Diastereoselection during allylindium addition to norbornyl α-diketones

We report a highly regio- and diastereoselective allylindium addition to norbornyl α-diketones that leads to acyloins. The diastereoselection in the case of monosubstituted derivatives greatly depends on the nature of the 5-endo substituents. Non-chelating groups direct the addition from the sterically less congested exo-face, diagonal to the substituent, while chelating substituents, such as an alkoxy or acetoxy units, induce a complete reversal in the selectivities. The presence of an oxygen atom directly linked to the norbornyl framework is crucial for eliciting a chelating effect because an acetoxymethyl (-CH2OAc) group exhibits normal behavior and acts as a non-chelating group. The dramatic influence (reversal) of an apparently innocuous exo-Me substituent is noted