Enamines singlet oxygen cleavage

Singlet oxygen failed to produce cleavage of enamines bearing electron-withdrawing groups9. 

Electron-rich olefins, especially vinyl ethers, " ketene acetals, thioalkyl-substituted olefins,and enamines, " react readily with singlet oxygen to give dioxetane cleavage products. Under carefully controlled temperature conditions, the intermediary 1,2-dioxetanes can be isolated. The first 1,2-dioxetanes prepared in this manner were the cis and trans isomers (8a) and (8b), respectively (Eq. 14). 

An interesting application to the synthesis of a-diketones, a-ketoesters and a-ketoa-mides, starting from ketones, esters and amides, was achieved by Wasserman and Ives . The method involves conversion of the starting carbonyl compound to the corresponding enamino carbonyl compound, followed by oxidative cleavage of the enamine double bond by singlet oxygen (Scheme 7). 

Pyrroles are very sensitive to the action of singlet oxygen ( O2). Wasserman (1970) has observed a novel type of oxidation in the case of aryl-substituted pyrroles exemplified by the photooxidation of 2,3,4,5-tetraphenylpyrrole in methanol. There is special interest in the reactions of imidazoles with O2 since it has been shown that photooxi-dative inactivation of certain enzymes involves destruction of histidine residues, and more specifically oxidation of the imidazole ring (Wasserman and Lenz). These heterocyclic system behave in many respects like furans and pyrroles, but are more prone to cleavage through reactions resembling the oxidation of enamines by 02 (Wasserman et al. 1968). 

---