A new and highly efficient catalyst for the enantioselective Mukaiyama–Michael reaction between (E)-3-crotonoyl-1,3-oxazolidin-2-one and 2-trimethylsilyloxyfuran

The Mukaiyama–Michael reaction between 2-trimethylsilyloxyfuran and (E)-3-crotonoyl-1,3-oxazolidin-2-one has been stereoselectively catalysed by several optically active complexes based on bis(oxazoline) (box) or pyridine bis(oxazoline) (pybox) chiral ligands and metal cations. The catalysts derived from the newly synthesised 2,6-bis[(4′R,5′R)-diphenyl-1,3-oxazolin-2′-yl]pyridine and the triflates of EuIII, LaIII, CeIV were highly efficient: the diastereoselectivity was entirely anti and the enantioselectivity was excellent (ranging from 98 to >99%). A mechanistic insight into the nature of the activated substrate–catalyst complex was inferred studying the lanthanum complexes with 1H and 13C NMR spectroscopy. Based on these results and on the crystallographic structure of the complex between pybox and La(OTf)3, a stereochemical model is proposed to rationalise the crucial role of the substituent in position 5, suitably placed to blind the Si-face of the coordinated reagent.