-onc 56 revealed the /ram-rclationship between the substituents in positions 2 and 3 <2003OL4595>. The (Oil)0-6 (3)-C(2)-0(0Ft) dihedral angle confirmed this geometric feature, whereas it has been shown that the imidazole ring is nearly planar. 

Some of the ring expansion reactions discussed in Section 2.03.3.3.1 can be extended to five-membered heterocycles containing two or more heteroatoms. Reaction of imidazoles and pyrazoles with dichlorocarbene, for example, gives chloropyrimidines together with small amounts of chloro-pyrazines or -pyridazines, and oxidative ring expansion of 1-aminopyrazole with nickel peroxide gives 1,2,3-triazine (this, in fact, constitutes the only known synthesis of the unsubstituted triazine). There are, however, a number of interesting and useful transformations which are unique to five-membered polyheteroatom systems. 

The removal of the iV-etheno group from the fused imidazole ring of tricycle 102 was achieved using rV-bromosuccin-imide (NBS) in 1 1 DMF-water to yield the 2-azanucleoside (pyrazolo[3,4-4][l,2,3]triazine) 103 (Scheme 79) <2004JOC4695, 20050BC1714>. An analogous tricycle transformation to imidazo[4,5-r7 [l,2,3]triazines has been reported earlier <1996CHEC-II(7)489>. 

The removal of the W-etheno group from the fused imidazole ring of tricycles, diimidazo[l,2-rt4, 5 -r ][l,2,3]tria-zines and imidazo[l,2-f]pyrazolo[4,3-i ][l,2,3]triazines, is a reliable method for making imidazo[4,5-rf [l,2,3]triazines <1996CHEC-II(7)489> and pyrazolo[3,4-r/ [l,2,3]triazines, respectively (Section 10.13.10.1.2). 

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