Conformations of Oxocane

Article abstract of DOI:10.1021/jo00027a051

Conformational analysis of oxocane (oxacyclooctane) has been examined by molecular mechanics (MM2), variable-temperature 13C NMR, and lanthanide-induced shift (LIS) 1H and 13C NMR.MM2 calculations find the BC-3 conformer and its enantiomer BC-7 to be favored, with the four next best forms and their energies relative to BC-3-being BC-1 (1.1 kcal/mol), TBC-1(1.1), BC-4(1.5), and TCC-1(1.6).Barriers to pseudorotational interconversion of BC-3 and BC-7 are calculated to be 5.0 kcal/mol through BC-5 and 6.7 kcal/mol through BC-1.The former would allow fast BC-3/BC-7 equilibration even at -170 deg C, which would leave reported low-temperature 1H NMR spectra compatible with a BC-3 structure as well as BC-1.Calculated barriers for BC ring inversion and interconversion of the BC family with the crown family (TCC-1) are 8.2 and 8.5 kcal/mol, respectively.A new two-step synthesis of oxocane and its 2,2,7,7-d₄ analogue is reported, the latter allowing unequivocal assignment of chemical shifts. 13C NMR spectra of oxocane between 138 and 290 K show BC-family/crown-family inter-conversion in the vicinity of 215 K (ΔG(excit.) = 10.0 +/- 0.3 kcal/mol), with the crown family comprising 4percent of the equilibrium at 174 K (ΔG⁰ = 1.1 +/- 0.1 kcal/mol).The 1H and 13C LIS induced by Yb(fod)3 on oxocane agree well with BC-3 and BC-7 being the predominant conformers at room temperature but do not acceptably fit a BC-1 structure.Thus, all available data from calculation and experiment are in accord with BC-3 being the favored conformation of oxocane.