Abstract
Prostaglandin H synthases (PGHSs) are heme-containing enzymes that catalyze the conversion of arachidonic acid (AA) to prostaglandin H2 by dioxygenation. This biosynthetic pathway is interesting in that the complex organic molecule can be produced in one step from a simple one. The trigger of this reaction is the H abstraction from AA by a tyrosyl radical. There, however, are no preceding examples of the organic reactions, which utilize an aryloxyl radical as a mediator, because of its instability. Here. we report the mimic of the active sites in PGHS using ferric 'twin-coronet' porphyrin (TCP), which contains four equivalent naphthol moieties in its hydrophobic and chiral cavities on both faces of the porphyrin ring. The mCPBA oxidation of TCP in CH2Cl2 at -40°C successfully gave the corresponding oxidized species having a naphthoxyl radical and an iron(IV) porphyrin, which were characterized by UV-vis, ESR, and resonance Raman spectroscopies. The oxygenase reaction of an 1,4-diene by the generated species proceeds through regioselective H radical abstraction at the 3-CH2 and stereoselectively gives the corresponding cis,trans-dienyl hydroperoxide in a catalytic manner. Thus, the present system is considered to be a good modeling reaction of PGHSs. This reaction is also the first example, which utilizes unstable Ar-O radical being susceptible to oxidation as a mediator in catalytic oxygenation.
