Thin-layer chromatography reaction, monitoring

The reaction was generally carried out at room temperature after the exothermic starting period had subsided. Progress of the reaction was monitored by thin layer chromatography. Reaction times are not optimized. 

Chromatography Johnson et al. [54] used thin-layer chromatography to monitor the reaction of naproxen with (-f-)-2-octanol using silica gel plates eluted with hexane tetrahydrofuran acetic acid (50 50 1). In the same paper, the reaction mixture was analysed by liquid chro-... 

The progress of the reaction was monitored hy injecting after each 24-hour period an aliquot into a gas chromatograph and checking the peak corresponding to isophorone. Alternatively, thin-layer chromatography (E. Merck 0.25-mm. silica gel plates developed with ethyl acetate) can be used,... 

Most of the reactions described in the following chapters were monitored by Thin Layer Chromatography (TLC) using plastic TLC plates coated with a thin layer of Merck 60 F254 silica gel. The products were detected by using an ultraviolet lamp or the TLC plates were treated with p-anisaldehyde reagent, prepared as explained below, and then heated to 120 °C to stain the spots. After visualization and measurement, the Rf values were recorded. 

From the analytical point of view, reactions were efficiendy monitored using a combination of FTIR (Fourier transform infrared), TLC (thin-layer chromatography), and MALDI-TOF mass spectroscopy analysis of crudes resulting from the cleavage of small resin samples. 

When these reactions were monitored by means of thin-layer chromatography, the initial formation of an amidine (see Schemes 26 and 27, 247 or 248) could be detected. The amidine then either reacted directly with diethyl malonate or was converted into another reactive intermediate, N-arylformimidate (249), which next reacted with diethyl malonate to give A,-(3-chlorophenyl)aminomethylenemalonate (250). 

Stearyl bromide (5.5 mmol) and triphenylphosphine (5.8 mmol) were heated under reflux for 20 hours in 30 mL anhydrous xylene. Upon completion of the reaction (as monitored by thin-layer chromatography) the solvent was removed followed by purifying the obtained crude yellowish oil by silica gel column chromatography using methanohchloroform (5 95) as an eluent. Purified STPP, obtained as colorless oil, crystallized on standing and was recrystallized from ether to yield pure STPP in 35% to 45 /o yields. 

B. General Oxidation Procedure for Alcohols. A sufficient quantity of a 5% solution of dipyridine chromium (VI) oxide (Note 1) in anhydrous dichloromethane (Note 7) is prepared to provide a sixfold molar ratio of complex to alcohol. This excess is usually required for complete oxidation to the aldehyde. The freshly prepared, pure complex dissolves completely in dichloromethane at 25° at 5% concentration to give a deep red solution, but solutions usually contain small amounts of brown, insoluble material when prepared from crude complex (Note 8). The alcohol, either pure or as a solution in anhydrous methylene chloride, is added to the red solution in one portion with stirring at room temperature or lower. The oxidation of unhindered primary (and secondary) alcohols proceeds to completion within 5 minutes to 15 minutes at 25° with deposition of brownish-black, polymeric, reduced chromium-pyridine products (Note 9). When deposition of reduced chromium compounds is complete (monitoring the reaction by gas chromatography or thin-layer chromatography analysis is helpful), the supernatant liquid is decanted from the (usually tarry) precipitate and the precipitate is rinsed thoroughly with dichloromethane (Note 10). 

Yields sometimes drop when an old bottle of borane-dimethyl sulfide is used. Reaction progress can be monitored by thin layer chromatography (silica gel, eluting with 10 10 1 chloroform-methanol-concentrated ammonium hydroxide). Any remaining phenylalanine stains heavily when exposed to ninhydrin (Rf 0.35). If phenylalanine is detected after 5 hr of reflux, an additional 10 ml (0.10 mol) of borane-dimethyl sulfide is added via syringe, and the solution is heated at reflux for 1 additional hr. 

Disappearance of the starting material may be monitored by thin layer chromatography (R, = 0.6, 2 1 hexane ethyl acetate). It is important that the reaction time not be extended past the point when all of the p-keto ester is consumed, because long exposure to the zinc carbenoid has been shown to result in substrate decomposition and reduced yields. 

The progress of the reaction is monitored by thin layer chromatography, eluting with ethyl acetate (product Rf = 0.06, visualized and developed using UV light and KMn04 respectively). 

The cleavage of a,a -disubstituted SAMP- or RAMP-hydrazones to spiroacetals is performed without racemization under acidic conditions (4-methylbenzenesulfonic acid in chloroform) by careful monitoring of the reaction by thin layer chromatography and immediate workup of the reaction after completion in order to avoid racemization. Racemic mixtures are obtained when the reaction mixture is stirred for a longer time20. 

The reaction was monitored by thin-layer chromatography and the product was extracted with CH2CI2 (3 x 150 mL). The combined extracts were washed with 10% aq. Na2S03 (3 x 150 mL) and water (3 x 150 mL) and dried with anhydrous sodium sulfate. The filtrate was concentrated in vacuo to give a white powder (18.4 g, 99% yield). 

The step 1 product (0.11 mol), 4-dimethylaminopyridine (0.06 mmol), hydroquinone (0.35 g), and triethylamine (0.375 mol) were added to 275 ml dry acetone and then treated with methacrylic anhydride (0.14 mol). The mixture was refluxed for 90 minutes while the reaction progess was monitored by thin-layer chromatography. Since some hydroxyl starting material still remained, an additional 5 ml of methacrylic anhydride was added and the mixture refluxed an additional hour. Once the reaction was completed, the solution was cooled to ambient temperature and a yellow solid isolated. The solution was further treated with 100 ml cold methanol to further precipitate the product. The material was collected, washed with cold methanol, air dried, and 78.3% product isolated with a 96% purity. 

A pressure vessel containing a tyrosine derivative dissolved in dimethyl formamide containing 2 (2 eq) was treated with the dropwise addition of ethyl bromide (1.1 eq) and then heated to 60°C for 30-minute intervals while being monitored by thin-layer chromatography (TLC). The reaction was then quenched with water and the aqueous layer extracted and then dried over Na2S04. The mixture was filtered, concentrated, and the product isolated. 

The reaction can be conveniently monitored in the infrared by observing the decrease in the 0=N stretching absorption at 1660 cm. 1 or by thin-layer chromatography [silica, developed with hexane-ethyl acetate (9 1) or (4 1)]. 

The reaction may be followed by thin-layer chromatography. The ratio of the Rf values for cyclohexene and c/s-diol 2 is 2 1 (commercial silica gel plates, ethyl acetate). The plates are best visualized by first spraying with 1 % aqueous potassium permanganate, then with methanolic sulfuric acid, followed by charring with heat. The checkers found that, if the procedure is followed exactly, monitoring the reaction by thin-layer chromatography is unnecessary. 

After 1 hour, the reaction is complete as monitored by gas chromatographic analysis on an Hewlett-Packard F M 5751 research chromatograph (1.85 m. x 0.313 cm. stainless-steel column of 10% Apiezon L on Chromosorb W AW/DMCS column temperature 250°). Thin-layer chromatography was found to be of little use in monitoring the reaction, as the Rf values for benzyl disulfide and benzyl sulfide are virtually identical for a variety of solvent systems tried. Proton magnetic resonance (benzene) shows that these two compounds have coincidental chemical shifts for the benzylic protons 8 3.4 (singlet). 

The amount of Na[Ph2CO] solution required depends on how dry the reaction mixture is. When the reaction appears visually to be complete, a small aliquot should be examined (solution IR spectrum) to ensure that the precursor cluster complex has reacted completely [using the 2096 cm-1 v(CO) band of Ru3(CO)j j(PMe2Ph)]. Alternatively, the progress of the reaction may be monitored by TLC (thin layer chromatography) (silica gel, petroleum ether eluant). 

Monitoring reaction progress throughout a multistep synthesis is a relatively difficult task.22 Typical methods used for solution-phase synthesis, including thin-layer chromatography (TLC), GC, and most types of mass spectrometry (MS), are less informative for solid-phase methods. However, Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) are particularly useful in solid-phase strategies. 

---