CHIRAL LEWIS-ACID CONTROLLED DIELS-ALDER REACTION - FACTORS AFFECTINGTHE STEREOSELECTIVE FORMATION OF BOTH ENANTIOMERS IN THE BIS(OXAZOLINE)-MAGNESIUM CATALYZED PROCESS

The chiral Lewis acid controlled Diels-Alder (CLAC-DA) reaction betwee n cyclopentadiene and 3-acryloyl-or (E)-3-crotonyl-1,3-oxazolidin-2-on es (2a,b), to give the endo adducts 3a,b has been studied. The use of magnesium perchlorate (MP) or magnesium triflate (MT), and three chira l 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 additio n of 2 equiv X, octahedral giving rise to the endo (R) adducts. The MT -based intermediates always have an octahedral configuration in the pr esence of two triflate ligands, and the endo (R) adducts are obtained enantioselectively. The controlled use of the different reagents to as semble 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 ca n be prepared with ee up to 95%. (C) 1998 Elsevier Science Ltd. All ri ghts reserved.