Silica column chromatography general methods

Optical resolution methods with carane-3,4-diol are noteworthy for wide generality. Esters of various cyclopropane carboxylic acids with (1,S, 3A>,4A>,6A>)-carane-3,4-diol were prepared and all (lR)-isomers could easily be obtained by a simple silica gel column chromatography. 

This procedure is general for the conversion of epoxides to dichlorides with inversion of configuration at each of the two carbons and, in effect, provides a method for the cis-addition of chlorine to a double bond.2 ci.v-1,2-Dichlorocyclohcxane has also been prepared from 1,2-epoxycyclohexane and sulfuryl chloride,3 but the stereospecificity of the reaction appears to be extremely sensitive to reaction conditions, and the yield is lower than that obtained by the method described here. Other methods give cis-1,2-dichlorocyclohexane contaminated with considerable amounts of the trans -isomer. This method has been used to convert cis- and trans-4,5-epoxyoctanes to meso- and d/-4,5-dichlorooctanes, respectively, and trans-7,8-epoxyoctadecane to fhreo-7,8-dichloro-octadecane. These conversions were carried out on smaller amounts of material, and the products were purified by column chromatography on silica gel. Yields were 51-63%. 

Column chromatography is generally used for compositional analyses (ASTM D-2007 ASTM D-2549). The former method (ASTM D-2007) advocates the use of adsorption on clay and clay-silica gel followed by elution of the clay with pentane to separate saturates, elution of clay with acetone-toluene to separate polar compounds, and elution of the silica gel fraction with toluene to separate aromatic compounds. The latter method (ASTM D-2549) uses adsorption on a bauxite-silica gel column. Saturates are eluted with pentane aromatics are eluted with ether, chloroform, and ethanol. 

The most commonly used methods for isolation and purification of phospholes are crystallization, generally performed at low temperature, and column chromatography on alumina or silica gel. A wide variety of solvents have been used as eluents, including protic solvents such as methanol. 

General procedure for preparation of 2-amino-4-(phenylthio)-4//-chromene-3-carbonitrile 6 In an oven-dried screw cap test tube, a mixture of sahcyhc aldehyde 1 (1 mmol), malononitrile 2 (1 mmol), thiophenol 5 (1 mmol) and sodium formate (10 mol%) was stirred with 4 mL of ethanol at room temperature for 2-3 h. On completion of the reaction as monitored by TLC, the product was precipitated out and filtered the filtrate was preserved for reuse. The crude residue was subjected to column chromatography using silica gel (60-120 mesh) and petrol ether-ethyl acetate mixture to obtain pure product 6, characterized by analytical and conventional spectroscopic methods. 

Chromatography. A number of HPLC and TLC methods have been developed for separation and isolation of the brevetoxins. HPLC methods use both C18 reversed-phase and normal-phase silica gel columns (8, 14, 15). Gradient or iso-cratic elutions are employed and detection usually relies upon ultraviolet (UV) absorption in the 208-215-nm range. Both brevetoxin backbone structures possess a UV absorption maximum at 208 nm, corresponding to the enal moeity (16,17). In addition, the PbTx-1 backbone has an absorption shoulder at 215 nm corresponding to the 7-lactone structure. While UV detection is generally sufficient for isolation and purification, it is not sensitive (>1 ppm) enough to detect trace levels of toxins or metabolites. Excellent separations are achieved by silica gel TLC (14, 15, 18-20). Sensitivity (>1 ppm) remains a problem, but flexibility and ease of use continue to make TLC a popular technique. 

General procedure - with telluronium salt method A mixture of trimethylsilylprop-2-enyl(di-isobutyl)telluronium bromide (0.33 g, 0.75 mmol), cesium carbonate (0.25 g, 0.75 mmol), chalcone (0.5 mmol), and DME (5 mL) and water (5 mm ) was heated at 70°C for specific periods of time. When the reaction was complete (monitored by TLC), the resulting mixture was eluted with ethyl acetate through a short column of silica gel. Removal of the solvent and flash chromatography on silica gel gave the desired pure product, of purity >98% (GC). 

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