Synthesis of a Family of Fine-Tunable New Chiral Ligands for Catalytic Asymmetric Synthesis. Ligand Optimization through the Enantioselective Addition of Diethylzinc to Aldehydes

Abstract

A family of enantiomerically pure (1R,2R)-1-(dialkylamino)-1-phenyl-3-(R-oxy)-2-propanols (4) has been synthesized from (2S,3S)-2,3-epoxy-3-phenylpropanol (1a), arising from the Sharpless epoxydation of cinnamyl alcohol, by two alternative sequences involving either the regioselective ring opening of the epoxide by a secondary amine (C-3 attack) and subsequent chemoselective protection of the primary hydroxy group or the reverse of these operations. A total of 19 different derivatives 4 have been prepared in an iterative process aimed at the optimization of their catalytic properties in the enantioselective addition of diethylzinc to benzaldehyde. In doing this, the steric bulk of the R-oxy group and the choice of the dialkylamino substituent as a nitrogen-containing six-membered ring have been identified as the key structural parameters for high catalytic activity and enantioselectivity in 4. Two optimized ligands fulfilling these structural requirements, 4d-Tr (R-oxy = trityloxy, dialkylamino = piperidino) and 4i-Tr (R-oxy = trityloxy, dialkylamino = 4-methylpiperazin-1-yl), depict a convenient activity and selectivity profile in the addition of Et2Zn to a structurally diverse family of aldehydes. These results show how chiral ligands based on non-natural starting materials can accommodate subtle variations of the steric/electronic characteristics key to the fine tuning of catalytic properties and thus represent a convenient alternative to ligands based on natural products.