Flash silica

The crude material was purified using flash silica gel chromatography eluting with pentane/ether (3 1). This provided 0.31 g (85%) phenethanol. 

After 20 min, HMPA (6.0 mmol, 0.6 mL) and A-phenyltrifluoromethanesulphonamide (2.6 mmol, 0.6 g) in THF (5 mL) were added and the solution was allowed to warm to room temperature. The reaction mixture was stirred for 4 h and then heated with 1 1 solution of saturated aqueous NH4CI/NH4OH (20 mL), exhacted with ethyl acetate (3x20 mL), dried, evaporated and the residue was purified by flash silica gel chromatography using hexane as eluent affording the vinyl triflates. 

The oil is taken up in diethyl ether and absorbed onto approximately 15 g of flash silica (Merck silica gel 60 mesh 0.040-0.063 mm), then applied to a 10-cm diameter column packed with 500 g of flash silica eluting the product with pentane/diethyl ether 85 15. (The checkers used 10 1 hexanes/ether for improved resolution.)... 

AflaskconlainingasolutionofDBH(l.l mmol) and H20 (1.1 mmol) in 1,4-dioxane (4 mL) was connected to a balloon filled with SiF4 ( 100 mL) and the mixture was stirred at rt for 15 min. A solution of alkene (1.0 mmol) in 1,4-dioxane (1 mL) was added, and the mixture was stirred at rt for 2-3 h. Sat. aq KF (5 mL) was added to the mixture, and after normal workup, the resulting crude mixture was purified by flash silica gel column chromatography to give the bromofluorinaled product. 

Stoichiometric amounts (1.75 mL) of 0.1 M solutions of the four Ugi components were combined in order of their participation in the Ugi reaction and stirred at room temperature for 20 h. The solvent was evaporated in vacuo at 40° and dried under high vacuum. A 10% solution of AcCl in MeOH (7 mL) was added to the crude material and stirred at room temperature for 15 h. The solvent was then evaporated in vacuo at 40°. The crude material was preabsorbed onto flash silica and purified by flash column chromatography (EtOAc hexane) to yield the desired product. The following benzodiazepines were prepared via this procedure. 

A suspension of 5-iodouridine (27 mmol) and 0.5 g of toluenesulfonic acid in 600 ml acetone were refluxed 4 hours and then passed through an addition funnel packed with 4 A molecular sieves. The reaction volume was then reduced to approximately 150 ml and passed through a 125 ml pad of flash silica gel eluting with 500 ml acetone. The solution was concentrated, then recrystallized using 225 ml ethyl alcohol, and the product isolated in 87% yield as a white crystalline solid. 

Flash silica is quite expensive but if you can buy it in bulk (25kg at a time) the cost will be reduced by nearly half. Another way to save money is to recycle it, using the procedure given below. The material generated by this method is very reliable. 

Column chromatography with hexanes on flash silica gel gave a few fractions of with 95% purity, as determined by HPLC, along with later fractions containing mixtures of Cio/C a in various ratios. Because of the poor solubility of C o and C70 in these alkanes, only limited amounts of pure C o were made available this way, in insufficient quantity for a reliable C NMR spectrum to be measured (see below). However, column chromatography on neutral alumina with hexanes gave an excellent separation in analytical quantities. Thus, pure fractions containing C (99.85%) and C70 (>99%) were obtained, as indicated by mass spectrometric measurements described below. 

As mentioned above, silica is the most common stationary phase, and as such, it is available in a host of shapes and sizes. Underivatized silica is most commonly seen in one of three sizes low-performance, flash, or HPLC. Low-performance silica stationary phases have large diameters (and often with a large particle-size distribution) and are packed into low-pressure glass or stainless steel columns. HPLC silica is typically 10 pm particles that are packed in finely polished stainless steel tubes. Flash silica particles are 30-70 pm and have low enough pressure drop that laboratory nitrogen... 

Compounds 13 are fairly stable under neutral conditions and the nucleoside derivatives can be isolated by flash silica gel column chromatography. It was found that the 2-thio-l,3,2-oxathiaphospholanes 13, similarly to their 2-oxo counterparts 10, react with alcohols in the presence of base catalyst (preferably DBU) under rigorously anhydrous conditions with the cleavage of endocyclic... 

A. Furan-2-ylcarbamic acid tert-butyl ester. In a 250-mL, one-necked, round-bottomed flask equipped with a magnetic stirring bar are placed 10 g (0.07 mol) of 2-furoyl chloride (Note 1), 80 mL of tert-butyl alcohol (Note 2), and 5.1 g (0.08 mol) of sodium azide (Note 3). After the flask is stirred at 25°C for 20 hr under an argon atmosphere, it is placed behind a protective shield (Note 4) and the solution is heated at reflux for 15 hr under a constant flow of argon. The solvent is removed with a rotary evaporator at aspirator vacuum to provide a white solid that is purified by flash silica gel chromatography (10% ethyl acetate/hexane) to give 10.8 g (81%) of furan-2-ylcarbamic acid tert-butyl ester as a white solid mp 98-99°C (Note 5). 

One half of the crude mixture obtained from the reaction between NjOs (14.4 g, 0.13 mol) and ethyl diazoacetate (30.3 g, 0.26 mol) was diluted with CHjCl (10 mL) and added dropwise over 1 h from an addition funnel cooled with ice/water to styrene (15.0 g, 0.13 mol) and Rh2(OAc)4 (100 mg, 0.25 mmol) in CHjClj (5 mL) at 5°C. After 2 h, the evolution of Nj had ceased the mixture was stirred for an additional hour, then concentrated and filtered through flash silica gel (5x5 cm), eluting first with pentane to remove excess styrene and then with 20% EtjO/hexane, to afford, after concentration, 21.0 g (0.11 mol, 85%) of the diastereomeric nitrocyclopropylcarboxylates. 

To a solution containing the ketone (0.2 g, 0.6 nunol) in dry CI FCb (20 inL) were added 1,2-ethanedithiol (0.3 g, 3.0 nunol) and boron trifluoride etherate (0.3 g, 2.3 mmol). The mixture was stirred at room temperature for 24 h and quenched with aqueous NaHCOs (10 mL). The aqueous layer was extracted with ( I bt b, and the combined organic layers were dried over magnesium sulfate and concentrated under reduced pressure. The crude residue was purified by flash silica gel chromatography to give 0.2 g (85%) of the dithiane as a white solid. 

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