Highly enantioenriched chiral products may be obtained by using a combination of two moderately selective catalysts. Sequential enantioselective transformations comprising an asymmetric reaction followed by a kinetic resolution of the scalemic product mixture obtained in the first step are well known. In such processes, the minor, undesired enantiomer is transformed to a compound that can be more easily separated from the major enantiomer. Although chiral compounds may be obtained with high enantiopurity by such coupled processes, the yield of the desired product necessarily suffers. Recycling processes, whereby the minor enantiomer is transformed to prochiral starting material, avoid this limitation. In this Minireview, different types of sequential catalytic processes using two reinforcing catalysts are surveyed and their advantages and limitations discussed in relation to recycling processes.
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