sentences:  They differ in that PGE2 has, while PGE1 lacks, a double bound between carbons 5 and 6.Both PGs undergo a dehydration reaction across their 11-hydroxyl-carbon 10 regions to form a new double bond between carbons 10 and 11 to become PGA2 and PGA1, respectively, with a cyclopenenone ring replacing the cyclopentaonon rings or their precursors and a newly established electrophilic site at carbon 11.This electrophilic site is probably less electrophilic that the carbon 9 sites of Δ12-PGJ2 and 15-deoxy-Δ12-PGJ2
  
  The cyclopentenone structures of PGA2, PGA1, PGJ2, Δ12-PGJ2, and 15-d-Δ12,14-PGJ2 possess α,β-unsaturated carbonyl groups (see Carbonyl group#α,β-Unsaturated carbonyl compounds) which serve to establish high levels of chemical reactivity at nearby carbons 9, 11, and/or 13.