In order to examine the steric consequences of the trialkyl lock, a series of halogenated lactones have been synthesized, including the severely crowded iodo lock. Studies on the kinetics of opening and closing of these systems are in progress. Halogenated acyclic locked systems have been synthesized, after considerable effort, in order to determine the free energy of the barrier to free rotation in such compounds. A locked five-membered lactone has been obtained by novel synthesis. The opening and closing of this system has beem compared with that of a six-membered lactone; closure to the five-membered lactone is faster, and is the fastest esterification reaction ever observed in aqueous solution at neutral pH.