Ab Initio Study of Intramolecular Ring Cyclization of Protonated and BF(3)-Coordinated Trans- and Cis-4,5-Epoxyhexan-1-ol

The potential energy surface for the rearrangement of cis- and trans-4,5-epoxyhexan-1-ol with acid and the Lewis acid BF(3) to five- and six-membered cyclic ethers has been investigated by ab initio methods. The transition structures involving both inversion and retention of configuration at the reaction center at the HF/6-31G and B3LYP/6-31G levels are characterized. The preference for furan formation over pyran is attributed to the more favorable O-C(ep)-O bond angles at the transition structures for furan formation. The torsional O-C(ep)-C(ep)-O angles associated with tetrahydrofuran and tetrahydropyran formation vary with structure and do not directly correlate with the preferred pathway.