Normal phase chromatography diastereomers

Abbreviations o-HSPh, or(/io-mercaptophenyl, Diastereomers of unknown stereochemistry are characterized by chromatographic behaviour I = faster eluting, 2 = slower eluting diastereomer in normal-phase chromatography. With / -mercaptoethanol in the assay medium. 

Faster-eluting diastereomer (normal-phase chromatography). Slower-eluting diastereomer (normal-phase chromatography)... 

Solutes that are labile (i.e., reacts with protic solvents) or exhibit poor solubility in aqueous media are prime candidates for normal-phase chromatography. Normal phase is well-suited for the separation of isomers and diastereomers, as well as for separating compounds with saturated and unsaturated side chains. Generally, the greater is the amount of unsaturation the greater the retention due to increased polarizabihty of double bond. 

Non-polar supports like polystyrene/divinylbenzene copolymers or carbon are also used as column materials. Alumina is polar and acidic while TiOi, and zirconia are much more neutral. They all have good aqueous stability compared to silica. Normal phase chromatography is restricted to the separation of stereochemical isomers, diastereomers, low molecular weight aromatic compounds and functionalized long chain aliphatic compounds. 

Lewis acid-mediated ene reaction of di- —)- / ,25)-2-phenyl-1-cyclohexyl diazenedicarboxylate with cyclohexene using tin(IV) chloride in dichloromethane at —60 °C for 5 min afforded the azo-ene adduct in 80% yield after purification by flash chromatography (eq 2). The H NMR spectrum of the azo-ene adduct recorded at 380 K in deuterated toluene established the presence of only one diastereomer. Further analysis of the ene adduct by HPLC on a Whatman Partisil 5 normal phase silica column using hexane-ethyl acetate (9 1) as eluent confirmed the presence of only one diastereomer. 

Additionally, the separation of diastereomers by reversed-phase liquid chromatography is preferred over normal-phase liquid chromatography, because the former method does not require the need to switch to chirally selective columns. The diastereomers used in reversed-phase liquid chromatography are relatively large molecules whose optimized energies vary widely. Their molecular shapes can be used to derive the differences in their contact surface areas using a model phase. 

The normal-phase (NP) mode is not often used in high-performance liquid chromatography, probably because many analysts suppose that it is too complicated or even unreliable. This is by no means true rather, it is a valuable tool for the separation of a large group of analytes. Sihca, as the prototype of an NP stationary phase, is superior for the separation of isomers ds/trans, positional isomers, and diastereomers). An example is shown in Fig. 1. The silica surface is rigid (in contrast to the flexible hydrocarbon chains of the usual reversed-phase materials) and the analytes interact in a stericaUy defined maimer with the polar groups that... 

Maleic anhydride reacts with cyclopenta-1,3-diene in a Diels-Alder reaction. Since there is a plane of symmetry, the reaction can lead to two achiral compounds, which are diastereomers of each other, containing an endo- or exo-oriented dicarboxylic anhydride group. These differ in absolute and relative configuration at the bond shared by both rings. Under normal conditions the Diels-Alder reaction proceeds stereospecifically to yield preferentially the endo product. Note that in the tricyclic product no trans fusion in the ring system is possible as a consequence of the reaction mechanism. Subsequent reduction of the products therefore affords two diols, which are also diastereomers of each other. These may be separated by chromatography on an achiral stationary phase. 

---