Abstract
A reliable procedure for the preparation of the chiral Lewis acid catalysts, (−)-menthoxy- ((−)-1) and (−)-bornyloxy-aluminum dichloride ((−)-2), has been established for the study of diene additions. The geometry of conformationally mobile α,β-unsaturated carbonyl compounds complexed with these catalysts has been probed using the α-methylene ketones 3 and 4 and 2,6-dimethyl-1,4-benzoquinone 5 as dienophiles of fixed geometry. The addition of the first two to cyclopentadiene and (−)-1 yields exo and endo adducts that exhibit a range of 28–54% enantiomeric excess (ee); with 5, only the endo isomer, showing 8% ee, is formed. In the corresponding additions catalyzed by (−)-2, adducts displaying ee in the range of 3–18% are formed.
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