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 ¹J_P–C was found to be diagnostic for the configuration assignment since the value ¹J_P–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).