Chiral designer phase-transfer catalysts for practical asymmetric synthesis*
Keiji Maruoka
Department of Chemistry, Graduate School of Sciece, Kyoto University, Kyoto 606‑8502, Japan
Abstract: A series of C2-symmetric, spiro-type chiral quaternary ammonium bromides have been designed as new, purely synthetic chiral phase-transfer catalysts, and readily prepared from commercially available optically pure (R)- or (S)-1,1'-bi-2-naphthol as a basic chiral unit. The structures of the assembled N-spiro chiral quaternary ammonium bromides were unequivocally determined by single-crystal X-ray diffraction analysis. The reactivity and selectivity of these chiral ammonium bromides as chiral phase-transfer catalysts have been evaluated in the asymmetric alkylation of N-(diphenylmethylene)glycine tert-butyl ester under mild liquid–liquid phase-transfer conditions, and the optimization of the reaction variables (solvent, base, and temperature) has also been conducted. Furthermore, the scope and limitations of other asymmetric transformations have been thoroughly investigated with a variety of substrates, in which the advantage of the unique N-spiro structure of our chiral phase-transfer catalysts and dramatic effect of the steric as well as the electronic properties of the aromatic substituents on the 3,3'-position of one binaphthyl moiety have been particularly emphasized.
Keywords: phase-transfer catalysts; asymmetric synthesis; amino acids; aldol reactions; peptides.
*An issue of reviews and research papers based on lectures presented at the 15th International Conference on Organic Synthesis (ICOS-15), held in Nagoya, Japan, 1-6 August 2004, on the theme of organic synthesis. Other presentations are published in this issue, pp. 1087-1296. This paper is based on the acceptance lecture for the Nagoya Silver Medal.