Etude des petits cycles—XXVII : Mecanisme de la regression de cycle des hydrates,hemicetals et cetals des bromo-2 (et tosyloxy-2) cyclobutanones

Solvent effect, common-ion effects and leaving group effects, and kinetic data for the ring contraction of 2-bromo (and 2-tosyloxy) cyclobutanones into cyclopropanecarboxylic acid by water alone, show the slow formation of the hydrate, followed by the fast expulsion of the leaving group in a S_n2 type reaction. The stereochemistry and kinetic data for the thermal ring contraction of 2-bromo-cyclobutanone ketals into cyclopropanecarboxylic esters show an intermolecular halogen transfer and exclude the occurrence of a pericyclic reaction.