Iodolactonization of 2-substituted (N-sulfonylamino, hydroxyl and hydroxymethyl groups)-4-pentenoic acid with NIS or I2 in the presence of Ti(Oi-Pr)4 gave the gamma-lactone with increased 1,3-cis-selectivity. Stereoselectivity in the haloetherification of 2-hydroxymethyl-4-penten-1-ol was reversed by the addition of Ti(Oi-Pr)4.