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Solvent-free asymmetric aldol reaction organocatalyzed by (S)-proline-containing thiodipeptides under ball-milling conditions
Authors:José G Hernández  Víctor García-López  Eusebio Juaristi
Institution:Departamento de Química, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Apartado Postal 14-740, 07000 México D.F., Mexico
Abstract:An efficient, solvent-free ball-milling protocol for the asymmetric aldol reaction between cyclohexanone and cyclopentanone with various aromatic aldehydes using a novel series of (S)-proline-containing dipeptides and thiodipeptides 1af as organocatalysts is reported. In general, (S)-proline-containing thiodipeptides proved to be better organocatalysts relative to their analogous amides. In particular, thiodipeptide (S,S)-1d catalyzed the stereoselective formation of the expected aldol products with excellent diastereo- and enantioselectivity (up to 98:2 anti/syn dr and up to 96% ee). This observation may be ascribed to the increased N–H acidity of the thioamide segment that leads to stronger H-bonding interaction with the aldehyde carbonyl at the transition state and thus higher stereoinduction. Furthermore, thiodipeptide 1f proved to be an efficient organocatalyst for the aldol reaction of acetone with isatin and isatin derivatives (ee 56–86%).
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