Mixing fraction

Elution with a petroleum ether-benzene (1 1) mixture (2.2 liters) affords 5.8 g of starting material. Further elution with petroleum ether-benzene (1 1) (400 ml) gives initially 0.57 g of a mixed fraction, followed by 0.47 g of a crude product (eluted with another 400 ml of the same solvent mixture). Crystallization of the latter fraction from methylene dichloride-petroleum ether gives (20 R)-18,20-cyclo-5a-pregnane-3/5,20-diol 3-acetate mp 164-166.5° [ ]q 9°. Elution with benzene (400 ml) gives 1.57 g of an oil. Continued elution with benzene (600 ml) and benzene-ether (9 1) (800 ml) provides 1.81 g of crude (20 S)-18,20-cyclo-5a-pregnane-3j9,20-diol 3-acetate mp 139-140° [a]o 15°. 

The electronic spectrum of the fractions containing the pure tridehydro [18]annulene exhibits the strongest absorption maximum (in benzene) at 342 nm. (e 155,000) and the spectroscopic yield, based on the molar extinction coefficient, is 1.17 g. (2.40% from 1,5-hexadiyne). The yield of tridehydro[18]annulene in the mixed fractions, based on the 342 nm. maximum,is 0.27g.(0.55%). The tridehydro [18]annulene is best stored in solution in the refrigerator. 

Figure 19.7. Cyclic batch elution chromatography obtaining high product purity and high throughput by using incomplete resolution (overlapping bands) and recycling the mixed fraction (mf) to the feedstock (a) Control of band separation and cut points determines fractional impurities t mij mr and Ami /mr2-<4l)> (b) Chromatogram for separation of pure ds- and trans- 1,3-pentadiene. Components 1, isoprene 2, trans- 1,3-pentadiene 3, cis-l,3-pentadiene 4, cyclo-pentadiene. Component 1 is eluted at almost the same time as component 4 of the...

The checkers attempted chromatography of the three-component mixture on a 150-mmol scale using 55- and 75-mm diameter columns. However, mixed fractions were obtained even with seemingly large differences for the components. 

In one run (5.0-mmol scale), the checkers obtained a crude product that contained ca. 40% of recovered 1. Attempts to purify the aldol product from this mixture by using the described crystallization procedure was unsuccessful. Accordingly, the crude product (2.36 g) was purified by flash chromatography on 170 g of silica gel using 1000 ml of a 9 1 1 mixture of hexane, ethyl acetate and methylene chloride. This provided 0.72 g of starting ester 1, 1.39 g of pure aldol 2, and 0.259 g of mixed fractions containing 2 and the minor aldol diastereoisomer (4 1 by H NMR analysis). 

The maximum production rate, however, often results in nnacceptable recovery yields. Low recovery yield requires further processing by recycling the mixed fractions. The recovery yield at the maximum production rate strongly depends on the separation factor. In the cases of difficnlt separations, when the separation factor under linear conditions is aronnd or lower than a= 1.1, the recovery yield is not higher than 40%-60%. Even in the case of a=1.8, the recovery yield at the maximum production rate is only about 70%-80%. The situation is still less favorable in displacement chromatography, particularly if the component to be purified is more retained than the limiting impurity. In this case, from one side the impurity, whereas from the other side the displacer, contaminates the product. 

Westphal, A. J., Fakra, S. C., Gainsforth, Z. et al. (2009) Mixing fraction of inner solar system material in comet 81P/Wild2. Astrophysical Journal, 694, 18-28. 

The overall process, illustrated in Scheme 10.4, intercepts the racemate (2) by crystallization from heptane. After separation of the enantiomers using the MCC process, the radafaxine free base is converted to the desired salt directly on treatment with anhydrous HCl. Any mixed fractions from the MCC separation are combined with the epimerized R,R)-enantiomer and fresh racemate for processing, hence generating further radafaxine free base for conversion to the hydrochloride salt. These results were subsequently confirmed in a Proof of Concept study performed on the medium- to large-scale in-house MCC equipment prior to scale-up. 

One way to achieve this is to replace the column by a loop of three to six smaller columns, as shown in Figure 12.10c. This is the principle of multi-column continuous chromatography (MCC). Since only pure fractions are collected, leaving mixed fractions to re-circulate through the columns, there is no need to achieve a complete separahon. Inlet (eluent, feed) and outlet (extract-most retained component, raffinate-least retained component) streams are moved periodically by one column according to the direction of the liquid flow and following the concentration profile inside the column. 

TLC was run on 10 x 20-mm silica plates (E. Merck) TLC solvent was 4 1 hexanes ethyl acetate visualization was with 5% (NH MoO. in 10% aqueous sulfuric acid, with heat. In the event that any mixed fractions are obtained, these are combined, evaporated, and the residue is rechromatographed in the same manner. 

Mix fractions to be tested with 10 pL sample buffer and boil for 10 min. Then load sample on the stacking gel and separate electrophoretically in the presence of 8 M urea. 

We are separating peaks by hand when bioassays have to be performed. It is our experience that it is easier to combine fractions than to rechromatograph wrongly mixed fractions. 

The reaction mixture is poured into a 2000-mL separatory funnel containing water (300 mL) and ether (300 mL). After thorough mixing, the aqueous layer is separated and extracted with ether (2 x 200 mL). The combined ethereal solutions are washed with water (300 mL) and brine (300 mL), dried over magnesium sulfate, and evaporated under reduced pressure to leave 8.5 g of a pale yellow oil. This material is subjected to flash chromatography on silica gel using 20% ethyl acetate in hexanes as the mobile phase (Note 9). The minor diquinane (230 mg, Note 10) is eluted first. A mixed fraction of the two isomers follows (570 mg, Note 11) in advance of the pure title compound (6.36 g, Notes 12, 13). 

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