Organic Synthesis - Stereoselective Synthesis

Stereoselective Synthesis

Most complex natural products are chiral, and the bioactivity of chiral molecules varies with the enantiomer. Traditional total syntheses targeted racemic mixtures, i.e., as an equal mixture of both possible enantiomers. The racemic mixture might then be separated via chiral resolution.

In the latter half of the twentieth century, chemists began to develop methods of stereoselective catalysis and kinetic resolution whereby reactions could be directed to produce only one enantiomer rather than a racemic mixture. Early examples include Sharpless epoxidation (K. Barry Sharpless) and stereoselective hydrogenation (William S. Knowles and Ryōji Noyori). For their achievement, these workers went on to share the Nobel Prize in Chemistry in 2001. Such reactions gave chemists a much wider choice of enantiomerically pure molecules to start from, where previously only natural starting materials could be used. Using techniques pioneered by Robert B. Woodward and new developments in synthetic methodology, chemists became more able to take simple molecules through to more complex molecules without unwanted racemisation, by understanding stereocontrol. This allowed the final target molecule to be synthesised as one pure enantiomer without any resolution being necessary. Such techniques are referred to as stereoselective synthesis.