Imidazoles 1,2,4-oxadiazoles

A/-Chloroamidines are usehil for preparation of biocidal imidazoles (106) and thiadiazolines (107). Ai-Chloroguanidines, RNHC(=NC1)NHR, serve as starting materials for synthesis of imidazoles, oxadiazoles, and thiadiazoles (108,109). 

Heterocycle = quinoxaline, imidazole, oxadiazole, benzoxazole, benzimidazole... 

Pyrazolines Imidazoles Thiazoles Oxadiazoles 1 -Phenylpyrazoles Purines Dioxolanes Ox azolines 10% Cyanethylated mannite on Celite 545. 5% OV-17 on Chromosorb W, AW-DMCS (H.P.). Carbowax 4000, dioleate on firebrick, 190 °C. Silicone grease on Chromosorb P. Apiezon L on firebrick C-22, 220 °C. 15% Hallcomid M-18 on firebrick. Carbowax 20M on Gas-Chrom P. Ethyleneglycol succinate on Diatoport-S. 

In azole chemistry the total effect of the several heteroatoms in one ring approximates the superposition of their separate effects. It is found that pyrazole, imidazole and isoxazole undergo nitration and sulfonation about as readily as nitrobenzene thiazole and isothiazole react less readily ica. equal to m-dinitrobenzene), and oxadiazoles, thiadiazoles, triazoles, etc. with great difficulty. In each case, halogenation is easier than the corresponding nitration or sulfonation. Strong electron-donor substituents help the reaction. 

The protected 5-amino-l-ribofuranosyl-4-(5-methyl-l,2,4-oxadiazol-3-yl)imidazole 58 (Equation 4) undergoes MHR to afford the 3-acetamidoimidazopyrazole 59 in dimethyl formamide (DMF) or DMSO as solvent at... 

The photolysis of 2-methyl-5-nitro-l//-imidazoles 132 in a water-containing solvent80 gives oxime 133, which is in turn hydrolysed to 134 followed by a dehydrative cyclization to the 1,2,4-oxadiazole 135. Light-induced hydrolysis of 135 gives 136 (equation 68). 

Heating of 4-hydroxyiminoimidazoles 260 in boiling water causes the hydrolytic opening of the imidazole ring and by subsequent cyclization leads to 5-amino-1,2,4-oxadiazoles 261 in 79-86% yields (equation 112). ... 

Enaminoesters and enaminoketones, for example (40), rearrange to imidazoles (41) or oxa-diazolium salts (42) (Scheme 14) <74T3859>. For R = H mixtures of imidazoles and pyrazoles, for example (43), have been obtained <78JA4208>. The formation of (43) is an example of a ring transformation, where the attacking nucleophile is the first atom of the side-chain in the 3-position of the oxadiazole (Scheme 14). 

Oxo-l-propenylamino)-l,2,4-oxadiazole lagern sich mit Natriumethanolat in Dimethyl-formamid in Ausbeuten von 60-80% in 4(5)-Acyl-2-acylamino-imidazole um392 ... 

Unsubstituierte 2,4(5)-disubstituierte Imidazole, bei denen mindestens einer der beiden Sub-stituenten ein Phenyl-Rest ist, werden mit Wasserstoffperoxid in Wasser/Natronlauge in 1,2,4-Oxadiazole umgewandelt (35- 56% Ausbeute)1243 ... 

Some other data on, 5N chemical shifts, referred to internal MeN02, are recorded in Table 25. Comparison of pyrrole with imidazole demonstrates that the deshielding of the pyrrole-type nitrogen is about 15 ppm. Increased nitrogen shielding is observed for 2,1,3-benzothiadiazole ( + 49 ppm) and the parent 1,2,5-thiadiazole ( + 34 ppm). In contrast, the corresponding oxadiazoles were deshielded, as in 2,1,3-benzooxadiazole ( — 36 ppm) and 1,2,5-oxadiazole (— 34 ppm). 

A rearrangement that does not fall into the pattern described in this section has been found by Cusmano and Ruccia.191 This time a 1,2,4-oxadiazole is the final product formed by nitrosation of an imidazole. What intermediates lie between the nitroso compound (103) and the final product (104) have not been determined, but the reagent is alcoholic hydrochloric acid. Other examples of this type of rearrangement are known191,192 that also resemble the nitrosopyrrole to isoxazole rearrangement.192"... 

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