Preparação e aplicação de ligantes selenoimínicos quirais em alquilações enantiosseletivas

Abstract

In this work, we describe the synthesis of a new series of chiral-seleno imine 7a-k from amino acids 1. These compounds were used as ligands in palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate. To synthesis these ligands, it was initially done the reduction of the respective commercially available aminoacids (valine, phenylalanine, leucine and isoleucine) using aluminium hydride in THF, leading to give aminoalcohols 2. These were further converted into the Boc-protected derivatives by reaction with di-tert-butyl dicarbonate in acetonitrile giving the respective N–Boc aminoalcohols 3. The chiral aziridines 4 were obtained in good yields by treatment of N-Boc aminoalcohols with tosyl chloride and potassium hidroxide in boiling THF. Finally, the selenium moieties were efficiently introduced by regioselective nucleophilic ring opening by attack of aryl selenide anion, generated from R1SeSeR1 and NaBH4 in mixture of THF and EtOH, at the less hindered carbon of the aziridines 4, furnishing the chiral amino selenide. Deprotection of the chiral selenide 5 using TFA proceeded smoothly at room temperature to give the free amino selenide. Subsequent imines 7a-k were synthesized with the corresponding aromatic aldehyde in the presence of magnesium sulfate. After filtration, and removal of solvent and the excess of aldehyde in high vacuum at elevated temperature, the compounds 7a-k were obtained as practically pure materials and single stereoisomers. The imines were unstable towards moisture as well as SiO2, thus, no further attempts at purification were undertaken....