Prostaglandin analogs, formation

Corey and Shimoji <83JA1662> have described the cyclization shown in Equation (19). Presumably, transetherification of (89) is catalyzed by the presence of pyridinium salts generated in the reaction mixture resulting in the formation of (90). The entropic factors are no doubt mitigated by the presence of the c -double bond in (89). An analogous procedure has been used to prepare intermediates in the total synthesis of major metabolite of prostaglandin D2 <83JA1662>. 

Since peroxyl radicals add so easily to double bonds (see Sections VIII.2.D and XI.4.B.a) 3-cyclopentenyl peroxyl radicals appeared to be attractive models for prostaglandin bicyclic endoperoxide formation, by analogy with the cycliza-tion discussed in Scheme 116. Under the conditions used, however, disproportionation was preferred to intramolecular cyclization. ... 

Of course, such a conclusion must not be generalized without care. For instance it has been proposed that the biosynthetic formation of the cyclopen-tanol ring of brefeldin A, a Cjj macrolide antibiotic, would be analogous to the prostaglandin biosynthesis, but such a free radical pathway has recently been ruled out Nevertheless, the experimental evidence is indirect, since this conclusion was arrived at using a biosynthetic path involving cyclization of an allyloxyl radical. Indeed, such a path appears to be a forbidden process (see Section VIII.2.A), and hence another pathway, ionic or radical, must be sought. 

The Chemistry of Cyclic Peroxides The Formation and Decomposition of Prostaglandin Endoperoxide Analogs... 

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