Intermediate compounds represented as formula (I) useful for the synthesis of 16-phenoxy-prostatrienoic acid derivatives and a preparing method thereof are disclosed. ##STR1## wherein R is tetrahydropyranyl, tetrahydrofuranyl, 2-ethoxyethyl, 3-t-butyldimethylsilyl triisopropylsilyl or triethylsilyl group; R.sup.1 and R.sup.2 are independently hydrogen or ester-forming group; P is hydrogen, trimethylsilyl or tri-n-butyltin; and wavy line means epistereoisomeric mixture.