Catalytic Enantioselective Reaction

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An enantioselective rraction ffimugh a catalytic process is now recognized as one of the most important and challenging problems in organic synthesis. There have been reported a number of chiml Lewis acids modified by a variety of chiral ligands such as chiml alcohols or amities. However, such an electron-donating ligand decreases the acidity of the Lewis acid. In order to realize efficient catalytic process, we reasoned that the rate accelomting effect or attractive effect of the catalyst seemed to be important in addition to the sterwwntrol and the facile exchange of the product with the substrate on the catalyst. Based on this consideration we became interested in modifying a Lewis acid by electron-withdrawing chiral ligands rather than chiral alcohols or amities. In such a modified Lewis acid, the chiral ligand will not only provide a chiral environment but also increase the acidity of the Lewis acid. Among various acidic compounds we selected C_2-symmetric sulfonamide as a chiral ligand. We have developed two types of catalytic enantioselective reactions. (1) Alkylation of an aldehyde with dialkylzinc, Ti(O-i-Pr)_4 and disulfonamide, and (2) Cyclopropanation of an allylic alcohol using CH_2I_2, Et_2Zn and disulfonamide. Significant features of alkylation are unprecedent high catalytic turnover (up to 2, 000 times) as well as excellent enantioselectivity. The present cyclopropanation is the first example of a catalytic and enantioselective cyclopropanation. And these results will strongly demonstrate that the concept of modifying a Lewis acid with chiral sulfonmft ligand is promising in developing other types of catalytic enantioselective reactions.