CONFORMATIONAL STUDY OF 2,4-DIARYL-3-AZABICYCLO[3.3.1]NONAN-9-ONES AND THEIR 3-METHYL DERIVATIVES BY QUANTUM-MECHANICAL CALCULATIONS, NMR, AND X-RAY CRYSTALLOGRAPHY

Some 2,4-diaryl-3-azabicyclo[3.3.1]nonan-9-ones 1 and their N-methyl derivatives 2 have been synthesized and studied by semiempirical quantum mechanical calculations and H-1-NMR spectroscopy (AM1/H-1 tandem) in order to establish their conformational preferences. The crystal structure of 3-methyl-2,4-diphenyl-3-azabicyclo [3. 3.1] nonan-9-one, 2a, has been determined by X-ray diffraction. It is found that compounds 1 present a CC-alpha preferred conformation in the gas phase and in CDCl3 solution, while the CC-beta form is observed in the solid state. This latter conformation, with the N-H bond in the equatorial position (CC-beta), seems to be favored by the formation of intermolecular hydrogen bonds in the crystal. Compounds 2 are practically monoconformational and display a slightly flattened CC conformation with the N-methyl group in the equatorial position (CC-beta).