1 -Methylpyrazole, irradiation

Following the action of extraordinary stimulants (hypoxic hypoxia, hypoxia + hyperoxia, hypodynamia + hyperthermia), animals demonstrate an accumulation of malonic dialdehyde with a simultaneous fall of antiradical activity of the liver tissue. A preliminary introduction to rats of acetylene amine 3,4,5-tris(morpho-linopropynyl)-l-methylpyrazole 103 and also of tocopherol antioxidant and gutumine antihypoxant averts activation of the lipid peroxidation processes. The inhibition of peroxidation with this agent is mediated by stabilization of ly-zosomal and mitochondrial membranes. Unsaturated amines prevent destruction of the organelle membranes provoked by UV irradiation and incubation at 37°C (pH4.7)(78MIl). 

Fluoro-substituted I-methylpyrazoles also phototranspose to fluoro-substituted 1-methylimidazoles <9IJOC6313>. Scheme 13 shows the primary products formed upon direct irradiation of each isomeric fluoro-1-methylpyrazole in acetonitrile. Although deuterium... 

A more significant difference between 1 -methylpyrazole and isothiazole photochemistry, however, appears to be the minor role of the N-2-C-3 interchange pathway in isothiazole chemistry. Thus, although N2-C3 interchange is a major transposition pathway in pyrazole chemistry, it is only a minor pathway upon irradiation of phenylisothiazoles in benzene solution. In fact, 4-phenylisothiazole (55), the compound most expected to react via the N-2-C-3 interchange pathway, was the only isomer that did not yield a transposition product upon irradiation in benzene solution (Scheme 27). This is not due to the photostability of the compound. Indeed, 55 is the most reactive of the six isomers. Nevertheless, even after consumption of 85% of 55, no phototransposition product could be detected. 

There are a number of examples of photochemically induced interconversions forming imidazoles. On irradiation, 3-methylpyrazole gives a mixture of 2- and 4-methylimidazoles, and 4-aroyl-I-arylpyrazoles yield the diimidazol-4-yl derivative 57 (Eq. 17). 

The phototransposition chemistry of iV-substituted pyrazoles and of isothiazoles has been of considerable interest [1] since the first reports that W-methylpyrazole 1 undergoes photoconversion to W-methylimidazole 2 [2] and that irradiation of isothiazole 3 leads to the formation of thiazole 4 [3]. 

There are examples of 1,2-azoles being converted into their 1,3-isomers by irradiation, though such processes are of limited preparative value. The conversion of cyanopyrazoles into cyanoimidazoles was studied using 3-cyano-5-deuterio-l-methylpyrazole, the resulting mixture of products requiring a duality of mechan-... 

On irradiation, pyrazoles rearrange to imidazoles and undergo ring-opening to P-aminoacrylonitriles. For these photoisomerizations, various mechanistic pathways have been discussed [431], as exemplified for the transformation of N-methylpyrazole leading to N-methyhmidazole (12) and the nitrile 14. 

Zarea et al. [126] reported an efficient multicomponent reaction for the synthesis of fused 6-amino-3-methyl-4-aryl-lH-pyrazolo[3,4-b]pyridine-5-carbonitrile (185) from substituted aldehydes (182), malononitrile (183), and 3-amino-5-methylpyrazole (184) (Scheme 48). The reaction has been carried out in the absence of catalyst and with the assistance of ultrasoxmd irradiation. Reported yields were good to excellent (85-98%) in short reaction times (8-10 min). 

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