Syntheses of isocarbacyclin (20) using highly regioselective alkylation of the new allylicalcohol intermediate 6 having an exocyclic double bond are described. The allylicalcohol 6 was prepared by (i) allylic rearrangement of the allylicalcohol 3 having an endocyclic double bond and (ii) reductive cyclization of γ-ethynyl aldehyde 11a which was synthesized from (R)-4-hydroxy-2- cyclopentenone derivative