3-Substituted and 3,4-disubstituted pyrroline nitroxides containing an ethynyl 20 group or two ethynyl groups were achieved by the reaction of a paramagnetic aldehydes with dimethyl (1-diazo-2-oxopropyl)phosphonate (Bestmann-Ohira reagent). The new compounds containing an ethynyl group were found to be useful building blocks in Sonogashira coupling, cyclization, and cycloaddition reactions producing potentially "azido-specific" cross-linking spin labels, paramagnetic ligands, and polyradical scaffolds. 25
An N-vinylation of imidazole and benzimidazole with a paramagnetic vinyl bromide was investigated. Among the tested procedures, Pd-catalyzed reaction was the most powerful one. The N-vinylation of 2-aminobenzimidazole with a β-bromo-α,β-unsaturated pyrroline nitroxide aldehyde offered 1,1,3,3-tetramethyl-1H-benzimidazo[1,2-a]pyrrolo[3,4-e]pyrimidin-2(3H)-yloxyl radical and the corresponding non-cyclized Schiff base. The reaction of a β-bromo-α,β-unsaturated pyrroline nitroxide aldehyde with imidazole gave β-imidazo-α,β-unsaturated pyrroline nitroxide aldehyde, which was reduced to the alcohol and converted to an unstable allyl chloride.
Stable nitroxide free radicals have traditionally been associated with 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) or its 4-substituted derivatives as relatively inexpensive
and readily accessible compounds with limited possibilities for further chemical
modification. Over the past two decades, there has been a resurgence of interest in stable
free radicals with proper functionalization tuned for various applications. The objective of
this review is to present recent results with synthetic methodologies to achieve stable nitroxide
free radicals fused with aromatic carbocycles and heterocycles. There are two
main approaches for accessing stable nitroxide free radicals fused with arenes, e.g., isoindoline-
like nitroxides: further functionalization and oxidation of phthalimide or inventive
functionalization of pyrroline nitroxide key compounds. The latter also offers the constructions
of versatile heterocyclic scaffolds (furan, pyrrole, thiophene, 1,2-thiazole, selenophene, pyrazole,
pyrimidine, pyridine, pyridazine, 1,5-benzothiazepine) that are fused with pyrroline or tetrahydropyridine nitroxide
rings. The possible applications of these new stable nitroxide free radicals, such as covalent spin labels
and noncovalent spin probes of proteins and nucleic acids, profluorescent probes, building blocks for construction
of dual active drugs and electroactive materials, and substances for controlled free radical polymerization,
are discussed.
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