Visualization and quantification of anisotropic effects on the 1H NMR spectra of 1,3-oxazino[4,3-a]isoquinolines-indirect estimates of steric compression

The anisotropic effects of the phenyl, α- and β-naphthyl moieties in four series of 1,3-oxazino[4,3-a]isoquinolines on the 1H chemical shifts of the isoquinoline protons were calculated by employing the Nucleus Independent Chemical Shift (NICS) concept and visualized as anisotropic cones by a through-space NMR shielding grid. The signs and extents of these spatial effects on the 1H chemical shifts of the isoquinoline protons were compared with the experimental 1H NMR spectra. The differences between the experimental δ (1H)/ppm values and the calculated anisotropic effects of the aromatic moieties are discussed in terms of the steric compression that occurs in the compounds studied.