Chiral Lewis acid controlled Diels-Alder reaction: Factors affecting the stereoselective formation of both enantiomers in the bis(oxazoline)-magnesium catalyzed process.

The chiral Lewis acid controlled Diels-Alder (CLAC-DA) reaction between cyclopentadiene and 3-acryloyl-or (E)-3-crotonyl-1,3-oxazolidin-2-ones (2a,b), to give the endo adducts 3a,b has been studied. The use of magnesium perchlorate (MP) or magnesium triflate (MT), and three chiral bis(oxazolines) (5-7) and, eventually, 2 equiv of achiral auxiliary ligands X [water or tetramethylurea (TMU)], induces a strong change of the enantioface selectivity. The intermediates are either tetrahedral or octahedral. The MP-based intermediates can be either tetrahedral, giving rise to the endo (S) adducts enantioselectively, or, by addition of 2 equiv X, octahedral giving rise to the endo (R) adducts. The MT-based intermediates always have an octahedral configuration in the presence of two triflate ligands, and the endo (R) adducts are obtained enantioselectively. The controlled use of the different reagents to assemble the catalysts gives either (S)-3a in 94% ee from a tetrahedral complex, or (R)-3a in 93% ee from an octahedral one, whereas (R)-3b can be prepared with ee up to 95%. (C) 1998 Elsevier Science Ltd. All rights reserved.