Synthesis of novel chiral bisoxazoline ligands with a spiro[4,4]-1,6-nonadiene skeleton

Abstract

A new type of chiral bisoxazoline ligands 1 based on spiro[4,4]-1,6-nonadiene backbone was easily prepared in six steps from racemic spiro[4,4]-nonane-1,6-dione, with the Pd-catalyzed coupling of the enol triflates with CO and amino alcohols as the key steps for the construction of the oxazoline moiety. The structure of the ligand (R,S,S)-1b was unambiguously established by X-ray crystallographic analysis. The chiral Cu(II) complex generated in situ from the combination of spiro bisoxazoline ligand (S,S,S)-1c and Cu(OTf)2 was effective in the catalysis of asymmetric chlorination of the β-ketoester, methyl 1-oxo-2,3-dihydro-1H- indene-2-carboxylate, affording the corresponding chlorinated derivative in 99% yield with 17% ee.