The occurrence of 13,14-dihydro and 13,14-cis-unsaturated prostaglandins in the coral, Plexaura homomalla. Synthesis of 13,14-cis-prostaglandin E2, 15-acetate methyl ester, and the 13,14-cis analogs of prostaglandin F2.alpha. and prostaglandin F2.beta.