Flash chromatography , isolation

Methoxy-l-methylethyl alkyl peroxides (1, X = OR , R = R = R" =, methyl R = decyl, undecyl, pentadecyl, 2-octyl) are Hquids that have been isolated using flash chromatography (131). Peroxyester derivatives (R = cyl) have also been prepared (131). 

Hyperaspine (1) was isolated from H. campestris by extraction with MeOH and subsequent flash chromatography (01TL4621). 

Flash chromatography is widely employed for the purification of crude products obtained by synthesis at a research laboratory scale (several grams) or isolated as extracts from natural products or fermentations. The solid support is based on silica gel, and the mobile phase is usually a mixture of a hydrocarbon, such as hexane or heptane, with an organic modifier, e.g. ethyl acetate, driven by low pressure air. (Recently the comparison of flash chromatography with countercurrent chromatography (CCC), a technique particularly adapted to preparative purposes, has been studied for the separation of nonchiral compounds [90].)... 

A solution of 1.5 mol equiv of butyllithium in hexane is added to 1.5 mol equiv of a 1 M solution of hexabutylditin in THF at 0 C under nitrogen, and the mixture is stirred for 20 min. The solution is cooled to — 78 °C and a solution of 1.5 mol equiv of diethylaluminum chloride in toluene is added. After stirring for 1 h at — 78 °C, a solution of 0.05 mol equiv of [tetrakis(triphenyl)phosphine]palladium(0) in THF is added followed by a solution of the allyl acetate in THF. The mixture is warmed to r.t., and stirred until the allyl acetate has reacted (TLC). The solution is cooled to 0°C, and an excess of aq ammonia slowly added. After an aqueous workup, the products arc isolated and purified by flash chromatography on silica gel using 1 % triethylamine in the solvent to avoid acid-induced loss of stannane. 

Measured by H NMK. h Isolated yield after flash chromatography. c Obtained by crystallization in cyclohexane. d c = 1 (acetone). 

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... 

An aqueous extract of P. hysterophorus (collected in Puerto Rico) was partitioned into methylene chloride at pH 7, pH 10 and pH 2. Bioassays of the methylene chloride soluble fractions, using the bean second internode bioassay (13), showed that the highest activity was concentrated in the methylene chloride extract at pH 7. Extensive chromatographic purification (flash chromatography, medium pressure LC, preparative TLC) monitored by bioassay led to the isolation of the four sesquiter-... 

JOC3172, 1996J(P2)2085>. Chemical electron transfer (CET) oxidation of these bicyclic housanes with tris(4-bromophenyl)aminium hexachloroantimonate performed at 0°C in the absence or presence of base (2,6-di-/-butyl-pyridine) leads to the formation of two olefinic products 369 and 370, which were isolable after flash chromatography on silica (Scheme 56) <1996JOC3172>. 

Compound 3 is isolated in 82% yield by flash chromatography on Si02 Si02 is Silica Woelm 32-63 active. Fa. Woelm Pharma, Eschwege or Aldrich Grade 951. 

Isolated yield of pure product after recrystallization or flash chromatography bDiastereomeric ratio 1.1 1... 

Scheme 7.126) [150], All experiments within this report were carried out in a domestic microwave oven, utilizing septum-sealed round-bottomed flasks. The alumina in the reaction mixture acts as a temperature moderator in order to prevent the reagents from extensively reacting with the metal catalyst. After the reaction, the mixture was suspended in hexane, filtered, and the products were isolated by evaporation of the solvent from the filtrate and subjecting the residue to flash chromatography. 

A solution of tri(p-methoxymethyl)phenylphosphine (0.500 g, 1.26 mmol) in dry methanol (5 ml) was placed in a pressure tube with a Teflon needle valve along with p-(methoxymethyl)bromoben-zene (0.255 g, 1.26 mmol) and anhydrous nickel(II) bromide (0.007 g, 0.032 mmol). The tube was flushed with dry nitrogen and closed. The tube was maintained in an oil bath heated at 180°C for 48 h. After cooling to room temperature, the solvent was evaporated under reduced pressure to give the crude product, which was purified by flash chromatography on silica gel (50 g) using 1 1 acetonitrile ethanol as eluent. In this manner was isolated pure tetra(p-methoxymethyl)phenylphosphonium bromide (0.050 g, 7%), which exhibited spectra and analyses in accord with the proposed structure. 

Mo02(acac)2 (30 mg, 0.1 mmol) and Adogen (89 mg, 0.2 mmol) are added to the alcohol (1 mmol) and Na,C03.1.5H202 (0.628 g, 4 mmol) in MeCN (10 ml). The mixture is stirred under reflux and then cooled to room temperature and filtered. The filtrate is evaporated and the carbonyl compound is isolated by flash chromatography from silica. 

Method A TBA-Oxone (0.98 g, 0.6 mmol) is added to the sulphide (0.2 mmol) in CH2CI2 (10 ml). The mixture is stirred at room temperature until all of the sulphide has been consumed, as shown by TLC analysis. The sulphone is isolated by direct flash chromatography of the reaction mixture on silica. 

The culmre solution was extracted with ethyl acetate (3 x 100 mL). Combined extracts were washed once with brine and dried with anhydrous Na2S04. The solvent was removed on a rotary evaporator and the residue was purified by flash chromatography over silica gel to afford the title compound as white crystals (36 mg, 65 % isolated yield). 

Once an assessment on a particular impurity has been made all process-related compounds will be examined to confirm that the impurity of interest is indeed an unknown. An easy way of doing this is to compare the retention times of known process-related compounds to that in question. If this analysis confirms that the compound is an unknown, the next step would be to obtain an LC-MS on the compound. Mass spectrometry provides structural information which aids in determining structure. In some cases, mass spectrometry will be enough to identify the compound. In other cases, more complicated methods like LC-NMR are needed or the impurity will need to be isolated in order to obtain additional information. Compounds that are not purified often contain high levels of by-products and can be used for this purpose. Alternatively, mother liquors from crystallizations also contain levels of by-products. Other ways of obtaining larger quantities of impurities include flash chromatography which is typically used for normal phase separations or preparative HPLC which is more common for reversed phase methods. Once a suitable quantity of the compound in question has been obtained a full characterization can be carried out to identify it. 

Method D (typical procedure).A mixture of phenacyl bromide (0.796 g, 0.4 mmol), bis(triphenylstannyl) telluride (3.30 g, 0.4 mmol) and KE-2H20 (0.103 g, 1.2 mmol) in MeCN (10 mL) is stirred at room temperature, under N2. Very soon a black solid is deposited. After stirring for 5 h, the reaction mixture is filtered through Celite and the filtrate evaporated. The product (acetophenone) is isolated by flash chromatography (hexane/EtOAc, 20 1). 

Cleavage of trichloro-t-butylcarbamates (general procedure) A 1% solution of the trichloro-t-butylcarbamate derivative and 24 mol% of bis(2-thienyl) diteUuride in THF is heated to 60°C under Nj. A 1% aqueous solution of NaBH4 (stabilized with three drops of a 10% NaOH solution) is added dropwise until the red colour of the catalyst disappears. This usually requires 4 or 5 equiv of NaBH4 and is carried out over a period of 30 min. The volatile components are removed in vacuo. The mixture is then poured into HjO and extracted with ether. The amines are isolated by flash chromatography or by acid extraction. 

Ratios were obtained by GC analysis of the crude reaction mixture. Products were separated and isolated by flash chromatography. Products were identified by comparison of their H and 13C NMR spectra with those reported in the literature. Typical isolated yields were 35-40% of 20 and 10-12% of 21... 

The allylic oxidation of the sesquiterpenoid (+)-valencene has been performed using f-BuOOH as the oxidant and BiCl3 as catalyst. Nootkatone was the major product, isolated in 35% yield by flash chromatography (ethyl acetate - light petroleum, boiling point 40-60 °C) (Scheme 17) [87, 88]. 

Their isolation by flash chromatography on silica gel was comparatively easy. The CD spectra of related pairs of diastereomers whose addition pattern represent pairs of enantiomers, reveal pronounced Cotton effects and mirror image behavior. It is the chiral arrangement of the conjugated Jt-electron system within the fullerene core that predominantly determines the chiroptical properties. Adducts with a C2-... 

---