Enantioselective formation of tert-alkylamines by desymmetrization of 2-substituted serinols

Novel enantioselective desymmetrization of 2-substituted 2-amino-1,3-propanediols has been established to generate asymmetric quaternary carbon centers comprising an amino group. Enantioselective as well as chemical conversion proved to be greatly dependent on the protecting group of the amino group in the substrate, desymmetrizing reagent, base, solvent, and naturally, catalyst. The highly effective desymmetrization has been implemented by using N-benzoylated substrates with benzoyl Chloride and triethylamine in the presence of tetraphenylbisoxazoline (24)-CuCl₂ complex in THF at ambient temperature. An extensive survey of catalysts revealed that dimethylmalonate- bridged bisoxazoline-CuCl₂ complexes were superior. Among them, the tetraphenylbisoxazoline (24)-CuCl₂ complex turned out to work most efficiently with a wide array of the substrates. All the examined substrates, with the exception of 2-phenylserinol 36, were desymmetrized in the presence of 24-CuCl₂ complex to give high enantioselectivities ranging from 85 to 95% ee. Complementary use of the diisopropylbisoxazoline (22)-CuCl₂ complex has remedied the mediocre desymmetrization of 36 to give a significantly improved enantioselectivity from 63 to 83% ee.