Stereochemistry of the Kabachnik‐Fields Condensation of Terpenic Amino Oximes with Aldehydes and Dimethyl Phosphite

Three‐component condensation of aldehydes and dimethyl phosphite with α‐amino oximes derived from (−)‐α‐pinene and (+)‐3‐carene (Kabachnik‐Fields reaction) resulted in amino phosphonates as pairs of diastereomers. Diastereomeric ratio depends on the catalyst used (SnCl2×2H2O, SiO2, Al2O3‐H+) and the heating type (conventional or microwave). The best results were achieved by MW irradiation with simultaneous cooling. Stereochemical assignment of the key derivatives was made by X‐ray diffractometry. According to quantum chemical calculations (DFT PBE0/def2‐TZVPP) and spectroscopic data, the terpenic α‐amino phosphonates should be conformationally inhomogeneous, exhibiting a tendency to form H‐bonded dimers (DFT M06/def2‐SVP, IR). One‐bond spin‐spin coupling 1JP–C was found to be diagnostic for the configuration assignment since the value 1JP–C depends on the dihedral angle between bond C−P and axis of the electron lone pair at the neighboring nitrogen (NMR, DFT PBE0/aug‐cc‐pVTZ−J). Condensation of aldehydes and dimethyl phosphite with terpenic α‐amino oximes (Kabachnik‐Fields reaction) resulted in stereoselective formation of amino phosphonates. According to DFT calculations and spectroscopic data, the α‐amino phosphonates are conformationally inhomogeneous, exhibiting a tendency to form H‐bonded dimers. One‐bond spin‐spin coupling 1JP–C was found to be diagnostic for the configuration assignment.