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

Abstract

The chiral Lewis acid controlled Die̊ls-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)- 3 a in 94% ee from a tetrahedral complex, or (R)- 3 a in 93% ee from an octahedral one, whereas (R)- 3 b can be prepared with ee up to 95%.