Hydroxylation possible mechanism

The reverse reaction (that is, the oxidation of a vinyl radical by Fe to the corresponding vinyl cation) may be involved in the reaction of the dimethyl ester of acetylenedicarboxyUc acid 261 with Fenton s reagent [Fe —H2O2, (217)] (216). When 261 was treated with Fe —H2O2 and the reaction mixture was extracted with ether, a small amount of furan 262 was isolated. A possible mechanism (216) for its formation may be addition of hydroxyl radical to the triple bond of 261, followed by addition of the intermediate vinyl radical to a second molecule of 261 and oxidation of the resulting radical with Fe to the corresponding vinyl cation, followed by cyclization to 262, as shown in Scheme XX. 

Figure 5 presents some possible mechanisms for substrate hydroxylation by intermediate Q additional possibilities are discussed elsewhere (59). Mechanisms A-C assume that this species is a diiron(IV) oxo... 

Fig. 5. Possible mechanisms for the MMO hydroxylation step. Pathway A insertion of the oxygen atom of Q into the C-H bond B concerted addition of the C-H bond to Q followed by reductive elimination C, D homolytic attack of Q on the C-H bond E reaction of the peroxo species with substrate.

There are various pathways for free radical-mediated processes in microsomes. Microsomes can stimulate free radical oxidation of various substrates through the formation of superoxide and hydroxyl radicals (the latter in the presence of iron) or by the direct interaction of chain electron carriers with these compounds. One-electron reduction of numerous electron acceptors has been extensively studied in connection with the conversion of quinone drugs and xenobiotics in microsomes into reactive semiquinones, capable of inducing damaging effects in humans. (In 1980s, the microsomal reduction of anticancer anthracycline antibiotics and related compounds were studied in detail due to possible mechanism of their cardiotoxic activity and was discussed by us earlier [37], It has been shown that semiquinones of... 

It should be noted that Reaction (4) is not a one-stage process.) Both free radical N02 and highly reactive peroxynitrite are the initiators of lipid peroxidation although the elementary stages of initiation by these compounds are not fully understood. (Crow et al. [45] suggested that trans-ONOO is protonated into trans peroxynitrous acid, which is isomerized into the unstable cis form. The latter is easily decomposed to form hydroxyl radical.) Another possible mechanism of prooxidant activity of nitric oxide is the modification of unsaturated fatty acids and lipids through the formation of active nitrated lipid derivatives. 

V-phenyIpmprionamidc is found to come directly from alfentanil and not from noralfen-tanil. The mechanism of how the carbon-nitrogen of the spiro center is cleaved has not appeared in the literature. A possible mechanism would entail initial hydroxylation of a ring carbon adjacent to the piperdine nitrogen, followed by elimination of hydroxide to form the imine then rearrangement to the enamine, and finally elimination of the amide as indicated in Figure 4.56. 

Chlorosulfate groups can be readily removed to give the corresponding hydroxyl groups, with retention of configuration, by treatment of a solution of the carbohydrate chlorosulfate in methanol with sodium iodide in aqueous methanol24 immediate liberation of iodine and evolution of sulfur dioxide occur.34 A possible mechanism for the dechlorosulfation reaction involves displacement by iodide at the chlorine atom the initially formed iodine monochloride would react with iodide ion to give iodine and chloride ion. Alternatively, an unstable iodosulfate could be formed as an intermediate.35... 

Figure 12-29 Drawing showing the hydrogen-bonding interactions between the guanidinium ions of arginines 35 and 87 of the micrococcal (staphylococcal) nuclease with the 5 -phosphate of the inhibitor thymidine 3, 5 -diphosphate in the complex of E + I + Ca2+. A possible mechanism is illustrated. A hydroxyl ion bound to Ca2+ carries out an in-line attack on the phosphorus. See Libson et al.S26...

The samples in which the Si02 was introduced in situ have higher decomposition temperatures. A possible mechanism for this improvement would be increased capability of the in situ produced silica to tie up hydroxyl chain ends that participate in the degradation reaction. 

These copper-mediated reactions very often involve dinuclear intermediates, but detailed mechanistic studies on stoichiometric systems are relatively few. The key features are the formation of p-peroxo or p-superoxo complexes by electron transfer from cop-per(i) to dioxygen. The co-ordinated oxygen may then act as an electrophile to the aromatic ring. A possible mechanism for the ortho-hydroxylation of phenol by dioxygen in the presence of copper catalysts is shown in Fig. 9-29. 

In the non-3-hydroxylation pathway, the initial step from GA] 2 alenzyme-catalyzed oxidation of carbon-7 which yields the acid, GAqg. This is followed by the loss of carbon-20 to give GAq. The mechanism for the loss of carbon-20 is still unresolved. There are several lines of evidence which eliminate a number of possible mechanisms. For instance the two oxygens in the lactone of GAq have been shown to have their origin from the 19-oic acid group of the Cgo precursor (25). 

Another possible mechanism of photolysis is through the formation of secondary photooxidants that can be formed from one of the photosensitizer routes shown above. For example, a possible mechanism with humic substances as photosensitizers [90] could involve the formation of hydrogen peroxide and, subsequently, hydroxyl radicals ... 

Another possible mechanism for the final step has been proposed. This requires a beta hydride shift from the hydroxyl bearing carbon to the alpha carbon with loss of PdCli- This mechanism cannot be ruled out at this time, but the first suggestion seems more consistent with the known chemistry of the transition metals. 

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