MOLECULAR SWITCHES - CALCULATIONS ON THE STEREODIVERGENT HETERO-DIELS-ALDER REACTION OF CHIRAL 1-OXA-1,3-BUTADIENES WITH DIFFERENT LEWIS-ACIDS

The stereodivergent facial differentiation of the intermolecular heter o Diels-Alder reaction of the 1-oxa-1,3-butadiene 1 with 2 in the pres ence of trimethylsilyl trifluoromethanesulfonate (TMS-OTF) or Me(2)AlC l is explained on the basis of semiempirical calculations (AM1 and PM3 ). The oxabutadiene 9 was shown to prefer the anti arrangement of the carbonyl groups of the imide moiety in the ground state by 1.8 kcal/mo l (Am1) or 2.2 kcal/mol (PM3). In the silylated compound 10 this prefe rence is even more pronounced. In contrast, in the chelate 11 the imid e moiety is fixed in a syn conformation. The orientation of the oxazol idinone rings in 10 and 11 is opposite: thus, the absolute configurati on of the cycloadducts is determined by a facial differentiation due t o an asymmetric induction under chelate control (11) or nonchelate con trol (10).