Hydroxylation of aromatic substrates

It has been found generally in a variety of living systems that hy-droxylation of aromatic substrates involves utilization of molecular 

---

The pattern of microbial hydroxylation of aromatic substrates appears to occur generally according to the usual rules of electrophilic aromatic substitution as shown by the hydroxylation positions of acronycine (1) at C-9 and C-11 (para- and ortho-positions to the amino group and mefa-position to the car-... 

The structural similarity of the catalytic domains of the enzymes of the AAH family, together with the identical reaction that they catalyze (i.e., hydroxylation of aromatic substrates) and the common dependency on BH4 and 02 (Section I), suggests that the mechanisms by which these enzymes operate are similar. It is assumed that the general AAH reaction mechanism follows a two-step reaction route in which a high-valent iron-oxo (FeIV=0) complex is formed in the first step, and that this intermediate is responsible for the hydroxylation of the aromatic amino acid substrate in the second step (15,26-28,50). The first step starts with 02 binding and activation and proceeds via a Fe-0-0-BH4 bridge and a subsequent heterolytic cleavage of the... 

The microbial hydroxylation of aromatic substrates with activating substituents appears to follow the normal rules of electrophilic substitution in that ortho and para products predominate (776,181), and the formation of 177 (major) and 178 (minor) in both the microbial and mammalian metabolism of acronycine has been attributed (777) to the directing influence of the nitrogen atom in a process involving a monooxygenase enzyme. 

Tzedakis T, Savall A, Clifton MJ. The electrochemical regeneration of Fenton s reagent in the hydroxylation of aromatic substrates batch and continuous processes. J Appl Electrochem 1989 19 911-921. 

This C(4a)-hydroperoxyflavin intermediate is the active form of the cofactor in a monooxygenation reaction, leading to the hydroxylation of aromatic substrates. The reaction mechanism is schematically depicted in Fig. 4.81. Table 4.3... 

Hydroxylations of aromatic substrates follow the rules of electrophilic substitution.12 L-Dihydroxyphenylalanine (L-DOPA),33 and hydroxyl-ated metabolites acronyane,34 5-anilino-l,2,3,4-thiatriazoles35 and a-methylfluoren-2-acetic acid36 have been prepared in this way. 

The mechanism for the hydroxylation of aromatic substrates by flavoprotein monooxygenases has been the subject of signiflcant research interest and controversy over the past decade. These enzymes (p-hydroxybenzoate hydroxylase, phenol hydroxylase, and melilotate hydroxylase) catalyze the initial step in the )8-ketoadipic acid pathway, the hydroxylation of substituted phenols into catechols (Scheme 55). Oxygen is required as cosubstrate, which is activated by the reduced FAD cofactor. The complex mechanism for the oxidative half-reaction is thought to consist of at least four steps and three intermediates 239-242) and to involve a controversial 4a,5-ring-opened flavin 242, 249, 250) (Scheme 56). The flavin C4a-hydroperoxy intermediate 64 and flavin C4a-hydroxy intermediate 65 have been assigned the structures shown in Scheme 56 based on the UV absorbance spectra of various model compounds compared with that of the modified enzyme cofactor alkylated at N(5) 243). However, evidence for the intermediacy of various ring-opened flavin species has been tentative at best, as model compounds and model reactions do not support such an intermediate 242). 

Scheme 55. General reaction scheme for the hydroxylation of aromatic substrates (S) by flavoprotein monooxygenases to give products (SO).

The hydroxylation of aromatic substrates by enzymes such as L-phenylalanine hydroxylase has been extensively studied in the National Institute of Health, Bethesda, USA, and it has been shown that the hydroxylation reaction is frequently accompanied by an intramolecular migration of the group or atom which is being displaced to an adjacent ortho position on the aromatic ring. This effect has been appropriately termed the NIH shift and detailed study of this type of reaction has led to some important conclusions regarding the precise nature of the chemical reactions involved in aromatic hydroxylation. In particular it has led to the suggestion that these enzymic reactions may proceed via arene... 

Daly, J. W., D. M. Jerina, and B. Witkop Migration of Deuterim During Hydroxylation of Aromatic Substrates by Liver Microsomes. I. Influence of Ring Substituents. Arch. Biochem. Biophys. 128, 517-527 (1968). 

---