Definition of l, u

1. Stereodescriptors of diastereoisomers with two chirality elements. When the two chirality elements are both R or both S the molecular entity is described as having an l relationship (l = like), when one chirality element is R and the other S the molecular entity is described as having a u relationship (u = unlike). The notation can be extended to molecules with more than two chirality elements whereby the l,u relationships of the chiral elements are considered in pairs. See V. Prelog and G. Helmchen, Angew. Chem. 94, 614-631 (1982), Angew. Chem. Internat. Ed. Eng. 21, 567-583 (19