Hexane gradient elution

Step 2 To a flame-dried round-bottom flask equipped with rubber septum and N2 line was added anhydrous DMF (2mL), skipped allenyl imine (0.689 mmol), and aryl iodide (0.758 mmol). N2 was bubbled through the solution for 30 min to eliminate O2. Pd(PPh3)4 (0.069 mmol), and NaOAc (2.067 mmol) were then added. The reaction was stirred at 80° C for 12 h under N2 then exposed to air and stirred for another 12 h, maintaining a temperature of 80°C. Subsequently, the mixture was cooled to room temperature. 10 mL of H2O was added and the product was extracted with Et20 (10 mL X 3). The organic layer was concentrated to yield a crude mixture which was purified via flash column chromatography on silica gel with 0 to 10% EtOAc in hexanes gradient elution system to obtain the pyridine products. 

Fig. 2.3.4. Normal-phase chromatograms of commercial mixtures of alkylphenol ethoxylates (a) OP[EO]g/g (b) NP[EO]2 (c) NP[EO]4 (d) NP[EO]10. Column 100 X 4.6 mm2 Hypersil 3 NH2 (3 pm), gradient elution with re-hexane-2-propanol-H20,...

D. (3S, 4E)-Methyl 3-(dimethylphenylsilyl)-4-hexenoate (4a). A stirred solution of 6.0 g (29.11 mmol) of alcohol 3a and 60 mL of dry toluene in a 200-mL flask equipped with a reflux condenser is treated with 15 mL (116.89 mmol) of trimethyl orthoacetate and 0.15 mL (2.01 mmol, 0.07 equiv) of propionic acid (Note 15). The reaction mixture is heated under reflux for 48 hr and then allowed to cool to room temperature. Solvents and volatile material are removed under reduced pressure to leave the crude (E)-crotylsilane 4a as a yellow oil. The crude residue is chromatographed on silica gel (gradient elution 5% EtOAc/hexanes). The eluant (Note 6) affords 6.58 g (25.1 mmol, 86%) of pure 4a as a faint yellow, viscous oil (Notes 6, 16 and 17). 

A. ( )-1-(Dimethylphenylsilyl)-1-buten-3-ol (2a). A solution of 10.0 g (0.143 mol) of racemic 3-butyn-2-ol (Note 1) dissolved in 255 mL of tetrahydrofuran (THF, Note 2) in a 1-L, round-bottomed flask equipped with a reflux condenser and nitrogen atmosphere is prepared. Dimethylphenylsilane (21.4 g, 0.157 mol) (Note 3) and a small piece of sodium metal (ca. 5 mg) (Note 4) are placed in the reaction mixture. The solution is stirred for 15 min and 12 mg (2.05 x 10 5 mol) of bis(q-divinyltetramethyldisiloxane)tri-tert-butylphosphineplatinum(O) (Note 5) is added. The reaction mixture is then heated under reflux for 12 hr. The orange solution is cooled to ambient temperature, and the solvent is removed under reduced pressure to yield a crude orange residue containing 2a. The oil is subjected to column chromatography on silica gel (Note 6) (gradient elution 5, 10, 20, 35% EtOAc/hexanes) providing 25.4 g (123.23 mmol, 86%) of pure 2a as a yellow oil (Note 7). 

Normal-phase HPLC has also found application in the analysis of pigments in marine sediments and water-column particulate matter. Sediments were extracted twice with methanol and twice with dichloromethane. The combined extracts were washed with water, concentrated under vacuum and redissolved in acetone. Nomal-phase separation was performed with gradient elution solvents A and B being hexane-N,N-disopropylethylamine (99.5 0.5, v/v) and hexane-2-propanol (60 40, v/v), respectively. Gradient conditions were 100 per cent A, in 0 min 50 per cent A, in 10 min 0 per cent A in 15 min isocratic, 20 min. Preparative RP-HPLC was carried out in an ODS column (100 X 4.6 mm i.d. particle size 3 jum). Solvent A was methanol-aqueous 0.5 N ammonium acetate (75 25, v/v), solvent B methanol-acetone (20 80, v/v). The gradient was as follows 0 min, 60 per cent A 40 per cent A over 2 min 0 per cent A over 28 min isocratic, 30 min. The same column and mobile phase components were applied for the analytical separation of solutes. The chemical structure and retention time of the major pigments are compiled in Table 2.96. 

The crude residue was purified by flash chromatography (gradient elution with 10-30 % EtOAc/hexanes). 179 mg (98 % yield). 

The mobile phases used to provide separations that interface cleanly with the MS are of great importance. Both isocratic and gradient elution can be used. High purity (HPLC grade) water, acetonitrile, and Ci to C4 alcohols are compatible with APTelectrospray and APCI. Less polar solvents such as hexane, cyclohexane, toluene, and ethyl acetate are also compatible with APCI. In general, it is advisable to always have an organic solvent present in the mobile phase to reduce surface tension, which enhances the formation of smaller, more uniform droplets and also aids vaporization and ionization and hence provides greater sensitivity. 

Another screening strategy using the same type of columns but with normal-phase gradient elution has been proposed by the pharmaceutical group Lilly.In this strategy, each compound is screened on four columns, i.e., Chiralcel OD-H, Chiralpak AD, Chiralpak AS and Chiralcel OJ. An n-hexane/2-propanol and an n-hexane/EtOH gradient elution system are... 

The resulting solid is extracted with 400mL of boiling absolute ethanol with stirring for 30 min and while hot it is filtered leaving behind a small amount of insoluble brown residue. The filtrate is then cooled in an ice bath to precipitate the crude product, II, and filtered (2.95 g, 52% of theoretical yield). Subsequent recrystallization from ethanol (with charcoal treatment), or alternatively, column chromatography on partially deactivated silica gel, gradient eluted with chloroform-hexane, will yield analytically pure product. 

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