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.
An oxygen adjacent to a carbonyl can coordinate the same metal as the carbonyl oxygen, constraining conformation. Nucleophilic attack on the less hindered face of the chelate can differ from an open Felkin–Anh prediction.
The Fürst-Plattner rule describes a preference for trans-diaxial opening of suitable bridged intermediates on six-membered rings. A chair-like pathway generally avoids the unfavorable geometry of a competing twist/boat pathway; substrate fusion and substitution determine which route is accessible.
The Felkin-Anh model predicts facial preference in nucleophilic addition beside a stereogenic centre under suitable open, non-chelating conditions. Orient the large alpha substituent approximately perpendicular to C=O, then favour approach opposite it and past the small substituent. It is a predictive model rather than a universal guarantee.
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