Chapter 4. Absolute Asymmetric Synthesis in the Soai Reaction

Abstract

Absolute asymmetric synthesis in the Soai reaction is described for the reaction between pyrimidine-5-carbaldehyde and diisopropylzinc without the involvement of any chiral factor. The absolute configurations of the chiral product, i.e., pyrimidyl alkanol, exhibit stochastic distribution of (R)- or (S)-pyrimidyl alkanol with detectable enantiomeric excess (ee). The initially formed statistically enantiomerically imbalanced product, i.e., the isopropylzinc alkoxide of 5-pyrimidyl alkanol, acts as an asymmetric autocatalyst to produce more of itself with the amplification of ee. The result stands as the first clear example of absolute asymmetric synthesis with pyrimidyl alkanol that gives significant, i.e., detectable, ee values in an organic reaction. These ee values of pyrimidyl alkanol can be amplified to near enantiopurity (> 99.5% ee)by subsequent asymmetric autocatalysis. The result is counterintuitive when considering conventional views of organic reactions;that is, a chiral product is generated as a racemate when achiral reagents are reacted without the intervention of a chiral factor. The present results shed light on the origin of homochirality of biomolecules.