Oxazoles reaction with singlet oxygen

Oxazoles may be used as masked forms of activated carboxylic esters since they readily form tertiary amides on reaction with singlet oxygen. Wasscrman et al. 

Although oxazoles form cycloadducts with singlet oxygen, no such reaction occurs with 1,3,4-oxadiazoles. 

The reaction of oxazoles with singlet oxygen is discussed in Section 4.18.3.1.2(vii). 

Several interesting applications of the reaction of singlet oxygen with oxazoles to form triamides... 

The oxazoles also display a number of characteristics that are typical of the furans and are explained by the structural similarity of these heterocyclic systems. The ease with which they undergo Diels-Alder reactions with dienophiles and autooxidation with singlet oxygen (see Sections IV, D and E) clearly demonstrates that oxazoles are not fully aromatic. This fact and ultraviolet data (Section III, E) suggest that oxazoles should be considered partly as conjugated dienes. 

Oxazoles are extremely susceptible to the action of singlet molecular oxygen and behave as 1,3-dienes, as they do in the Diels-Alder reaction. The wide variety of reactions observed with singlet oxygen and oxazoles take place, not by diverse modes of attack of the excited oxygen species with the substrate, but rather by a multitude of paths that appear to be open for the decomposition of the intermediate peroxide or hydroperoxide. The secondary decompositions are highly dependent on the structure of the oxazole, the nature of the functional groups in the immediate environment of the newly formed peroxide, the solvent, temperature, and other conditions. 

The highly reactive intermediate isoimides of the type 209 have actually been obtained as reaction products when fused-ring oxazoles are employed as substrates. For example, the photosensitized autoxidation of the phenyl-substituted oxazole 211 in methanol yields the iV-benzoylimino anhydride (212 ).126-362 Further studies on the reaction of oxazoles with singlet oxygen... 

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... 

Wasserman et al. [286] reported a closely related reaction of an oxazole as shown in Scheme 63. The major product was formed via [4 + 2] cycloaddition of singlet oxygen, followed by rearrangement to 5-cyanopentanoic benzoic anhydride 161 and methanolysis to the cyano ester 162. In the case of the oxazole-linked [60]fullerene, however, the treatment with methanol for a long time to ensure methanolysis might result in further conversion of the once formed cyano... 

Oxazoles are competent dienes for Diels-Alder reactions with alkenes, alkynes, and singlet oxygen however, the initial cycloadduct is unstable and decomposes to yield different products depending on the nature of the dienophile. 

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