Stereoselectivity

An example of modest stereoselectivity is the dehydrohalogenation of 2-iodobutane which yields 60% trans-2-butene and 20% cis-2-butene. Since alkene geometric isomers are also classified as diastereomers, this reaction would also be called diastereoselective. : Cram's rule predicts the major diastereomer resulting from the diastereoselective nucleophilic addition to a carbonyl group next to a chiral center. The chiral center need not be optically pure, as the relative stereochemistry will be the same for both enantiomers. In the example below the (S)-aldehyde reacts with a thiazole to form the (S,S) diastereomer but only a small amount of the (S,R) diastereomer: : The Sharpless epoxidation is an example of an enantioselective process, in which an achiral allylic alcohol substrate is transformed into an optically active epoxyalcohol. In the case of chiral allylic alcohols, kinetic resolution results. Another example is Sharpless asymmetric dihydroxylation. In the example below the achiral alkene yields only one of the possible 4 stereoisomers. : With a stereogenic center next to the carbocation the substitution can be stereoselective in inter- and intramolecular reactions. In the reaction depicted below the nucleophile (furan) can approach the carbocation formed from the least shielded side away from the bulky t-butyl group resulting in high facial diastereoselectivity: : == Stereoselective biosynthesis ==

Pinoresinol biosynthesis involved a protein called a dirigent protein. The first dirigent protein was discovered in Forsythia intermedia. This protein has been found to direct the stereoselective biosynthesis of (+)-pinoresinol from coniferyl alcohol monomers. Recently, a second, enantiocomplementary dirigent protein was identified in Arabidopsis thaliana, which directs enantioselective synthesis of (−)-pinoresinol. == See also ==