stereoselektiv

Stereoselectivity, or stereoselective synthesis, refers to the property of a chemical reaction to preferentially form one stereoisomer over others. This preference can concern enantiomers (mirror-image isomers) or diastereomers (stereoisomers that are not mirror images). When a reaction yields more of one enantiomer, it is enantioselective; when it favors one diastereomer, it is diastereoselective. The outcome is typically described by enantiomeric excess (ee) or diastereomeric ratio (dr).

The origin of stereoselectivity lies in the reaction pathway and its transition state. Control can be substrate-controlled, where existing chiral centers in the starting material influence the product, or catalyst- or reagent-controlled, where a chiral catalyst or auxiliary biases the formation of a particular stereoisomer. Conditions such as temperature, solvent, and additives can also affect selectivity. Reactions may operate under kinetic control, favoring the faster-forming product, or thermodynamic control, favoring the more stable product.

Common approaches to achieve stereoselectivity include the use of chiral catalysts (often metal complexes with chiral ligands or organocatalysts) and chiral auxiliaries (temporary chiral groups attached to the substrate). Classic examples are Sharpless asymmetric epoxidation, which converts allylic alcohols into enantioenriched epoxides, and the Corey–Bakshi–Shibata reduction for enantioselective ketone reduction. Other important strategies include asymmetric hydrogenation, enantioselective aldol and Mannich reactions, and enantioselective organocatalytic methods.

Stereoselectivity is central to the synthesis of biologically active compounds, where the ratio of stereoisomers can strongly influence efficacy and safety. Its measurement relies on analytical techniques such as chiral chromatography and spectroscopic methods.