Double column chromatography

There was some concern that the solvent peak might shorten column life. Double column chromatography with trapping overcomes this problem and further opens up the possibility of selectively looking at portions of the sample on a capillary column by means of heart-cutting. These techniques show promise for the future although present systems appear somewhat complex for the average user. 

Fig. 3.2. System for double-column chromatography with intermediate trapping and re-injection, suitable also for direct injection of aqueous solutions. 1, carrier gas 2, pressure regulator 3, flow controller 4, vent for back-flushing S, injection port for heart-cut and back-flushing 6, precolumn (packed) 7, injection port for aqueous solutions 8, control flame ionization detector for pre-separation 9, vent for cutting 10. leak for make-up gas 11, trap 12, outlet of splitter 13, glass capillary column 14, flame ionization detector for main separation. Reproduced from [35].

In the synthesis of the squalenoid glabrescol (72 originally attributed structure), containing five adjacent (all cis) THF rings, the necessary precursor of the polyepoxide cascade, the pentaepoxide 71, was achieved by epoxidation of each of the trisubstituted double bonds of the known (R)-2,3-dihydroxy-2,3-dihydrosqualene (70) by the Shi epoxidation approach (Scheme 8.18) [34]. Treatment of 71 with CSA at 0 °C and subsequent purification by column chromatography provided the pure polycyclic ether 72 by a cascade process reasonably initiated by the free secondary alcohol functionality [35a]. 

The lluorous biphasic technique has also been applied to the Wittig reaction, one of the most effective methodologies for the production of C=C double bonds [13] (Scheme 10.11). One of the major drawbacks of this reaction is the separation of the alkene from the phosphine oxide by-product. This is commonly achieved via recrystallization or column chromatography, but recently it has been shown... 

Similarly, the bromination of 2-bromobenzobarrelene 377 gives a mixture of tribromo products which were separated by column chromatography (equation 135)189. The major product 378 was isolated in 58% yield (whereas the combined yield of the rest of products was 37%). By using XH and 13C NMR it was shown that bromine was added to the unsubstituted double bond only. 

The DCA-sensitized irradiation of 107a for 13 hr affords, after column chromatography on silica gel, the rrans -cyclopropane derivative 108a (10%) as a 1 1 mixture of C=N bond fiZ-isomers. Similarly, irradiation of the oxime acetate 107b under these conditions for 2.5 hr affords, after chromatography, the rrans -cyclopropane derivative 108b (12%). These results show that the novel 1-ADPM rearrangement promoted by electron-transfer sensitization can be extended to other C—double-bond derivatives. 

Alai/r[(Z)-CF=C]-Pro containing N, 0-diacylhydroxamic acid type protease inhibitors have been prepared as shown in Scheme 18 [63,64], The synthesis is based upon the use of fert-butyl-a-fluoro-trimethylsilylacetate in a variation of the Peterson olefination procedure to construct the necessary functionalized fluoroolefin. Treatment of 51 with 4 equiv. of lithium diisopropylamide (LDA) and 6equiv. of chlorotri-methylsilane at 78°C formed 52 in 71% yield. The key step is the Peterson olefination reaction of the TBDMS-protected 2-(hydroxymethyl)cyclopentanone (53) with tert-butyl-a-fluoro-a-trimethylsilylacetate (52). The fluoroolefin product was obtained as a mixture of (Z) (E) isomers (54). Separation of the double-bond isomers by column chromatography provided (Z) isomer (54) in 43% yield. Further... 

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%. 

Selectivity is more important than efficiency (AO in determining resolution because Rs is directly proportional to selectivity, but is proportional only to the square root of efficiency (see Fig. B4.2.5 along with Equation B4.2.1 and Equation B4.2.2. Hence, a four-fold increase in efficiency is required to double resolution, as compared with a two-fold increase in selectivity. In practice, selectivity depends partly on the chromatographic technique employed but can usually be controlled by manipulating experimental conditions, such as the pH and ionic strength of the mobile phase. Because this can be done easily and predictably, selectivity is the factor that is exploited to achieve maximum resolution in column chromatography rather than efficiency, which is fixed by the particle size and uniformity of the medium selected. 

A palladium-cataly/ed 1,6-enyne cycli/ation of the Tmst20 type provides the desired bicyelic system as a mixture of hexahydro-5//-1-pyrindine 14-Z and its double-bond isomer 15-Z in a ratio of 97 3. The two can be separated by column chromatography In the process, the exocyclic alkylidene sidechain is created with Z selectivity, and the rings are stereo specifically joined cis. It should be noted that only with the use of BBEDA [A V -bis(benzylidene)ethylene diamine] as a powerful a-donor ligand is high Z selectivity achieved Other catalytic systems, such as Pd(OAc) or Pd(OAc) ifPPhOVHOAc produce EfZ mixtures of 14 and 15 in ratios of 45 25 27 3 or 9 69 6 16 (14-Z 15-Z 14-E 15-E). 

The gaseous products were analyzed by gas chromatography with a double column of Poropak-Q and... 

Figure 5. Comparison of Sephadex LH-20 column chromatography of DMBA-deoxyribonucleoside adducts formed by enzymatic digestion of calf thymus DNA that had been treated for 2 h in the presence of Aroclor-induced mouse liver microsomes with (a) 2 or (b) 353 nmol [ HJ-DMBA per mg microsomal protein. The single-headed arrow is as defined in Figure 2. The double arrow denotes the position of elution of added DMBA K-region epoxide- deoxyribonucleoside uv-absorbing markers.

In this process, epoxidation of the double bonds was followed by reduction to obtain the tert-alcohol which was esterified with methacryloyl chloride in the subsequent step. While epoxidation was found to be close to quantitative based on double bond content, reduction was incomplete and the residual epoxy functional PIB (24-47%) had to be separated by column chromatography before esterification. It should be noted that this macromonomer was a tert-ester which might be quite unstable in acidic conditions, and is also more hindered than the... 

High conversion of double bonds was found only with Glissopal, but it was necessary, even in this case, to separate from non-functional PIB before esterification. While the feasibility to synthesize methacrylate functional PIBs by all three methods was demonstrated, due to the necessary column chromatography step to obtain high functionality PIB macromonomers, the utility of the method is questionable. 

While you are waiting for the paper chromatogram to develop (step no. 7), you can perform the column chromatography experiment. Take a 25-mL buret. (You may use a chromatographic column, if available, of 1.6 cm diameter and about 13 cm long see Fig. 29.8. If you use the column instead of the buret, all subsequent quantities below should be doubled.)... 

In a dry nitrogen flushed 100 ml flask sealed with a septum were placed the phenol (5 mmol) and dry methanol (10 ml). A solution of DIB (1.61 g, 5 mmol) in methanol (25 ml) was transferred via a double-ended needle to the stirred phenolic solution at room temperature over 40 min. After 40 min, the reaction mixture was concentrated and the residue was purified by column chromatography (silica gel, light petroleum to dichloromethane) to afford the title compounds in 65-99% yield. 

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