Enantio-face control by molecular sieves in the asymmetric Diels–Alder reaction

The presence of molecular sieves 4Å (MS 4Å) significantly influenced enantioselectivity in the Diels–Alder reaction of 2-methoxy-5-methyl-1,4-benzoquinone with 1-acetoxy-1,3-butadiene promoted by chiral TADDOL–Ti complexes. In the absence of MS 4Å the (R)-adduct was formed in 72% ee, and in the presence of MS 4Å the (S)-enantiomer in up to 53% ee. Capture of HCl by MS 4Å has been demonstrated to play a critical role.