Reevaluation of Bruice’s proximity orientation

Ab initio molecular orbital calculations at HF/6-31G, HF/6-31G (d,p) and DFT at B3LYP/6-31G (d,p) levels and molecular mechanics calculations of thermodynamic and kinetic parameters for Bruice’s systems 1-6 indicate that the remarkable acceleration in the cyclization of di-carboxylic semi-esters 1-6 is solely the result of a strain effect and not proximity orientation stemming from the ‘reactive rotamer effect’.