Diastereoselectivity is preferential formation or reaction of one diastereomer among competing possibilities. Unlike enantiomers, diastereomers can have different energies and physical properties even in an achiral environment.
For example, an addition reaction can favor one relative arrangement of two stereocenters, producing a diastereomeric ratio. This ratio describes product composition, while chemical yield describes how much product is obtained. Steric interactions, chelation, and conformational preferences can affect the competing transition-state energies, so the favored product depends on substrate and conditions.
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