[化学]chapter 3-2003.ppt

Example (CSR) Water soluble 3.2 Determination of enantiomer diastereomer composition Advantages vs.Disadvantages CDA: Most widely used, much higher Δδ. excluding racemization or kinetic resolution; avoiding potential selective enrichment of one of the diastereomers when purification. CSA: Quick and relatively simple to perform, no problem associated with kinetic resolution, not necessary be enantiomerically pure. Low solubility of salt in less polar solvents (usually required). CSR: Besides those mentioned above in CSA, yielding info on the constitution, configuration and conformation of molecules. syntheses complicated, very expensive. 3.2 Determination of enantiomer diastereomer composition A stereogenic center which has two identical enantiomorphic ligands plus two other non-identical ligands, but which actually lies on a plane of symmetry, is termed a pseudoasymmetric center. Stereodescriptor: r, s 3.3 Pseudoasymmetric centers ! An R stereogenic center has priority over an S one (R before S in the alphabet). 3.3 Pseudoasymmetric centers e.g Prochirality An achiral compound which becomes chiral upon either of the following: 1) addition to one face of a double bond; 2) replacement or modi?cation of one of two identical ligands. Prochiral molecules contain either ligands whose separate replacement yields enantiomers or a double bond where separate addition to the two faces yields enantiomers. The descriptors pro-R and pro-S distinguish ligands, while Re and Si distinguish faces. 3.4 Prochiral centers Necessity for designation of prochiral center Cabc2 A group Cabc2, where a, b and c are different substituents, can be made chiral by a single desymmetrization step. The two groups of c are enantiotopic. 3.4 Prochiral centers The example of introducing further chirality: 3.4 Prochiral centers Pro-R, Pro-S The two groups c in Cabc2, are distinguished as pro-R and pro-S: this is done by notionally assignin