Imidazole 3-oxide ring

Imidazole as base, weak 21, 22 Imidazole 3-oxide ring 23, 509... 

Palladium-carbon/acetic acid Imidazole 3-oxide ring... 

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. 

Oxo dithioic acids react with a combination of (Me3Si)2S and A-chlorosuccinimide in the presence of imidazole to give oxidative ring closure to the 3//-l,2-dithiole-3-thiones (equation 21)39. 

The oxidative ring-closure of (178) gave (179), and similarly (180) was obtained from 3,4-diamino-2,5-dithiocarbamino-thieno[2,3-6]thiophen. The synthesis of substituted (3//)-thieno[2,3-rf]imidazoles has been achieved by a Curtius rearrangement of (181). ... 

The pyrazole ring is resistant to oxidation and reduction. Only ozonolysis, electrolytic oxidations, or strong base can cause ring fission. On photolysis, pyrazoles undergo an unusual rearrangement to yield imidazoles via cleavage of the N —N2 bond, followed by cyclization of the radical iatermediate to azirine (27). 

Imidazole rings also survive most oxidation conditions, but photosensitized oxidation of imidazoles can give diarylbenzamidines through a hydroperoxide (136) (70AHC(12)103). 

As shown in Scheme 2, two heteroatom-carbon bonds are constructed in such a way that one component provides both heteroatoms for the resultant heterocycle. By variation of X and Z entry is readily obtained into thiazoles, oxazoles, imidazoles, etc. and by the use of the appropriate oxidation level in the carbonyl-containing component, further oxidized derivatives of these ring systems result. These processes are analogous to those utilized in the formation of five-membered heterocycles containing one heteroatom, involving cyclocondensation utilizing enols, enamines, etc. 

The only example of ring-chain tautomerism of imidazoles is 1-hydroxy-4 -imidazoline-3-oxide (69MI86 78MI2, pp. 155,182). 

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... 

Diamine chelate complexes are more stable than the monodentate amine heterocycles and, therefore, can be studied under physiological conditions. The imidazole complexes are unstable in aqueous solution and decompose rapidly to technetium oxide hydrate. Six-membered ring chelates are significantly less stable than five-membered ones. Lesser flexibility of the ligand, such as 1.2-diamino-cyclohexane, parallels somewhat lower stability of the complex [53] ... 

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