Highly Enantioselective [4 + 2] Cycloaddition Reactions Catalyzed by a Chiral <i>N</i>-Methyl-oxazaborolidinium Cation

The reaction of lithium aryl borohydrides with salts of β-amino alcohols provides a new route for the synthesis of oxazaborolidines. This method also leads to the first synthesis of hitherto elusive N-methyl oxazaborolidine cations, specifically the cationic proline derivative 3. Compound 3 is a strong chiral Lewis acid which is very effective for catalysis of [4 + 2]-cycloaddition reactions in good yield and with high enantioselectivity. Several diverse examples illustrate the scope of these catalytic reactions.