Conformational studies of dibenzo-16-crown-5 lariat ethers

Conformations of sym-(R)(OR')dibenzo-16-crown-5 compounds with R = decyl and R' = H, CH₃, CH₂CO₂H, and CH₂CO₂CH₃, as well as R = alkyl, fluoroalkyl, alkenyl, and alkynyl and R' = CH₂CO₂H, in solution have been probed by NMR spectroscopy. The difference in chemical shifts of the diastereotopic hydrogens for the methylene groups adjacent to the geminally substituted ring carbon is utilized to assess the ease of inversion of the three-carbon bridges. For lariat ethers with R = C₁₀H₂₁, the chemical shift difference is much larger when R' = CH₂CO₂H and CH₂CO₂CH₃ than H and CH₃. In non-hydrogen bonding solvents, the chemical shift difference for lariat ethers with R = C₁₀H₂₁ is much larger when R' = CH₂CO₂H than CH₂CO₂CH₃ which demonstrates intramolecular hydrogen bonding of the carboxylic acid-containing side arm with a crown ether oxygen.