Threo-
Definition: A relative stereochemical descriptor for diastereomers with two adjacent chiral centers having substituents on opposite sides in a Fischer projection.
Context: The opposite of erythro-; used in carbohydrate and amino acid stereochemistry.
Example: Threo-2,3-butanediol has hydroxyl groups on opposite sides.
Related Terms: Erythro-; Diastereomer; Relative Configuration.
Reference: Eliel & Wilen (1994).
Topicity
Definition: Relationship of identical groups/sites: homotopic, enantiotopic, diastereotopic.
Context: Guides selectivity predictions in synthesis and NMR.
Example: Diastereotopic methylene hydrogens in chiral environments.
Related Terms: Enantiotopic, Diastereotopic.
Reference: Eliel & Wilen (1994).
Topological Chirality
Definition: Chirality arising from molecular topology (e.g., knots, catenanes) rather than stereocenters.
Context: Inspires novel drug-like architectures and materials.
Example: Molecular trefoil knots.
Related Terms: Axial Chirality, Helicity.
Reference: IUPAC Gold Book.
Twist-Boat Conformation
Definition: A less strained intermediate form of cyclohexane between the boat and chair conformations.
Context: Has lower energy than pure boat form; relevant in conformational equilibria.
Example: Cyclohexane adopts twist-boat at high energy transition states.
Related Terms: Boat Conformation; Chair Conformation; Conformational Isomer.
Reference: Eliel & Wilen (1994).