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

Three-component condensation of aldehydes and dimethyl phosphite with alpha-amino oximes derived from (-)-alpha-pinene and (+)-3-carene (Kabachnik-Fields reaction) resulted in amino phosphonates as pairs of diastereomers. Diastereomeric ratio depends on the catalyst used (SnCl(2)x2H(2)O, 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 alpha-amino phosphonates should be conformationally inhomogeneous, exhibiting a tendency to form H-bonded dimers (DFT M06/def2-SVP, IR). One-bond spin-spin coupling(1)J(P-C)was found to be diagnostic for the configuration assignment since the value(1)J(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).