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Catalytic Parallel Kinetic Resolution under Homogeneous Conditions Trisha A. Duffey, James A. MacKay,† and Edwin Vedejs* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109. †Present Address: Department of Chemistry and Biochemistry, Elizabethtown College, 1 Alpha Drive, Elizabethtown, PA 17022
[email protected] Received April 10, 2010
Two complementary chiral catalysts, the phosphine 8d and the DMAP-derived ent-23b, are used simultaneously to selectively activate a mixture of two different achiral anhydrides as acyl donors under homogeneous conditions. The resulting activated intermediates 25 and 26 react with the racemic benzylic alcohol 5 to form enantioenriched esters (R)-24 and (S)-17 by fully catalytic parallel kinetic resolution (PKR). The aroyl ester (R)-24 is obtained with near-ideal enantioselectivity for the PKR process, but (S)-17 is contaminated by ca. 8% of the minor enantiomer (R)-17 resulting from a second pathway via formation of mixed anhydride 27 and its activation by 8d.
Introduction The kinetic resolution (KR) of racemic mixtures is a powerful method for obtaining enantioenriched substances.1 Even with relatively modest enantioselectivity, s (kfast/kslow) = 20 or above, a substantial fraction of the less reactive enantiomer can be recovered with >90% ee if the reaction is run to g55% conversion. However, this approach usually results in
