Amino acids reactions with singlet oxygen

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). 

The rate constants for reaction of singlet oxygen vary significantly between biomolecules with greatest reactivity towards amino acid side chains, thereby affording some specificity of response [ 15]. Nevertheless, it can be considered as a powerful short-lived oxidant. The most frequent type of reaction initiated by this species is addition across unsaturated bonds yielding a cyclised product. This has been extensively reviewed by Davies [16] and will not be discussed further here. 

I.B.C. Matheson, J. Lee (1979). Chemical reaction rates of amino acids with singlet oxygen. Photochem. Photobiol, 29, 875-878. 

Irradiation of the herbicide metamifop (50) leads to A-(2-fluorophenyl)-A-methyl-2-hydroxypropionamide as major product. Minor amounts of A-methyl-2-fluoroaniline and A-methyl-A-phenyl-2-oxopropionamide are also obtained." Laser flash photolysis of p-lapachone-3-sulfonic acid (51) gives the triplet excited state, which is efficiently quenched by electron rich amino acids via electron transfer followed by proton transfer. No measurable quenching is observed in the presence of thymine or thymidine by contrast, the reaction with 2 -deoxyguanosine is fast, although not diffusion-controlled. The singlet oxygen quantum yield, determined by time-resolved near-IR emission, is ca. 0.7. "... 

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