Oxygen, singlet with oxazoles

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

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... 

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

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. 

Wasserman has completed two syntheses of recifeiolide, both employing the use of oxazoles as acetic acid dianion equivalents. The first (Scheme 1.10) utilized a route previously developed by Corey for his synthesis of 1. Cuprate 4 was alkylated and then converted to iodide 38 in a straightforward manner. Alkylation with oxazole anion 39 and removal of the THP with acid then led directly to alcohol 40. Liberation of the carboximide moiety was achieved by treatment with singlet oxygen to afford 41, which led to the isolation of synthetic 1 after an acid-mediated lactonization step, and separation of the olefin isomers. 

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 photooxygenation with singlet oxygen, when applied to 4,5-cycloalkeno-l,3-oxazoles, e.g. 21, leads via 22 to mononitriles of dicarboxylic acids 23 ... 

Oxydation with singlet oxygen co-Cyanocarboxylic acids from oxazole ring... 

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... 

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

Oxazole cycloadditions have been reported with alkyne dienophiles " (tandem Diels-Alder addition and retro Diels-Alder loss of a nitrile leads on to furans), benzyne (the primary adduct can be isolated), and with typical alkene dienophiles. The primary adducts from addition of singlet oxygen rearrange, by a mechanism... 

Oxazoles also react with dilute solutions of ozone to produce products similar to those obtained from singlet oxygen. In 1997 Kashima and co-workers reviewed in detail their investigations on the ozonolysis of oxazoles. This group initially... 

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. 

---