Conformational Studies of Some 1-Hetera-2,6-diphenylcyclohexan-4-one Oximes Using NMR Spectra-Evidence for Boat Form Contributions to trans-2,6-Diphenyl Systems

Abstract

Abstract 1H NMR spectra have been recorded for cis- and trans-2,6-diphenyltetrahydrothiopyran-4-ones (1 and 5), cis- and trans-2,6-diphenyltetrahydropyran-4-ones (2 and 6), 3-methyl- and 3-ethyl-2,6-diphenyltetrahydropyran-4-ones (3 and 4) and their oximes (7 — 12) at 270 MHz. For the oximes, 13C NMR spectra have also been recorded. The coupling constants suggest that the oximes 7, 8, 9, and 10, which have the phenyl groups in cis orientation, largely exist in chair conformations with equatorial disposition of the substituents. Analysis of the vicinal coupling constants of trans-2,6-diphenyltetrahydrothiopyran-4-one oxime (11) suggests that boat forms 11c and 11d must make significant contributions to it. Moreover, chair conformation 11a, which has the axial phenyl group in the syn-side, should be more populated than chair conformation 11b, which has the axial phenyl group in the anti-side. These conclusions are corroborated using 13C chemical shifts. The NOESY spectrum of 11 gives definite evidence for the contribution of 11c. The coupling constants of 12 suggest that the chair conformation 12a with axial phenyl group in the syn-side should be more populated than chair conformation 12b with the axial phenyl group in the anti-side. By analogy boat form should contribute to 12 also.