Asymmetric Diels−Alder Reactions Catalyzed by a Triflic Acid Activated Chiral Oxazaborolidine

This paper reports a new method for the generation of chiral Lewis superacids by protonation of a non-Lewis acidic oxazaborolidine (1) with triflic acid. The resulting cationic species (3) are powerful and highly enantioselective catalysts for the Diels-Alder reaction of various 1,3-dienes with alpha,beta-enals. An optimization study (see Table 1) led to the selection of reaction conditions and catalysts (6A and 6B) which are very effective. The reactions are simple to conduct, reproducible, and economical, since only ca. 6 mol % of catalyst is required. In addition, the chiral catalyst precursor is readily recovered for reuse (>95% efficiency) and is commercially available. The broad scope of the process is documented by the 14 examples listed in Table 2. The absolute stereochemical course of the Diels-Alder reactions catalyzed by 6A and 6B was successfully predicted on the basis of the mechanistic principles which have recently been formulated for this type of catalytic enantioselective reaction involving re-face attack by the diene on complex 7. The mode of generation of Lewis superacids 6A and 6B allows an approximate comparison (or scale) connecting the catalytic power Lewis and protic acids.