Medium pressure chromatography

Synthetic chiral adsorbents are usually prepared by tethering a chiral molecule to a silica surface. The attachment to the silica is through alkylsiloxy bonds. A study which demonstrates the technique reports the resolution of a number of aromatic compoimds on a 1- to 8-g scale. The adsorbent is a silica that has been derivatized with a chiral reagent. Specifically, hydroxyl groups on the silica surface are covalently boimd to a derivative of f -phenylglycine. A medium-pressure chromatography apparatus is used. The racemic mixture is passed through the column, and, when resolution is successful, the separated enantiomers are isolated as completely resolved fiactions. Scheme 2.5 shows some other examples of chiral stationary phases. 

Haliclonacyclamine E (13) and arenosclerins A (14), B (15), and C (16) have been isolated from the marine sponge Arenosclera brasiliensis, endemic in Brazil. Crude extracts of this sponge displayed potent cytotoxic and antibiotic activities, and were subjected to fractionation by sihca-gel flash chromatography, medium pressure chromatography on a SiOH cyanopropyl-bonded column, and reversed-phase Cis column chromatography to give compounds 13-16 [18]. The structure elucidation was based on spectroscopic analysis, including HRFABMS, COSY, HSQC, HSQC-TOCSY, and HMBC NMR... 

A similar condensation of (+)-norephedrine (121) gave the corresponding 2-methyl-1,3,2 oxazaphospholidine-2-thiones 122a,b which were separated by rapid, medium pressure chromatography (Scheme 35) [67],... 

Another approach is based on the copolymerisation of a mixture of two acrylic monomers. One is of the anionic type (or cationic) and the other one is poly-hydroxylated (Fig. 4.3). The latter is used to ensure the hydrophilic character necessary for the stationary phase. A limitation of these resins is their variable swelling, which depends on the composition of the mobile phase. They are normally used for medium pressure chromatography and certain biochemical applications. 

Starting from (R)-pulegone we present herein an efficient three-step synthesis furnishing (-)-phenylmenthol as an easily separable 87 13-mixture of diastereomers in 55-80% overall isolated yield. Separation of the two dlastereomers 1s achieved either by careful medium-pressure chromatography as Corey and Ensley stated, a,b or, less tediously for greater quantities, by fractional crystallization of the chloroacetic acid esters and successive saponification as described herein. 

Sample preparation. For the study of trace compounds we used two different sample preparation procedures SPI and SPII for wine no. 1 and wine no. 2, respectively. The first flavor extract SP I was obtained by liquid-liquid extraction with fluorochloromethane and dichloromethane (9+1) from 45 L Scheurebe wine. For further analysis a portion of 1/3 was used. After separation on silica gel (pentane/diethyl ether) 6 fractions were analyzed. For the second flavor extract SP II we started from 200 L wine stripping off volatile compounds with vapour in a spinning cone column (SCC) system (5). The condensate was sequentially collected in two main portions of 8 and 2 L, respectively. The first condensate was discarded. The second condensate (2 L) was subjected to liquid-liquid extration with fluorochloromethane and dichloromethane (9+1). After separation on silica gel (pentane/diethyl ether) using medium pressure chromatography (MPLC) 4 fractions were analyzed. 

A solution of l-but-3-enyl-2,3-dihydro-li/-inden-l-ol (2.0 g (10.6 mmol) in rert-butyl methyl ether (350 mL) was irradiated in a quartz vessel with a 500-W high-pressure Hg lamp for 2.5 h. After workup, the yellow oil was separated by medium-pressure chromatography (hexane//err-butyl methyl ether 1 1) to give 0.45 g (23%) of 4 and 0.85 g (43%) of 5. 

A solution of Pd(PPhj)4 (767 mg, 0.665 mmol), Af-phenyldiphenylketenimine (2.93 g, 10.9 mmol) and (l-methylethylidene)cyclopropane (2.03 g, 35.6 mmol) in anhyd toluene (30 mL) was placed in a 200-mL stainless steel autoclave. The autoclave was pressurized with 4 MPa of Nj at rt and then heated to 123 °C for 30 h with stirring. After the reaction was complete (most easily monitored by the disappearance of the ketenimine stretching vibration in the IR spectrum), the mixture was filtered through a pad of alumina (EtjO) in order to remove the solid elemental palladium. After removal of the readily volatile components (bp up to 30 C/10" Torr), the brown, vi.scous residue was purified by medium-pressure chromatography (silica gel, CHjClj/cyclohexane 1 1). Under these conditions, most of the impurities were washed off, while the cycloadduct remained on top of the column and was finally eluted with pure Et20 yield 3.3 g (93%) brown needles 98% pure (as determined by GC) mp 148°C (toluene). 

The appropriate vinylcyclopropane was evaporated through a horizontally situated hot tube (Vycor or Pyrex) at a specified temperature. The glass had been conditioned prior to use with a slurry of PbCO, in HjO. The actual distillation of material through the columns took 2-5 min. The pyrolysate was condensed under vacuum (0.05-0.10 Torr) in a liquid Nj cooled trap. The crude mixture was then purified by medium-pressure chromatography (silica gel, hexane/EtjO). 

In addition to traditional flash column chromatography, there are now commercially available low and medium pressure chromatography systems. Manufacturers... 

Compound 4 was separated from the reaction mixture by medium pressure chromatography on silica, using Et20/light ether (1 3) as... 

From a practical point of view, the availability of all 16 dinucleotide blocks made possible a rapid buildup of any deoxyribonucleotide sequence. The basic strategy of a synthetic plan is to extend the chain from the 3 -end towards the 5 -end by block condensation in the presence of arylsulfonyl tetrazole as a coupling reagent (Figure 8). The reaction is generally over within 30 minutes after workup, a pure, fully protected product was isolated by short-coluttm medium-pressure chromatography on RP-2 absorbent or by a simple but very effective chromatography on deactivated silica gel with aqueous solvent systems such as acetone-water-dichloromethane. 

While lome sutionaiy phases are dominated a single nu mecha alsm, and subsequently can be placed near one of the apices of the triangle, other stationary phases function via two or more mqjor retention mecha nisms. These multimodal phases, henceforth referred to as mixed-interaction or mixed-mode stationary phases, have historically bMn successful in separating such biomolecules as nucleic adds and protdns. Most often, phases of these types have been used in low- to medium-pressure chromatography, and have often been of polymeric base. 

Gentamicin C, Ci,. and Ca, the three major components of the antibiotic from Micromonospora purpurea culture filtrates, were isolated by medium pressure chromatography on silica gel. ... 

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