Nuclear magnetic resonance, force field, and MNDO studies on 1-methoxycycloalkenes

Article abstract of DOI:10.1016/S0040-4020(01)85647-7

The ¹H, ¹³C, and ¹⁷O NMR spectra of 4- to 9-membered 1-methoxycycloalkenes have been recorded in CDCl₃ solution. The ¹⁷O NMR chemical shifts reveal unexpectedly large and irregular variations with ring size. The other NMR shift data suggest that these variations are due to differences in the relative amounts of planar and nonplanar conformers about the O-C(sp²) bond. The same NMR data also suggest that whereas the MeO groups of the 5- to 9-membered ethers adopt mainly the expected planar s-cis conformation, the 4-membered ether exists predominantly in the exceptional s-trans conformation. The geometries of and charge densities in these ethers have been studied by force field (COSMIC, MM2) and MNDO calculations.