Synthesis, characterisation, stereochemistry and dynamic NMR studies of N-nitroso and N-formyl-t-3-isopropyl-r-2,c-6-bis(4-methoxyphenyl)piperidin-4-ones

• Two new N-nitroso- and N-formylpiperidin-4-ones were synthesized.
• Assignment of 1H and 13C NMR signals are made using 2D NMR spectra.
• Doubling of all NMR signals is due to syn-anti rotational equilibrium.
• Detailed conformational analysis has been carried out.
• Energy barriers are determined using dynamic 1H NMR spectral experiments.

The synthesis, characterisation and stereochemical investigations were made for N-nitroso (2) and N-formyl (3) derivatives of t-3-isopropyl-r-2,c-6-bis(4-methoxyphenyl)piperidin-4-one (1) using IR, mass, 1D and 2D NMR spectral studies. The compound 1 prefers to adopt a chair conformation with equatorial orientations of all substituents. The compounds 2 and 3 showed doubling of all the NMR spectral signals corresponding to syn and anti rotamers at RT and 223 K, respectively. The syn and anti rotamers of 2 exist in an equilibrium between alternate chair (CA) and twist boat (TB1) conformations, respectively. The syn and anti rotamers of 3 exist in TB1 conformation. The stereodynamics of compounds 2 and 3 have been investigated using dynamic 1H NMR spectra and the calculated average energy barriers for N-NO and N-CHO rotations are found to be 77.76 kJ mol−1 and 68.18 kJ mol−1, respectively.

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