Evaluation of exo-endo ratios in the halolactonization of omega-unsaturated acids.

The reaction of 2-(omega-alkenyl)benzoic acids with bis(collidine)iodine and bis(collidine)bromine hexafluorophosphate was examined. Except with 2-but-3-enylbenzoic acid, for which only the exo lactone was obtained, for the other acids a mixture of exo-endo lactones was always obtained. The proportion of endo lactone was important for the acid chain length of 11 carbons (formation of a 12-membered ring endo lactone) and for the acid chain lengths higher than 14 carbons. The formation of the endo lactones was explained, on the base of molecular calculations, by competition between electronic and steric effects. These latter were developed by transannular interactions (for the acid chain lengths 8-11) and/or the conformations adopted by the chains (for the acid chain lengths > or = 14,) which disfavored the formation of the exo lactones. The larger proportion of endo lactones observed with the bromo reagent compared to the iodo reagent seemed due to electronic factors.