Enantioselective addition of cyclic enol silyl ethers to 2-alkenoyl-pyridine-N-oxides catalysed by Cu(II)-bis(oxazoline) complexes.

Asymmetric reactions involving (E)-3-aryl-1-(pyridin-2-yl-N-oxide)prop-2-en-1-ones and cyclic enol silyl ethers show good yields and excellent enantioselectivities (up to 99.9 % ee) when catalysed by bis(oxazoline)-Cu(II) complexes. Different reaction pathways can be followed by different enol silyl ethers: with 2-(trimethylsilyloxy)furan, a Mukaiyama-Michael adduct is obtained, whereas a hetero Diels-Alder cycloadduct was formed by using (1,2-dihydronaphthalen-4-yloxy)trimethylsilane. In the latter reaction, the absolute configuration of the product is consistent with a reagent approach to the less hindered Re face of the coordinated substrate in the reactive complex.

Enantioselective cycloadditions of 2-alkenoylpyridine-N-oxides catalysed by a bis(oxazoline)/Cu(II) complex: structure of the reactive intermediate.

Diels-Alder and 1,3-dipolar cycloadditions involving (E)-3-aryl-1-(pyridin-2-yl-N-oxide)prop-2-en-1-ones as the 2π components are efficiently catalysed by bis(oxazoline)-Cu(II) complexes. The cycloadducts are obtained in quantitative yields with up to 98 % ee; absolute configurations were determined by X-ray analysis. The structure of the reactive complex, determined by X-ray analysis, is fully consistent with the stereochemical outcome of the catalysed process (approach of the diene or nitrone to the less hindered face of the coordinated pyridine-N-oxide derivative).