Conformational study of the 1-azabicyclo[3.3.1]nonan-2-one system. Molecular-mechanics calculations and X-ray structure of 5-phenyl-1-azabicyclo[3.3.1]nonan-2-one

Abstract

Molecular-mechanics calculations for 5-methyl-1-azabicyclo[3.3.1]nonan-2-one indicate that the boat–chair conformation is more stable than the twin-boat conformation by 4.0 kJ mol^–1. An endo methyl substituent at position 7 causes the twin-boat conformation to be favoured by 5.2 kJ mol^–1. The boat–chair conformation of 5-phenyl-1-azabicyclo[3.3.1]nonan-2-one has been characterized by X-ray crystal-structure analysis. The bridgehead amide is appreciably distorted from planarity, with N(1) displaced by 0.36 Å from the plane of C(2), C(8), and C(9). The crystal structure provides an example of enantiomeric disorder: on average, each molecular site is occupied by 0.8 of a molecule of one configuration and 0.2 of the mirror-image molecule. Crystallographic data are a= 12.253(3), b= 8.372(2), c= 11.665(3)Å, Z= 4, space group Pca2₁. X-Ray intensity measurements were made on a four-circle diffractometer and least-squares adjustment of the atomic parameters converged at R 0.050 for 1 023 reflections.