Ring cleavage extradiol

From all of these substrates, catechol is formed, and this is metabolized by extradiol (2 3) or intradiol (1 2) ring cleavage mediated by dioxygenases and... 

Figure 9 Proposed mechanism for ring cleavage by extradiol dioxygenases.

No special evidence has been reported to discriminate the mechanisms of 2,3-PCD from that of 4,5-PCD. It is said that the mechanisms of proximal and distal extradiol dioxygenase are fundamentally the same and that the site of ring cleavage is determined by the influence of the protein structure on the proximity of an activated oxygen species to the site of attack on the aromatic ring of catecholic substrates [41]. 

Scheme 10. Proposed mechanism for extradiol ring cleavage of catechol by 2,3-CTD [132].

The distal extradiol cleavage of L-dopa 12, catalyzed by an iron-dependent dioxygenase, gives an alanyl muconic semialdehyde derivative 18 which, on cyclization and lactol oxidation, yields stizolobic acid 16. The pyrone ring is then ammonolyzed24 to give 3-(6-carboxy-2-oxo-4-pyridyl)alanine 17 (Scheme 3). 

Component 13 has been suggested to arise from prior hydroxylation of the ring followed by intradiol cleavage and lactonization. Though plausible, this mechanism does not satisfactorily explain why 15 is not observed at all in several cases. A more reasonable suggestion involves the extradiol cleavage of the ring, subsequent oxidation of the aldehyde function, and lactonization. 

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