Eder et al. and Hajos et al. separately reported an intramolecular asymmetric aldol reaction which employed proline as the only asymmetric catalyst at the beginning of the 1970s.1) This reaction was considered to be a special case at that time. Later in 2000, List et al. reported an intermolecular asymmetric aldol reaction which used proline as a catalyst.2) List’s report received broad attention and served as a trigger to make research active for asymmetric organic catalysts. Therefore, varieties of asymmetric organic catalysts have been developed and the research has been applied to many areas.3) The proline derivative 1, having the heterocyclic ring was developed by Nakamura et al. It shows high TON as an asymmetric catalyst for obtaining a chiral aldol adduct.4)
Compared with conventional metal complex catalysts, asymmetric organic catalysts are considered to be environmentally friendly and are expected to be further developed because they are stable, easy to handle and free of harmful metals.
- 1)a) U. Eder, G. Sauer, R. Wiechert, Angew. Chem. Int. Ed. Engl. 1971, 10, 496.
- 2)B. List, R. A. Lerner, C. F. Barbas III, J. Am. Chem. Soc. 2000, 122, 2395.
- 4)S. Nakamura, N. Hara, H. Nakashima, K. Kubo, N. Shibata, T. Toru, Chem. Eur. J. 2008, 14, 8079.