Synthesis of chiral naphthyl beta-aminoalcohol ligands for catalytic asymmetric reactions

Abstract

N-Salicyl-l-naphthyl and N-Salicyl-2-naphthyl- β-aminoalcohol-based chiral ligands were synthesized. α –Bromination of 1- and 2-acetonaphthone both afforded the corresponding α-bromoacetonaphthones in 98%. Subsequent asymmetric reduction by (-)-β-chlorodiisopinocampheylborane (DIP-chloride), followed by a reaction with sodium hydroxide solution resulted in optically active ®-1-naphthyl and ®-2-naphthyl oxiranes in 43% and 67% yields, respectively. The results showed that ®-1-naphthyl and ®-2-naphthyloxirane were obtained in higher than 99 %ee. The following nucleophilic ring-opening of oxirance by sodium azide yielded benzylic-azidoalcohols in 65% and 65% yields for (S)-1naphthyl and (S)-2-naphthyl azidoalcohols. The %ee values were more than 99 %. The next step is a reduction to chiral naphthylaminoalcohol, followed by a reaction with salicylaldehyde and a reduction. Through this sequence, chiral-N-salicyl-l –naphthyl and 2-naphthyl- β-aminoalcohols were obtained in 57% and 70%, respectively. These products had been evaluated as potential chiral ligands for catalytic asymmetric Strecker reaction, Michael reaction, and Pudovik reaction. For the Strecker and Michael reaction, chiral-N-salicyl-l-naphthyl- β-aminoalcohols could induce products up to 97% and 91%ee, respectively. Moreover, this ligand has been shown to be more efficient than 2-naphthyl- β-aminoalcohols. Asymmetric models of these reactions were proposed. Unfortunately, they could not induce to any enantioselections in Pudovik reaction.