Oxazole rings fully substituted

Finally, 2-iodonitrobenzene has also been effectively utilized in the heteroaryl Heck reaction to provide a fully substituted oxazole ring. ... 

Oxazoles 1, benzoxazoles 2, oxazolium salts 3, and oxazole A -oxides 4 are fully conjugated compounds (Figure 1). In addition, the two mesoionic structures l,3-oxazolium-5-olates 5 and (l,3-oxazolium-4-olates) 6, commonly known as miinchnones and isomiinchnones, respectively, are also considered to be conjugated rings. There are five systems of hydroxyl-substituted oxazoles and they exist in their oxo forms the 2(3H)-, 2(5H)-, 4(5//)-, 5(2//)-, 5(4//)-oxazolones 7-11. Three forms of dihydrooxazoles are known 2,3-, 2,5-, and 4,5-dihydrooxazoles respectively 12-14. The fully saturated ring is called oxazoline 15. The monooxo derivatives are 2-oxazolidinone 16,4-oxazolidinone 17, and 5-oxazolidinone 18. The three variants of oxazolidinediones are 19-21 and the fully oxidized oxazolidi-netrione is 22. 

Reactions of oxazoles indicate that they are not fully aromatic substances and that the azadiene system present readily reacts with dienophiles by the Diels-Alder mechanism. The bicyclic adducts formed in these condensations undergo facile aromatization, particularly in acid media, by elimination of a molecule of water, an alcohol, hydrocyanic acid, a nitrile, or hydrogen, forming substituted pyridine bases. The course of the reaction is highly dependent on the substituents on the oxazole ring, the nature of the dienophile, and the reaction conditions. 

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