Imidazoles, mesoionic

Imidazole, 4-formyl-5-hydroxy-l-phenyl-mesoionic, 5, 372 Imidazole, 2-formyl-1-methyl-mass spectra, S, 360 Imidazole, 2-formyl-l,4,5-trimethyl-... 

The mesoionic imidazole 166 on refluxing with DMAD in benzene gave the pyrrole 168 almost quantitatively143 the postulated intermediate 167 is expected to lose phenyl isocyanate readily. A number of similar reactions involving both EP143 and DMAD144 have been described. 

Mesoionic 1,2,3,5-thiatriazoles 24 that are fused to an imidazole ring were successfully prepared from 1,2-diaminoimidazoles 23 and thionyl chloride (Scheme 3) <1981JOC4065>. The bcnzimidazo[l,2-t ]thiatriazole was prepared in an analogous manner. 

Carbonyl compounds and other dipolarophiles containing heteroatoms react with miinch-nones. Thus benzaldehyde forms the betaine (245) which suffers ring-cleavage to yield the enamine (246 equation 65). Thiocarbonyl compounds and nitrosobenzene behave in an analogous manner. The action of dipolarophiles containing cumulative double bonds results in the formation of new mesoionic systems. Thus carbon disulfide and phenyl isothiocyanate afford a zwitterionic thiazole and imidazole, respectively (Scheme 25). 

Numerous structures containing the thiocarbonyl ylide dipole are conceivable. Incorporation of the thiocarbonyl ylide dipole into a bicyclic heterocyclic system is possible by the conversion of the cyclic thione (203) into the ring-fused mesoionic system (204). The thiocarbonyl ylide dipole (205) undergoes cycloaddition with both alkenic and alkynic electron-poor dipolarophiles in refluxing benzene or xylene so that, after extrusion of hydrogen sulfide or sulfur, respectively, from the initial 1 1 cycloadducts (206) and (207), a ring-fused pyridinone is formed. The method has been used for the annelation of pyridinones to the imidazole, 1,2,4-triazole, thiazole and 1,3,4-thiadiazole systems... 

Syntheses of fused mesoionic heterocycles such as [l,2,3]triazolo[l,5-a]-quinoline, -quinazoline, -quinoxaline, and -benzotriazine derivatives have been described <02T3185>. Cyclizations of alkyl 2-benzoylamino-(4,5-dicyano-1 //-1,2,3-triazol-1 -yl)propenoates gave [1,2,3]triazolo[l, 5-a]pyrazines <02H(56)353>. Reaction of triethyl N-( 1 -ethy 1-2-methy 1-4-nitro-l//-imidazol-5-yl)phosphorimidate with aryl isocyanates provided a route to 2-aryI-2//,4//-imidazo[4,5-fif][l,2,3]triazoles <02JCS(P 1)1968>. 2-(A,A-Diphenylamino)-4-hydrazino-6-... 

Mesoionic imidazoles (15) are accessible from the reaction of N-methyl-Al-(Al -phenylbenzimidoyl)aminoacetonitrile (14) with an add chloride, followed by sodium bicarbonate treatment. Dry hydrogen chloride converts 14 into the imidazolium salt 16 which can also be transformed into 15, or which, with dilute potassium hydroxide, undergoes a Dimroth rearrangement to a 4-anilinoimidazole (Scheme 4). 

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