Title : Use of 1H NMR spectroscopy in finding unusual conformations of saturated six-membered heterocyclic compounds
Abstract:
Saturated six-membered heterocyclic compounds largely adopt chair conformation with equatorial orientations of the bulky substituents. However, there are exceptions. Vicinal proton-proton coupling constants are largely useful in deriving information about the conformations of organic compounds. Also, NOE between protons furnishes information about the conformation. The NOESY spectrum and vicinal proton-proton coupling constants of t(4)-acetoxy-3,3-dimethyl-r(2),c(6) diphenyl-N- acetylpiperidine (1) suggest that it adopts chair conformation 1C with axial phenyl groups. The vicinal proton-proton coupling constants of t(4)-acetoxy-r(2),c(6)-diphenyl-N acetylpiperidine (2) could be accounted for by an equilibrium mixture of chair conformation 2C and boat conformation 2B with a phenyl group in the flagpole position. Analysis of the vicinal proton-proton coupling constants of trans-2,6-diphenyltetrahydrothiopyran-4-one oxime (3) suggests that boat forms 3B and 3B' should contribute to it in solution. Also, chair conformation 3C, with the axial phenyl group in the syn side, should be more populated than chair conformation 3C' with the axial phenyl group in the anti side. These conclusions are corroborated using 13C chemical shifts. The NOESY spectrum of 3 strongly suggests contribution of boat conformation 3B.
