ASYMMETRIC AUTOCATALYSIS AND AMPLIFICATION OF ENANTIOMERIC EXCESS OF A CHIRAL MOLECULE

THE homochirality of natural amino acids and sugars remains a puzzle for theories of the chemical origin of life(1-18). In 1953 Frank(7) proposed a reaction scheme by which a combination of autocatalysis and inhibition in a system of replicating chiral molecules can allow small random fluctuations in an initially racemic mixture to tip the balance to yield almost exclusively one enantiomer. Here we show experimentally that autocatalysis in a chemical reaction can indeed enhance a small initial enantiomeric excess of a chiral molecule. When a 5-pyrimidyl alkanol with a small (2%) enantiomeric excess is treated with diisopropylzinc and pyrimidine-5-carboxaldehyde, it undergoes an autocatalytic reaction to generate more of the alkanol. Because the reaction involves a chiral catalyst generated from the initial alkanol, and because the catalytic step is enantioselective, the enantiomeric excess of the product is enhanced, This process provides a mechanism by which a small initial imbalance in chirality can become overwhelming.