Oxazole properties

This chapter is an attempt to present the important results of studies of the synthesis, reactivity, and physicochemical properties of this series of compounds. The subject was surveyed by Bulka (3) in 1963 and by Klayman and Gunther (4) in 1973. Unlike the oxazoles and thiazoles. there are few convenient preparative routes to the selenazoles. Furthermore, the selenium intermediates are difficult to synthesize and are often extremely toxic selenoamides tend to decompose rapidly depositing metallic selenium. This inconvenience can be alleviated by choice of suitable reaction conditions. Finally, the use of selenium compounds in preparative reactions is often complicated by the fragility of the cycle and the deposition of metallic selenium. 

Photolysis, luminiscent properties, and laser activity of oxazole derivatives 97MI27. 

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]. 

McDonald, N. A. and Jorgensen, W. L. (1998) Development of an all-atom force field for heterocycles. Properties of liquid pyrrole, furan, diazoles, and oxazoles. J. Phys. Chem. B 102, 8049-8059. 

PPP calculations have been used to predict the properties of the t3rpe B meso-ionic l,2-oxazol-4-one (447), although the possibility that die valence tautomer 448 may be more stable was not considered. So far, the three predictions that this as yet unknown compound (447) (i) should show two absorption maxima at about 460 and 440 mp (approximation 30 mp) (ii) should appear as an orange or red substance and (iii) should be handled and kept at low temperatures have not been confirmed. We leave to the judgment of others the extent to which their confidence can and should be placed in these three predictions. 

The facile proton abstraction of activated oxazolines offers a convenient method to convert cis oxazolines to the thermodynamically more stable trans isomer. Suga, Ibata, and co-workers took advantage of this property to correctly determine the stereochemistry for the oxazoline obtained from the [3 + 2] cycloaddition of 5-methoxy-2-(p-methoxyphenyl)oxazole with benzaldehyde (Scheme 8.122). 

Part A is devoted specifically to the synthesis, reactions, and spectroscopic properties of oxazoles and encompasses four chapters Chapter 1—Synthesis and Reactions of Oxazoles Chapter 2—Spectroscopic Properties of Oxazoles Chapter 3—Oxazole Diels-Alder Reactions and Chapter 4— Mesoionic Oxazoles. 

The biosynthesis 237,5381 involves enzymatic dehydration of serine and threonine residues in a manner similar to the formation of thiazoles and dihydrothiazoles vide supra) with or without subsequent oxidation to yield the 2-(l-aminoalkyl)oxazole-4-carboxylic acid and 2-(l-aminoalkyl)dihydrooxazole-4-carboxylic acid shown in Scheme 38. These cyclic peptides display interesting physiological properties such as cytotoxicity/541, 569,5831 antitumor activities, or antineoplastic effects/523,5291 but as for the sulfur-containing compounds the mechanism of action is not yet understood despite extensive SAR studies. 515,521,540,5431... 

Renewed interest in 1,2,4-thiadiazoles is not merely part of the general intensive activity in contemporary heterocyclic chemistry. It is obviously desirable to compare this ring system with closely related important heterocycles (including thiazoles, oxazoles, pyrimidines, etc.), the chemistry of which is known in much greater detail. The iso-steric relationship between pyrimidine and 1,2,4-thiadiazole (but not with any of its isomers) foreshadows similarities in certain physical properties of the two series. The question of the biochemical function and physiological activity of heterocyclic compounds of this general pattern has also served to reinforce interest in the 1,2,4-thiadiazoles. 

The expected adduct (430) from the Diels-Alder reaction of the oxazole (429) with diphenylcyclopropenone could not be isolated (Scheme 145) (70JCS(C)552). Instead the pyran-4-one (431) is obtained, resulting from elimination of acetonitrile. This process is essentially irreversible because the pyranone lacks diene properties and nitriles are poor dienophiles. 

Their presence has only been reported in a limited number of foods coffee, cocoa, meat products, barley and soy sauce. Very few sensory properties have been reported for oxazoles and oxazolines. 

Sulfur Heterocyclics. Sulfur containing compounds (thiols, thiophenes, thiazoles,. .. etc.) play a major role in the flavor of raw and processed foods. These compounds have characteristic flavor notes and the flavor thresholds are mostly low. Several reviews (ill, 112, 113) demonstrate the important role of sulfur compounds in food flavors. Organoleptic properties of these compounds may be pleasant, strong nut-like odor of U-methyl-5-vinylthiazole which is present in cocoa (llU) objectionable pyridine-like odor of thiazole (115) quinoline-like odor of benzothia-zole (ll6) strong tomato leaf-like odor of isobutylthiazole (117) and bread crust flavor of acetyl-2-thiazoline (ll8). A mixture of oxazoles, thiazoles, thiazolines, imidazoles, trithiolanes and... 

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