Pyrrole ring cleavage

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... 

The ring cleavage of 3-aryl-2-substituted-2//-azirines by molybdenum hexacarbonyl has been described earlier in regard to the synthesis of pyrroles, pyrazoles and isoxazoles. In contrast to this behavior, analogous reactions of 2-unsubstituted derivatives lead to the formation of mixtures of 2,5-diarylpyrazines (139) and isomeric 3,6- and 1,6-dihydropyrazine derivatives (140,141) (Scheme 163).47,53 It is possible that the pyrazine products are formed by an intermolecular nitrene mechanism akin to the intramolecular processes described earlier (see Scheme 22 in Section IV,A,1). 

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. 

The five-membered ring heteroaromatics (furan, thiophene, and pyrrole) show very similar ring cleavage patterns. The first step in each case is cleavage of the carbon-heteroatom bond. 

Catalytic reduction of pyrroles gives successively A3-pyrroles and pyrrolidines. Tetrahydrofurans are formed by the catalytic reduction of furans with Raney nickel and hydrogen ring cleavage products... 

In addition to acyclic and monocyclic enamines, bicyclic and tricyclic enamines also undergo cycloaddition with dihalocarbenes. Endocyclic enamines , such as pyrrole and indole, add dichforocarbene and the adducts rapidly undergo ring cleavage to afford 3-chloropyridine and 3-chloroquinoline, respectively, in moderate yields (c/. Section 4.7.3.9).75-77... 

Photoreaction of l-vinyl-4-alkyl-5-aminotriazolines leads to unexpected pyrrolines and pyrroles.459 The 1-vinylaziridine formed by ring closure of the 1,3-diradical undergoes a selective ring cleavage at one of the C—N bonds (bearing the amino group) followed by cyclization with the j -vinyl carbon to give the products (Scheme 157)459... 

Pyrrole, like thiophene, does not react with benzophenone to give the corresponding oxetane. However, pyrrole reacts with aliphatic aldehydes and ketones to the corresponding 3-pyrryl carbinols. The alcohols derive from the cleavage of the corresponding oxetanes (Scheme 3.48) [94]. The yields increase when A -methylpyrrole is used as substrate, while the reactivity is depressed in the presence of substituents on the pyrrole ring. 

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