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 hetero Diels‐Alder reaction of the 1‐oxa‐1,3‐butadiene 1 with 2 in the presence of trimethylsilyl trifluoromethanesulfonate (TMS‐OTf) or Me 2 AlCl 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/mol (Am1) or 2.2 kcal/mol (PM3). In the silylated compound 10 this preference is even more pronounced. In contrast, in the chelate 11 the imide moiety is fixed in a syn conformation. The orientation of the oxazolidinone rings in 10 and 11 is opposite: thus, the absolute configuration of the cycloadducts is determined by a facial differentiation due to an asymmetric induction under chelate control ( 11 ) or nonchelate control ( 10 ).