Pyrrole ring oxidative cleavage

The reaction mechanism for the aerobic oxidation of the pz to seco-pz can be attributed to a formal 2 + 2 cycloaddition of singlet oxygen to one of the pyrrole rings, followed by cleavage (retro 2 + 2) of the dioxetane intermediate to produce the corresponding seco-pz (160). This mechanism is shown in Scheme 29 for an unsymmetrical bis(dimethylamino)pz. Further photophysical studies show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product, 159. As a consequence, the kinetics of the process is shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. 

An example of the synthetic utility of the oxidative cleavage of the pyrrole C-2 to C-3 bond is the ring expansion of bicyclic pyrroles (127) to lactams (128) (83S390). 

When the donor character of the amino substituent at the transition-metal-carbene carbon atom is reduced, it should be possible to influence the thermal stability and reactivity in favor of the desired cyclopropanation process. Indeed, pyrrol-1 -ylcarbene complexes 18 of chromium, molybdenum and tungsten do exhibit the desired reactivity. In the last step, the pyrrole ring of 19 can be converted to the NH2 function in 20 after oxidative cleavage with ozone.In this respect, the pyrrole heterocycle represents a synthetic equivalent of the amino function. 

The first step of tryptophan catabolism is the oxidative cleavage of the indole ring of L-tryptophan, which is catalyzed by members of the family of pyrrole dioxygenases. A key member of this family, indoleamine-2,3-dioxygenase (IDO, see Fig. 1, EC 1.13.11.17), is expressed in all tissues except in the liver and produces the central metabolite kynurenine (KYN). Two different and competing branches of the pathway then further metabolize KYN the first pathway includes a family of enzymes called kynurenine aminotransferases (KATs), which produce kynurenic acid (KYNA) in a terminal branch. In a second arm, KMO (or kynurenine... 

The addition of thiol or amino groups of proteins, e.g., HS-groups of L-cysteine or -amino groups of L-lysine residues, to the quinoid structures yields covalent bindings between melanins and proteins. Oxidative cleavage of the quinoid rings causes formation of pyrrol moieties, which are frequently part of the melanin molecule (Fig. 290). 

Nakagawa et al. (275) believe that when photosensitized oxidations of tryptophan are carried out in solvents such as formic acid, acetic acid and water, participation of the ethylamino side chain is unfavorable and that the reaction consequently proceeds via the alternative path A of pyrrole ring cleavage. However, Savige (339) recently found that photooxidation of tryptophan in water, in the presence of methylene blue... 

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