Until a few years ago, it was generally accepted that
transition metal complexes and enzymes were the two
main classes of very efficient asymmetric catalysts. Synthetic
chemists have scarcely used small organic molecules
as catalysts throughout the last century, even
though some of the very first asymmetric catalysts were
purely organic molecules. Already in 1912, Bredig reported
a modestly enantioselective alkaloid-catalyzed cyanohydrin
synthesis. In the 1960s, Pracejus showed that
organocatalysts can give significant enantioselectivities.
The 1970s brought a milestone in the area of asymmetric
organocatalysis, when two industrial groups led by Hajos
at Roche and Wiechert at Schering published the first and
highly enantioselective catalytic aldol reactions using the
simple amino acid proline as the catalyst. The cinchona
alkaloids and proline stood as the only familiar organocatalysts
for some time