Singlet oxygen imidazoles

In a study of the photosensitized oxygenation of imidazoles, Wasserman et al.i2S found that 2,4,5-triphenylimidazole (82) reacts with singlet oxygen to yield an V.V -diaroylbenzamidine (84),... 

Imidazole reacts very slowly with singlet oxygen to form the imida-zolidone (86)42S through an elimination reaction involving proton loss and cleavage of the oxygen-oxygen bond of the transannular peroxide (87). On the other hand, 4-phenylimidazole forms a hydantoin derivative (88) and V-benzoyl-V -methoxycarbonylurea (89). This... 

Cleavage of imidazoles dehydroamino acids,3 2,4-Disubstituted imidazoles4 undergo a Diels-Alder-like reaction with singlet oxygen in the presence of DBU (1-2 equiv.) to provide imine diamides, which isomerize to dehydroamino acid derivatives in the presence of base. Hydrogenation in the presence of catalysts with chiral phosphine ligands results in optically active amino acid diamides. The overall process is illustrated for the synthesis of the N-benzylamide of N-acetylleucine (equation I). 

Jursic, B. S., Zdravkovski, Z., Reaction of Imidazoles with Ethylene and Singlet Oxygen An Ab Initio Theoretical Study, J. Org. Chem. 1995, 60, 2865 2869. 

Figure 2.9 Chemical quenching of singlet oxygen, (a) The ene reaction — addition of singlet oxygen to an olefin with allylic hydrogen (b) the ene reaction of cholesterol (c) endoperoxide formation by singlet oxygen to imidazole residue as in histidine.

Bridged Peroxides.—Routine procedures have been extended to the synthesis and characterization of photo-adducts of singlet oxygen with cyclo-octa-1,3,5-triene, cycloheptatrienes, 3,5-cycloheptadienone, cyclohepta-1,3-dienes, ° naphthalenes, anthracenes, a-pyrones, pyrazines and pyrimidines,imidazoles, and indenes. Transformation of some of these... 

Oxidation with singlet oxygen Oxidative opening of heterocycles Imidazole ring opening... 

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 which is not definitely established, to form triamides, themselves useful synthetic intermediates. The only example of this sort of process with an imidazole is an intramolecular example, the product in this case being a pyr-role. ... 

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). 

Wasserman, H.H. Stiller, K. Floyd, M.B. The reactions of heterocyclic systems with singlet oxygen. Photosensitized oxygenation of imidazoles. Tetrahedron Lett. 1968, 3277—3280. 

Hartman, P. E., Hartman, Z., and Ault, K. T. (1990). Scavenging of singlet molecular oxygen by imidazole compounds high and sustained... 

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