Udenfriend’s system

It is notable that flavone and its derivatives are substituted predominantly in the phenyl group at positions 3 and 4 whereas reaction with Fenton s reagent gives mainly the S-hydroxy isomer." " Similarly, comparison of the distribution of products from coumarin obtained with Udenfriend s system with that given by Fenton s reagent re-... 

The effect of SOD points to the intervention of Oj in the autoxidation of ascorbate. Proteins in large amounts could react with the strong oxidizing agent formed in this Udenfriend s system , thus protecting the enzyme. The ineffectiveness of boiled SOD could be due to its amino-acid composition (See Sect. 4.1.2). While O did not inactivate dopamine P-monooxygenase the rate pf inactivation in the presence of... 

Information of the Gif system has been summarized,1055 1123 and new results, including new oxidants such as bis(trimethylsilyl) peroxide,1124 the synergistic oxidation of saturated hydrocarbons and H2S,1125 studies with the Fe3+-picolinate complex encapsulated within zeolites,1126 and the use of Udenfriend s system under Gif conditions1127 were disclosed. Gif-type oxidations were found to be moderately stereoselective.1128 Iron/zinc-containing species involved in Gif-type chemistry were synthesized, and their reactivity and catalytic behavior were studied.1129... 

Transition-metal systems with macrocyclic nitrogen and oxygen donors are more analogous to biological 02-oxidation centers than the more organometallic type systems considered thus far. Studies on the macrocyclic ligand systems follow earlier ones such as Udenfriend s systems (e.g., Fe(II)-EDTA), and the use of other transition-metal salts, sometimes with added ligands (43, 113). 

Role of Ascorbic Acid in a Mono-oxygenase Model (Udenfriend s System)... 

The reaction sequences originally suggested for oxygen insertion in a substrate such as salicyclic acid in Udenfriend s System (19-22) in which molecular oxygen is the oxidant and an iron chelate such as Fe(II)-EDTA is the catalyst and ascorbic acid is the reductant, is indicated by Equations 14-18. In this reaction sequence oxygen insertion is considered to occur by direct reaction of the aromatic compound with HO2 and OH free radicals, and the ascorbate reductant merely serves the purpose of regenerating the Fe(II) chelate as indicated by Equations 16 and 17. 

Unlike Udenfriend s system, which is active with several different metal ions (29), hydroxylation of aromatic compounds using NBNH and oxygen occurs only in the presence of ferric ions. Ceric or cupric sulfate are unable to activate the system, and more surprisingly, ferrous sulfate is ineffective. Ferrous ions under the reaction conditions (pH 4.2) are relatively stable and are oxidized only slowly to ferric ions by molecular oxygen thus, it would seem that only negligible amounts of ferric ions are formed during the reaction period, and hence no hydroxylation occurs. 

One of the better known models is Udenfriend s system, consisting of a mixture of Fe(II), EDTA, ascorbic acid, and dioxygen [37-42]. At neutral pH, this system hydroxylates arenes to phenols under mild conditions. 

The fact that Udenfriend s system does not bring about the NIH-shift [45] points to a mechanism different from that operative in enzymic hydroxylations. However, deuterated arenes with Fenton s reagent produce an NIH-shift in non-protic solvents (acetonitrile, nitromethane) [46] to an extent related to the amount of water in the system. Taking into account Groves results, supporting the formation of oxoiron intermediates from Fenton s reagent [47-50] ... 

The addition of other polyaminocarboxylate type chelating agents such as NTA, IMDA, HEDTA, or CyDTA to Udenfriend s system increases both the rate of formation and the yield (up to 50 %) of phenol from benzene hydroxylation [51]. 

Other monooxygenase mimics are obtained if ascorbic acid in Udenfriend s system is replaced by other hydrogen donors, such as enediols [37], reduced nicotinamide coenzymes [41], thiosalicylic acid (Ullrich s system) [52], cysteine [53], pyrimidine [40], or tetrahydropteridine [54,55]. Low-valent transition metal compounds (e.g. FeCl ) with dioxygen and reducing agents (hydrazobenzene,... 

Udenfriend s system, consisting of a mixture of Fe(II), EDTA, ascorbic acid, and dioxygen [124-129] hydroxylates phenol and anisole in o- and p-position [124,130]. In a comparative study of phenol and benzyl alcohol, the m-position in the latter was also hydroxylated [131]. The same effect was observed with anisole and benzyl methyl ether. The explanation lies in the geometrical accessibility of the m-position for the Fe-O moiety in the benzylic derivatives 50 relative... 

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