Enantioselective conjugate addition of alkylzinc using monooxazoline-containing ligands

Abstract

We have studied the catalytic enantioselective conjugate addition of alkylzinc using monooxazoline-containing ligands to cyclic and acyclic enones. Because this reaction is one of the most widely used methods for the construction of carbon-carbon bond and synthetically useful reaction. First, P-containing chiral ligand 17 and derivatives were synthesized from L-tert-leucine 10. In the case of cyclic enones, chiral ligand 17 and 34 showed considerable results. Using 2 mol% of catalyst and substrate 7 at -30℃ ~ -40℃ for 6h reaction in toluene, 92% ee and 93% ee were obtained respectively. In the case of acyclic nitroalkenes, chiral ligand 22 showed the highest % ee. Using 2 mol% catalyst and substrate 57 under optimized condition, 91% ee was obtained. Second, O-containing chiral ligands 52, 55 and 56 were synthesized from salicylic acid 50 and 53. Using 2 mol% of catalyst and substrate 7 under optimized condition, the reaction showed very low enantioselectivity. Therefore we changed mol% of catalyst, but the enantioselectivity was not improved. Next, to prepare bisoxazoline chiral ligand 67, various synthetic conditions described were attempted but failed.