stereoselektive

Stereoselektive, in chemistry, describes a process in which a chemical reaction preferentially forms one stereoisomer over others. This term is used to indicate control over the three-dimensional arrangement of atoms in the product, which is crucial for the properties and activities of many compounds, especially in pharmaceuticals and natural products. Stereoselectivity can be contrasted with stereospecificity, where the reaction's outcome is determined by the stereochemistry of the starting material.

Stereoisomers include enantiomers, which are non-superimposable mirror images, and diastereomers, which are not mirror images. Enantioselectivity refers to preferential formation of one enantiomer, while diastereoselectivity refers to preferential formation of one diastereomer. The degree of selectivity is often quantified by enantiomeric excess (ee) or diastereomeric ratio (dr).

Achieving stereoselectivity commonly involves chiral catalysts or chiral auxiliaries that create a biased environment for bond formation. Approaches include catalytic asymmetric synthesis (using enzymes, organocatalysts, or transition-metal catalysts), substrate-controlled methods, and adjustments to reaction conditions such as temperature or solvent. Well-known examples include enantioselective hydrogenation, Sharpless asymmetric epoxidation, Jacobsen’s epoxidation, and the Corey–Bakshi–Shibata reduction.

Measurement and impact: ee and dr values indicate how selectively a reaction produces a single stereoisomer. High stereoselectivity reduces the need for resolution steps and is especially important when stereochemistry governs biological activity or material properties. Research in stereoselective methods aims to broaden scope, improve efficiency, and enable access to enantiomerically pure compounds.