Ring-Fission Reactions

The anaerobe Peptococcus (Micrococcus) aerogenes had a dehydrogenase that carried out specific hydroxylation at the 6-positions of 2- and 8-hydroxypurine, and was therefore distinct from xanthine dehydrogenase from which it could be separated (Woolfolk et al. 1970). It was also able to carry out dismutation of 2-hydroxypurine to xanthine (2,6-dihydroxypurine) and hypoxanthine (6-hydroxypurine). 

Although it had been assumed that only hypoxanthine dehydrogenase is required for the conversion of hypoxanthine (6-hydroxypurine) into uric acid, in Clostridium purinolyti-cum, two enzymes, both of which contain a selenium cofactor, are required. The enzymes differ in the molecular mass of their subunits, in their terminal amino acid sequences, in their kinetic parameters, and in their specific activities for purines (Self and Stadman 2000). Purine hydroxylase converts purine into hypoxanthine and xanthine (2,6-dihy-droxypurine), which is then further hydroxylated to uric acid (2,6,8-trihydroxypurine) by xanthine dehydrogenase (Self 2002). 

Derivatives of 1,3,5-triazine are important herbicides so that attention has been directed to their persistence particularly in the terrestrial environment. In some experiments when a growth substrate was supplied, they have been used as sources of nitrogen or sulfur  

Degradation of substituted triazines is accomplished by a sequence of hydrolytic reactions (Jutzi et al. 1982 Mulbry 1994). Atrazine, which is a representative herbicide, is degraded 

FIGURE 10.34 Aerobic degradation of 1,2,4-triazolone herbicides (a) metamitron and (b) metribuzin. 

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Analogous ring fission reactions have also been found in stndies on the metabolism of other aromatic compounds by the yeast Trichosporon cutaneum whose metabolic versatility is indeed comparable with that of bacteria. Examples inclnde the degradation of... 

Our findings lead us to assume that the oxidative degradation of guaiacyl compounds with saturated side chains in the 4-position (para to the hydroxyl group) is proceeding via 0,0 -coupling. Dimerization is then followed by ring fission reactions. 

Recently, some additional empirical evidence that the ring fission is a relatively low-energy decomposition pathway is provided by some work by Long et al. [32], They observed a gas phase decomposition reaction with Ea 33.5 kcal/mol. They suggested that this might be due to the ring fission reaction observed by Zhao et a/.[33], although there may be other explanations for the observations. 

A strong motivation for the Rice et al. [99] simulations was to try to interpret the Zhao et al. [33] observations that the HONO elimination channel dominates while the N-N bond rupture reaction does not occur. A possible explanation is that the nascent CH2NN02 product of the RDX ring fission reaction is highly excited and has a nonstatistical distribution of energy. Sewell and Thompson [35] estimated that it may be formed with 55 to 65 kcal/mol of energy, which is well in excess of the predicted energy... 

In contrast to the ring-fission reaction of 1,2,3-selenadiazoles in the presence of strong base, ethyl 4-methyl-l,2,3-selenadiazole-5-carboxylate (141) is hydrolyzed smoothly to the... 

The oxidation of pyridinium salts with alkaline potassium ferricyanide provides a facile route to A -subslituted pyridones (Scheme 4.29a), although the sensitivity of pyridinium salts to nucleophilic attack may lead to ring fission reactions (Scheme 4.29b). 

The aerobic degradation of y-hexachloro[flflaeee] cyclohexane (HCH) has been accomplished with a strain of Pseudomonas paucimobilis strain UT26 (Nagasawa et al. 1993), and involves a complex chain of elimination, hydrolytic displacement of chlorine, and ring fission reactions (Chapter 6, Sections... 

Of particular interest in this field are multitasked disposable tethers offering flexibihty in the ring fission reaction. 

The two decisive aspects of epoxide chemistry are the chemoselectivity and stereochemistry of their preparation and the efficiency and regioselectivity of the various ring-fission reactions. 

As in these last examples, the descriptions of diastereoselective epoxidations were accompanied already by details on their subsequent use in ring fission reactions, we shall now switch to the various options of controlled regioselective and diastereoselective epoxide transformations. The nucleophilic opening of epoxides shows a strong dependence on substrate structure, nature of the nucleophile, and the catalyst involved, similarly to nucleophilic substitutions. Lastly, one has to consider the solvent and other details of the reaction conditions including the transition state conformation. 

Applying well-established ring fission reactions (see there) this optically pure intermediate will prove to be a very flexible building block. 

With a-sulfonylcarbanions, this is documented with a few ring fission reactions opening lactones [180] and ejxtxides [18lj-... 

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