Imidazole from thiazole

These compounds are isomeric with the 1,2-azoles, e.g. isoxazole, pyrazole and iso thiazole. The aromatic characters of the oxazole, imidazole and thiazole systems arise from delocalization of a lone pair of electrons from the second hetero-atom. 

Oxazole, imidazole, and thiazole can be formally derived from furan, pyrrole, and thiophene respectively by replacement of a CH group by a nitrogen atom at the 3 position. The presence of this pyridine-like nitrogen deactivates the 1,3-azoles towards electrophilic attack and increases their susceptibility towards nucleophilic attack (see later). These 1,3-azoles can be viewed as hybrids between furan, pyrrole, or thiophene, and pyridine. 

We have previously discussed the reduced reactivity to electrophiles of oxazole, imidazole, and thiazole, as compared to furan, pyrrole, and thiophene, which results from the presence of the pyridine-like nitrogen atom. This behaviour is paralleled by increased reactivity to nucleophiles. Nucleophilic attack on furan, pyrrole, and thiophene derivatives only occurs when an additional activating group is present, as in the displacement reaction giving thiophene 3.41. 

Thiazol-2-ylidenes are less well studied than their imidazole and triazol analogues. Only one isolated and structurally characterised example is known in the literature, the 3-(2,6-diis opropylphenyl)thiazol-2-ylidene synthesised by Arduengo et al. in 1997 [2] (see Figure 6.8). The parent compound, 2,3-dihydrothiazol-2-ylidene, was generated in an argon matrix at 10 K from thiazol-2-carboxylic acid as the starting material [33] (see Figure 6.9). 

Imidazole and thiazole ring from oxazole ring... 

Oxazoles, Imidazoles, and Thiazoles. Resin-bound benzenesulfinic acid 52 obtained from sodium benzenesulfinate resin 43 (Scheme 12.13) could be treated with excess TEA and aldehyde in the presence of thiazohum catalyst to provide a-ketoamide in situ. Further reactions of the a-ketoamide with PPha/I, Lawesson s reagent, and EtOH/amine generated substituted oxazoles 56, imidazoles 57, and thiazoles 58 (Scheme 12.14). 

Thiazoles. A mixture of 3-chloro-l,3-diphenyl-l-propyne, thiourea, and ethanol heated 1.5 hrs. 2-amino-4-benzyl-5-phenylthiazole. Y 87%. F. e., also imidazoles from guanidine, from a,/ -acetylenehalides s. Y. Yura, Chem. Pharm. Bull. 10, 376, 372 (1962) from other acetylene derivatives s. Chem. Pharm. Bull. 10, 1087, 1094. 

The benzoin condensation has been examined computationally as a function of the structure of three carbene organocatalysts, that is, imidazol-2-, triazol-3-, and thiazol-2-ylidenes. DFT studies have featured three aldehydes and the historical mechanism of Breslow appeared to be consistent with the calculated reaction energy profiles whatever the aldehydexarbene combination was. It is noteworthy that the Breslow intermediate (118) arising from thiazol-2-ylidene proved to be much more stable than (119)/(120)... 

Mercaptoselenazoies with Q to C,o cyclic hydrocarbon substituents in the 4-position have been mentioned as giving negative images from photosolubie emulsions (38. 39). Equally cited are other heterocycles such as thiazole. oxazole, or imidazole. 

Selenium heterocycles receive far less mention in the literature than do such homologs as oxazole, thiazole, or imidazole. In fact, preparative methods of selenium heterocycles are much more limited than for the other series, mainly because of manipulatory difficulties arising from the toxicity of selenium (hydrogen selenide is even more toxic) that can produce severe damage to the skin, lungs, kidneys, and eyes. Another source of difficulty is the reactivity of the heterocycle itself, which can easily undergo fission, depending on the reaction medium and the nature of the substituents. 

Pyrazoles and imidazoles carrying a substituent on nitrogen, as well as oxazoles, thiazoles, etc., are converted by alkyl halides into quaternary salts. This is Illustrated by the preparation of thiamine (89) from components (87) and (88). 

In addition to the reactions described in the preceding section, alkyl groups in the 2-positions of imidazole, oxazole and thiazole rings show reactions which result from the easy loss of a proton from the carbon atom of the alkyl group which is adjacent to the ring (see Section 4.02.3.1.2). 

C=N, C=S, C=C, and N N containing substrates. Thus oxa2oles, imidazoles, thiazoles, p rrroles, and 1,2,4-triazoles have been prepared, respectively. Furthermore, p-tolylsulfonylmethyl isocyanide has found use in a one-step conversion of ketones into cyan-idea and in a two-step synthesis of a-hydroxyaldehydes from ketones. ... 

Hetero-benzylic anionic reagents, derived from 2-alkyl-l,3-oxazoles, -1,3-thiazoles and -imidazoles and related compounds, are not covered in this section because these resemble metallo 1-azaenolates in their reactivity (Section D.l.3.5.). 

Comparison of thiazole-based regioisomers (464) and (465) has revealed a marked preference for the phenyl substitution of the former, a trend also observed in the subsequent imidazole series (data not shown), but not in the triazole series as evident from the data presented for (466) and (467) (Table 6.40). The thiazole (464) was approximately 8-fold less potent than the corresponding imidazole (470) (Table 6.41) suggesting that the seemingly subtle change of the central scaffold imparts a marked change in CBi receptor recognition. 

The most common ligands are those derived from imidazole and benzimidazol (Scheme 54), followed by the (benz)thiazols. The free Wanzlick-Arduengo carbenes can be isolated and employed for the synthesis of the complexes, but often it is more convenient to prepare the carbenes in situ from the dimers or the corresponding onium salts, or to use carbene-transfer reactions.256-259... 

Palladium chemistry involving heterocycles has its unique characteristics stemming from the heterocycles inherently different structural and electronic properties in comparison to the corresponding carbocyclic aryl compounds. One example illustrating the striking difference in reactivity between a heteroarene and a carbocyclic arene is the heteroaryl Heck reaction (vide infra, see Section 1.4). We define a heteroaryl Heck reaction as an intermolecular or an intramolecular Heck reaction occurring onto a heteroaryl recipient. Intermolecular Heck reactions of carbocyclic arenes as the recipients are rare [12a-d], whereas heterocycles including thiophenes, furans, thiazoles, oxazoles, imidazoles, pyrroles and indoles, etc. are excellent substrates. For instance, the heteroaryl Heck reaction of 2-chloro-3,6-diethylpyrazine (1) and benzoxazole occurred at the C(2) position of benzoxazole to elaborate pyrazinylbenzoxazole 2 [12e]. 

Frank Burnett Dains, 1869-1948. Lecture assistant to Dr. W. O. Atwater at Wesleyan University, Middletown, Connecticut, and later assistant professor at Northwestern University and professor at Washburn College in Topeka, Kansas. From 1911 until tire time of bis retirement in 1942 he was in charge of the department of organic chemistry at the University of Kansas, where he made notable contributions to the chemistry of the aldehydes, urea ethers, substituted ureas, thiazoles, imidazoles, and pyrazoles, and was an enthusiastic collector of books, portraits, and other memorabilia connected with the history of chemistry. He was a charter member of the Chicago Section of the American Chemical Society and served as Councilor of the Society, as Chairman of the Divisions of Organic Chemistry and History of Chemistry, and as contributing editor and abstractor for the Journal of Chemical Education. See also ref. (166). 

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