Prostanoids biosynthesis conversion

Conversion of arachidonate to PGH via cyclooxygenase catalysis is a key regnlatory step in prostanoid biosynthesis. 

Finally, there has been speculation and recent experimental support for the involvement of a Nazarov-type cyclization in the biosynthesis of c/ s-jasmonic acid ° and marine-derived prostanoids. Radiolabel tracer studies have demonstrated Ae intermediacy of 8-HPETE (104) in the biosynthesis of prostanoid intermediate preclavulone A (lO ). This remarkable conversion was proposed to proceed by formation of allene oxide (105) followed by isomerization to (107) via the 2-oxidocyclopentadienyl cation (106 Scheme 41). To demonstrate the chemical feasibility of this proposal, Corey reported the transformation of epoxysilane (108) to, inter alia, the cyclopentenone (111 Scheme 42). The reaction is presumed to involve formation of the allene oxide (109) followed by isomerization to the 2-oxi-dopentadienylic cation (110). Conrotatory closure of (110) is expected to produce the cis isomer of (111) as observed. 

The biosynthesis of prostanoids involves three important enzymatic reactions. In the case of PGE2, formation begins with the release of AA from phospholipids by phospholipase A2 (PLA2), followed by the synthesis of PGH2 by COX, and conversion of PGH2 to specific prostanoids (e.g., PGE2) by terminal PG synthases (66). 

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