Decomposition oxazoles, oxidative

The attacks of heterocyclic A -oxides, e.g. of pyridine, quinoline, isoquinoline, phenanthridine, etc., on activated alkynes (RC CR R = R = COOMe R = Ph, R = COOEt R = Ph, R = CN) pose similar problems . An acyclic intermediate has been postulated but is rarely detected. Some of the possibilities are illustrated in equation (126) . If the open intermediate is formed, then the paths to the ylid and the 2-substituted quinoline in equation (126) seem simple enough, but several possible mechanisms can lead to the 3-substituted products . Other workers regard the reaction of the nitrone (or azomethine oxide) with alkyne as simple cycloadditions - which yield 2,3-dihydro-l,2-oxazoles since these are often unstable, only decomposition products may be found (equation 127). The construction of the indolizine skeleton initiated by a similar process has been reviewed (equation 128). ... 

The oxidative transformation of dihydrooxazoles to oxazoles has also been achieved using the Kharasch-Sosnovsky reaction <94TL6803>. The first step is a hydrogen atom abstraction by a species generated from a copper-ion catalyzed (CuBr) decomposition of a peroxyester (Bu OOC(O)Ph). Yields are better when a copper(II) species is added, perhaps because of faster reaction with the radical intermediate (135) (Scheme 61). 

Reaction of oxazoles with singlet oxygen generated from the thermal decomposition of 9,10-diphenylanthracene peroxide gives oxidation products identical to those observed in dye-sensitized photooxygenations.866-368... 

From Table 1, below, no signs of thermal decomposition were observed below 400 °C for nitrile and sulfonyl substituted di-and triaryl oxazoles. Nitro compounds, in contrast, exhibited marked thermal decomposition exotherms with decomposition onset temperatures of 360-365 °C, conditions only marginally compatible with polyimide curing conditions. Nitro aromatics are also oxidants which may lead to long-term stability problems. As a result of these experiments nitrile and sulfonyl substituted diaryl oxazoles were selected for polymer doping studies. 

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