Short-column chromatography

Short column chromatography can be performed to purify the compound further if desired. 

F mol of electricity had been consumed, the mixture was poured into brine (50 niL) and extracted with CHjClj (3 X 30 mL). The combined extracts were dried (MgS04) and concentrated under reduced pressure, and the residue was purified by short column chromatography (silica gel. hexane, EtOAc 3 1) to give the corresponding 4-fluoroazetidin-2-one II. 

Flash chromatography is a quick preparation technique that is, in effect, a hybrid between medium pressure and short column chromatography. It uses a short, fat column (e.g., 1-5 cm i.d. x 45 cm) packed with silica gel and filled with solvent. Compressed air is used to compress and remove the air from the solvent which then elutes quickly. The sample is then added and the column filled again. Pressure is adjusted to achieve a separation in 5-10 minutes. It is a fast and inexpensive method for the preparative separation of mixtures requiring only moderate resolution. Use of 40-63 pm sihca gel and a pressure driven flow rate of 2.0 in/min are essential for successful separation [30]. 

Initially developed at Columbia University, flash chromatography is an air pressure driven hybrid of medium pressure and short column chromatography which has been optimized for particularly rapid separations. The resolution is measured in terms of the ratio of retention time (r) to peak width (w, w/2). It is measured by baseline resolution in the valley between the peaks and the distance from peak to peak. Sample size can be increased dramatically if less resolution is required. This innovative work by the Columbia University team demonstrated that column performance was quite sensitive to the rate of elution, and the best performance was accomplished with relatively high eluent flow rates. 

A heterogeneous mixture of solid potassium cyanide (0.651 g, 10 mmol), solid methylamine hydrochloride (0.753 g, 11 mmol) and neutral alumina (for column chromatography, 1.5 g) in acetonitrile (4 mL) is placed in a Pyrex test tube (25 mm diameter, 150 mm) with a Teflon -lined screw cap. The mixture is subjected to preliminary sonication at 50°C for 10 min in a thermostated Branson cleaner (100 W, 45 kHz). Benzaldehyde (0.531 g, 5 mmol) was then added and sonication was continued for 5 h. Solid materials were filtered off through a Celite pad and the filtrate was concentrated on a rotary evaporator. The crude product was purified by short column chromatography to give a-methylamino-phenyl-acetonitrile. IR (neat) 3380, 3320, 2225 cm-1. iR NMR (CDCI3, TMS) 6 7.45 (m, 5H), 4.80 (s, IH), 2.65 (s, 3H), 1.65 (br. s, IH) ppm. 

For each reaction, the oxime (5 mmol) and toluene (1 mL) were charged into a 50 mL two necked round-bottomed flask equipped with a magnetic stirrer and condenser. The reaction was heated to reflux and diethyl chlorophosphate (5 mmol) was added to the mixture. The reaction mixture was heated for 20-120 minutes and then cooled to room temperature. The crude mixture was neutralized with aqueous solution of sodimn hydroxide (10 mL, 5%) and then extracted with diethyl ether (10 mL). Drying the ethereal layer over anhydrous sodium sulfate and removal of the solvent gave the crude product, which was purified by short column chromatography over silica gel using -hexane and ethyl acetate (9 1-5 5) as eluent. " ... 

General procedure. Preparation of cyanides 1379 [1137] To a solution of an amide 1377 or aldoxime 1380 (1 equiv.) and CDC (1 equiv.) in an appropriate solvent, an amine (2 equiv.) is added dropwise at room temperature. The reaction mixture is stirred at room temperature, poured into water, and extracted with dichloro-methane. The organic solution is successively washed with 5% aq. HCl, saturated aq. NaHCOs solution, and water, then dried (MgS04) and concentrated to dryness. The residue is purified by short column chromatography (Si02) to give the cyanide 1379. 

General procedure. N-Vinylcarbodiimides 1732 [1268] To a solution of iminophos-phorane 1731 (5.27 mmol) in dry benzene (50 mL) was added dropwise a solution of the isocyanate 1730 (5.83 mmol) in benzene (50 mL). The reaction mixture was stirred at room temperature for 1-2 h (R = aryl, -styryl) or for 1-2 d (R = alkyl). Evaporation of the solvent and trituration of the residue with hexane afforded tri-phenylphosphine oxide as colorless crystals, which were filtered off. Removal of the solvent from the filtrate and short column chromatography of the residue (silica gel benzene/hexane, 1 5) gave the carbodiimides 1732 as viscous oils or solids in yields of around 70%, which were immediately used in the next reaction. Attempts to purify them by distillation under reduced pressure failed. 

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