4H-Imidazoles

H-Imidazole 1-oxide, 2,4,4,5-tetramethyl- H NMR, 5, 16 <83UP40100> 4H-Imidazole-2,4,5-triamine, hexaethyl-... 

D-er7f/zro-pentofuranosyl)amino]-4H-imidazol-4-one (40) [73, 74]. The mechanism of their production (Scheme 4) may be rationalized in terms of the transient formation of oxidizing guanilyl radicals 39 that arise from deprotonation of 38 [75]. The resulting neutral radicals of dGuo(-H) 39 may... 

The AH-i midazole-3-ox ides (223) have electrochemical potentials similar to 4H-imidazole-l,3-dioxides (219) AH -imidazole-1-oxides (224) are more difficult to oxidize (Fig. 2.17) (Table 2.6) (425). 

Unlike the 4H- imidazoles (219), (223), (224) electrochemical oxidation of the nitrone group in 4-R-3-imidazoline-3-oxides (228), (230-232), as in a-PBN and DMPO is of irreversible nature. Therefore, the formation of radical cations... 

Oxaziridines, 6-oxa-l,4-diazabicyclo[3.1.0]hex-3-enes (245) and their 4-oxides (246), obtained in the photolysis of 4H -imidazole-3-oxides (223) and 1,3-dioxides (219), underwent a quick thermal isomerization to the starting nitrones. Further... 

Owing to the existence of two centers for nucleophilic attack (at C2 and C5) in radical cations (220) obtained from the oxidation of 4-H -imidazole-1,3-dioxides (219), the formation of two products of methoxy group addition was observed, namely NNR (221) and NR of 3-imidazoline-3-oxide (222). The ratio of the products depends on the electronic nature of substitutes R1 and R2. Both, the donor character of R1 and acceptor character of R2 facilitate the formation of nitroxyl radicals (222) with the yield of (221) increasing with the inverted effect of the substituents. As was mentioned in Section 2.4, the results of preparative electrochemical oxidative methoxylation of 4H -imidazole-1,3-dioxides are similar to the results of chemical oxidation. 

Oxidation of mono-iV -oxides 4H -imidazole (223) and (224) with Pb02 in methanol leads to the formation of stable a,a-dimethoxy-substituted nitroxyl radicals (271) and methoxy substituted imino nitroxyl radicals (INR) (272)-(274) (Scheme 2.101) (514). 

In addition to the oxidative alkoxylation of 4H -imidazole and oxazolidine derivatives, the reaction was also used with other cyclic aldo-nitrones such as DMPO, derivatives of 3-imidazoline-3-oxide (228-232) (506), and derivatives... 

Oxidative Animation of Nitrones to a-Amino-Substituted Nitroxyl Radicals Similar to the oxidative methoxylation reaction, oxidative animation of 4H -imidazole TV-oxides, in amine saturated alcohol solutions, give stable nitroxyl (282), nitronyl nitroxyl (283), imino nitroxyl (284) and (285) radicals with the amino group at the a-carbon atom of the nitroxyl group (Scheme 2.107) (520, 521). The observed influence of substituents on the ratio of animation products at C2 and C5 atom is close to the ratio observed in the previously mentioned oxidative methoxylation reaction. It allows us to draw conclusions about the preference of the radical cation reaction route. 

Oxidative Fluorination of Nitrones to a-Fluorosubstituted Nitroxyl Radicals Formation of nitroxyl radicals by the radical cation route was observed in reactions of various nitrones with xenon difluoride in dry methylene chloride (520, 523). In this reaction, more than 40 nitrones, including 4H -imidazole N,N -dioxides (219), 4H -imidazole TV-oxides (223) and (224), 2H -imidazole N -oxides (225), 2H -imidazole TV,TV-dioxides (226), 3,3,5,5-tetramethylpyrroline N -oxide (TMPO), derivatives of 3-imidazoline-3-oxides (231) and (232), have been examined. ESR spectra of nitroxyl radicals containing one or two fluorine atoms at a-C have been registered (Scheme 2.108) (523). In the case of... 

For reviews dealing with chemistry of 2H- and 4H-imidazoles, see (a) Sammes MP, Katritzky AR. In Advances in Heterocyclic Chemistry, Katritzky AR (Ed.), Academic Press, New York, Vol. 35, 375-412, 1984 ... 

One interesting difference between the known examples of 2H- and 4H-imidazoles is that nearly all in the latter group contain at least one hetero-linked substituent. This arises because the major synthetic approach is via imidazolone derivatives (see next section) a second important route using 1,2-diketones places an OH group at C-4, and electrophilic additions to 1//-imidazoles also tend to introduce heteroatoms at this site. 

Iminothiones 18 have been transformed into the 4H-imidazoles 19 by two different routes, both involving Dimroth rearrangements (Scheme 6). ... 

Early claims to have prepared 4-hydroxy-4H-imidazoles from benzami-dine hydrochloride (27) and 2,3-butanedione (28), or phenylethane-1,2-dione (29), appear to be substantially correct (Scheme 9). Although this work has not been repeated, analogous intermediates have been observed subsequently in related reactions. Waugh and co-workers recognized that 30 might well have been the rearranged isomer 31. (see Section IV,A,l,b). 

For cyclic amides containing the NHCONHCO moiety (64, Scheme 17), the reaction follows a different course, not involving a medium-ring intermediate, and losing carbon dioxide to give products 65. When X = COCHj, yet another mechanism is observed and no 4H-imidazole is formed. ... 

On prolonged standing in acid solution, 83 is converted to 84 (Scheme 22). Salt 86, prepared readily from 85, ° can give 4H-imidazoles, including 87 and 88, on addition of nucleophiles. 

The H-NMR signal for the proton at C-4 in 88 (Scheme 26), the only 4H-imidazole known to be stable in this tautomeric form, is found at S 5.17. Only single examples of a base (79) and a cation (74) " with a proton at C-2 have had spectra recorded (Scheme 26) no data are available for protons at C-5. 

Aus a-Dicarbonyl-Verbindungen und Amidinen bilden sich zunachst 4,5-Dihydroxy-4,5-dihydro-imidazole, die isoliert werden konnen127 128. Setzt man eine disubstituierte a-Dicarbonyl-Verbindung und statt des Amidins ein Guanidin-Derivat ein, so erhalt man unter Abspaltung eines Aquivalents Wasser 4,5-disubstituierte 2-Amino-4-hydroxy-4H-imidazole, die in vielen Fallen ebenfalls isoliert werden konnen129,130 ... 

Dagegen tritt bei 2-Amino-4,5-diphenyl-4-hydroxy-4H-imidazol keine Umlagerung ein, da kei-ne Methyl-Gruppe vorhanden ist. Es wird stattdessen in konz. Salzsaure zu 2-Amino-4,5-di-hydroxy-4,5-diphenyl-4,5-dihydro-imidazol hydratisiert129. Mit Methanol/Salzsaure erhalt man 2-Amino-4,5-dimethoxy-4,5-diphenyl-4,5-dihydro-imidazol129 ... 

Beim Erhitzen von 4,4-Dimethyl-2-phenyl-4H-imidazol (unsubstituierte 5-Stellung) in Diphe-nylether auf 180° erhalt man in 93% Ausbeute 4,5-Dimethyl-2-phenyl-imidazoll2lb. 

Durch Einwirken von Isoamylnitrit auf 2-Amino-imidazole im sauren Milieu erhalt man die isolierbaren 4-Hydroximmo-4H-imidazole, die sich in heiBem Wasser recht schnell zu 5-Ami-no-3-aryl-l,2,4-oxadiazoIen umlagern232 233. 

Imidazo[2,l-c][l,2,4]dithiazole-3-thione H NMR, 6, 904 <71TL1317) 4H-Imidazol-5-amine, 4-cyano-4-methyl- H NMR, 5, 16 <83UP40100)... 

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