Oxidation of the Ring

The only examples of ring oxidation are the one-electron anodic oxidation of Nl-aryl[l,2,4]triazolo[4,3-a]pyridines such as compound 143 to give quaternary salts (88ZC187), and the voltammetric oxidation of the anti-depressant Trazodone (Section V.A) (87MI1). 

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Benzene with no alkyl side chain and no benzyhc hydrogens undergoes a different reaction under these conditions Oxidation of the ring occurs to convert benzene to its epoxide... 

The nitrosation of pyrroles and indoles is not a simple process. The 3-nitroso derivatives (84) obtained from indoles exist largely in oximino forms (85) (80IJC(B)767). Nitrosation of pyrrole or alkylpyrroles may result in ring opening or oxidation of the ring and removal of the alkyl groups. This is illustrated by the formation of the maleimide (86) from 2,3,4 -trime thylpyrrole. 

The one-electron oxidation of the ring of a metalloporphyrin to give the radical cation (Equation (5)) is very sensitive to the nature of the metal ion present. 

Oxidation of the ring was also observed in reaction of 163 with thionyl chloride <1997BCJ2209>, 164 was obtained in 81% yield (Scheme 19). 

Figure 7. Absorption changes during the electrochemical oxidation of FePc(py)2 in CH2CI2 in the presence of 0.05 M [(Et) N]C10 (1) The unoxidized, neutral species. (2) The spectrum recorded following metal oxidation at 0.69 V vs. see. (3) The spectrum recorded following oxidation of the ring at 0.88 V vs. see...

In conclusion, photolysis of ZnPc, RuPc and CoPc results in the formation of the it cation radioed species through oxidation of the ring, while in the case of FePc, meted oxidation occurs prior to ring oxidation, but n ring oxidation is demonstrated to take place following metal oxidation. 

In contrast to the above situation with respect to oxidation of the ring nitrogen atoms of 1,2,3-triazines, oxidation of derivatives of 3-amino-l,2,3-benzotriazin-4-(3/0-one (101a) has proved to be of considerable interest and has been investigated in some detail by Rees and his colleagues. Treatment of 101a with lead tetraacetate at 80° has been shown to give benzyne in very low yield, but at room... 

Nitrosation of pyrrole or alkylpyrroles may result in ring opening or oxidation of the ring and removal of the alkyl groups. This is illustrated by the formation of the maleimide (119) from 2,3,4-trimethylpyrrole. 

Several productive degradative transformations of chlorophyll a and b depend upon the ready oxidation of the ring E /3-keto ester. The so-called phase test involves aerial oxidation of the enolate anion produced by treatment of the ring e keto ester with alkali. One molecule of oxygen is consumed in this allomerization reaction to produce a hydroperoxide (111) which can fragment to give (112) after acidification, the unstable chlorin (113) is obtained, and this can be transformed into purpurin-18 (114) by evaporation, or into purpurin-7 trimethyl ester (115) by esterification with diazomethane. 

Oxidation of the ring system in thiins has also been achieved with hydride acceptors such as triphenylmethyl cations, and results in high yields of thiopyrylium salts. This is far more efficient than the disproportionation reactions discussed above, as the only byproduct is triphenylmethane. The reaction will be discussed in the section on synthesis. 

The third reaction is the oxidation of the ring to form a carbon-carbon bond. The reducing equivalents are transferred to a flavin cofactor of the enzyme dihydroorotate dehydrogenase. The product is orotic acid. 

Periodate oxidation of the ring in LXI led to sirupy n-2-phenyldigly-colaldehyde (LXIV), which in turn was oxidized with silver oxide to sirupy D-2-phenyldiglycolic acid (LXV). The latter was converted through its ester (LXVI), to crystalline D-(+)-2-phenyldiglycolamide (LXVII), m. p. 172-172.5°, [a]KD 102.2° (ethanol). 

Oxidation of the ring sulfur atoms of the derivatives (165) with m-chloroperbenzoic acid in CHC13 at 0°C gives mixtures of the sulfoxides (166) and the sulfones (167) (Equation (8)) <85S86i>. 

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