Droplet Countercurrent Chromatography

The technique of countercurrent chromatography (CCC) has seen a rapid expansion following the introduction of new methods such as droplet countercurrent chromatogra-phy (DCCC), rotation locular countercurrent chromatog-raphy (RLCC), and coil planet centrifugation. These methods have the advantage of being more rapid and less solvent consuming than traditional CCC. Furthermore, the advent of commercially available, compact apparatus has led to a widespread acceptance of these new liquid-liquid techniques as standard laboratory procedures for the separation of natural products. The detection of compounds that are eluted from a DCCC can be performed by three methods: (1) UV detection for suitable UV-active substances, (2) monitoring of the fractions that are collected by TLC, and (3) weighing of fractions after evaporation of solvent. The majority of DCCC separations involve polar compounds, especially glycosides, which are often difficult to purify. Chloroform-methanol-water systems of varying compositions remain the most widely used, in view of the good formation of droplets and the convenient viscosity of this combination. The most notable developments in the application of DCCC have occurred in the field of polyphenols, in particular in the separation of tannins. DCCC has also been applied in the separation of natural products such as alkaloids, triterpene glycosides, steroid glycosides, basic steroid saponins, and glycosides of flavonoids. Rotation locular countercurrent chromatog-raphy (RLCC) relies on the percolation of one layer of a two-phase solvent system through compartments (loculi) that contain the second layer. During passage of the mobile phase, the loculi (connected into tubes) are constantly rotated, to increase contact between the two phases. Basically, RLCC has the same advantages as DCCC. As in DCCC, the apparatus can be run in either ascending or descending solvent modes but the formation of droplets is not a necessary condition of RLCC. Consequently, a broader range of solvent system is possible, and a system containing ethyl acetate (often incompatible with DCCC) has been used, for example, in the separation of flavonoids— an important application of this method has been the separation of enantiomers of (±)-norephedrine on an instrument consisting of 16 columns and each column containing 37 loculi. The stationary phase was sodium hex-afluorophosphate solution at pH 4, and the mobile phase was(R, R)-di-nor-5-yl tartrate in 1,2-dichloroethane. Presumably, the enantiomers of (t)-norephedrine form different diastereotopic complexes with the tartrate ester, and these complexes are then partitioned differently between the two solvent phases. Separations by RLCC of a range of natural products, including flavones, xanthone glycosides, and antitumor antibiotics have been reported. RLCC provides a useful complementary method to DCCC in instances in which suitable solvent systems are not available.

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