Discovery and Development of Asymmetric Autocatalysis

Abstract

Abstract Asymmetric autocatalysis occurs when a chiral product operates as an asymmetric catalyst for its own production. We have discovered an asymmetric autocatalytic reaction in the enantioselective addition of dialkylzinc to pyridine-3-carbaldehyde, where the product, 3-pyridyl alkanol, acts as an asymmetric autocatalyst. A diol, ferrocenyl alkanol, 5-pyrimidyl alkanol, and 3-quinolyl alkanol were also found to be asymmetric autocatalysts for the enantioselective isopropylation of the corresponding aldehydes. Of these compounds, 5-(2-alkynyl)pyrimidyl alkanol was the best asymmetric autocatalyst, and the asymmetric autocatalytic isopropylation of the corresponding pyrimidine-5-carbaldehyde proceeded quantitatively ( >99%), affording itself as a near enantiomerically pure ( >99.5% ee) chiral product. When an asymmetric autocatalyst with an extremely low ee was used, the asymmetric autocatalytic reaction proceeded along with amplification of the ee, and a highly enantiomerically enriched product was obtained. When the enantioselective isopropylation of pyrimidine-5-carbaldehydes was examined in the presence of chiral organic compounds with a very low ee acting as chiral initiators, then highly enantiomerically enriched 5-pyrimidyl alkanols were obtained. Their absolute configurations were dependent upon the slight excess of enantiomer in the chiral initiators. Chiral inorganic crystals, such as quartz and sodium chlorate, also act as chiral initiators. In conjunction with asymmetric autocatalysis, the chirality of organic compounds with a high enantiomeric excess has been correlated for the first time with the circularly polarized light and chiral inorganic crystals. Moreover, when an enantioselective isopropylation was examined in the absence of a chiral initiator, asymmetric induction above the detection level was observed in the product, with the ratio of the resulting predominant S- and R-isomers being almost equal. This result fulfills one of the conditions necessary for spontaneous absolute asymmetric synthesis.