in the latter half of the twentieth century, chemists began to develop methods of asymmetric catalysis and kinetic resolution. these reactions could be directed to produce only one enantiomer rather than a racemic mixture. early examples include sharpless epoxidation (k. barry sharpless) and asymmetric 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 an organic synthesis. previously only natural enantiomer starting materials could be used. using techniques pioneered by robert burns woodward and other new synthetic methods, chemists became more able to make complex molecules without unwanted racemisation. this is called 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 asymmetric synthesis.