Asymmetric autocatalysis – Pathway to the biological homochirality
Amplification of enantiomeric excess (ee) is a key feature for the chemical evolution of biological homochirality from the viewpoint of the origin of chirality. We describe the amplification of ee in asymmetric autocatalysis of 5-pyrimidyl alkanol in the reaction between diisopropylzinc and pyrimidine-5-carbaldehyde. During the asymmetric autocatalysis, very low ee (ca. 0.00005%) can be amplified to become more than 99.5% ee. Because the proposed origins of chirality, such as circularly polarized light, chiral inorganic quartz and statistical fluctuations of ee in racemic compounds, can initiate asymmetric autocatalysis with amplification of ee, these proposed origins of chirality can be linked with enantiopure organic compounds via asymmetric autocatalysis. Spontaneous absolute asymmetric synthesis has been achieved for the first time. In addition, we describe how the crystal chirality of cytosine generated by the dehydration of crystal water can trigger asymmetric autocatalysis. Thus, the achiral nucleobase, cytosine, is one of the candidates for the origin of chirality. Asymmetric autocatalysis can correlate the chiral isotopomers to the highly enantioenriched 5-pyrimidyl alkanol, that is, D/H-, 13C/12C- and 18O/16O-substituted chiral isotopomers can act as chiral initiators of asymmetric autocatalysis. Therefore, the tiny isotope chirality could be another explanation for the origin of chirality.
Chirality is one of the most fascinating research topics in many scientific areas. The principal chiral system in chemistry consists of chiral molecules (1). Tetrahedral molecules with four different substituents on a carbon atom are archetypal cases in which molecules have chirality. It is well known that terrestrial life is composed of highly enantioenriched compounds such as L-amino acids (Figure 1).
Thus, homochirality of biomolecules has been considered as one of the prerequisites for the origin of life. There is great interest in how and when biomolecules achieved high enantioenrichment, including the origin of chirality (2). Several mechanisms have been proposed for elucidating the origins of the chirality of organic compounds, such a