Ethers versus acetals

Picolyl ethers are prepared from their chlorides by a Williamson ether synthesis (68-83% yield). Some selectivity for primary versus secondary alcohols can be achieved (ratios = 4.3-4.6 1). They are cleaved electrolytically ( — 1.4 V, 0.5 M HBF4, MeOH, 70% yield). Since picolyl chlorides are unstable as the free base, they must be generated from the hydrochloride prior to use. These derivatives are relatively stable to acid (CF3CO2H, HF/anisole). Cleavage can also be effected by hydrogenolysis in acetic acid. ... 

For substituted allylic systems, both a- and y-substitution can occur. Reaction conditions can influence the a- versus "/-selectivity. For example, the reaction of geranyl acetate with several butylcopper reagents was explored. Essentially complete a- or y-selectivity could be achieved by modification of conditions.28 In ether both CuCN and Cul led to preferential "/-substitution, whereas a-substitution was favored for all anions in THF. 

Acetylcholineesterase Miniaturized multichannel transduc-tor with planar Au electrode which was first covered with a choline-selective liquid membrane made from 66% PVC-polyvinyl acetate (PVA), 33% 2-nitrophenyl octyl ether plasticizer and 1% ion-pair choline phosphotungstate. A second layer of 2% AChE in the PVA-polyethylene dispersion was spread on the top. The electrode was used as working electrode versus Ag/AgCl for potentiometric measurement of Ch and ACh in 0.1 M Tris buffer at 7.4. Optimum pH range for the sensor was 7-9. The calibration graph was linear from 0.02-10 mm ACh and detection limit was 5 pM. Response time was 3-5 min. Sensor was suitable for determination of ACh in biological fluids. [86]... 

The similar behavior of 6c versus 0c Plols for silica is shown in Fig. 9a,b, where data for several C-solvents (ethyl ether, ethyl acetate, acetone, isopropanol) in admixture with hexane or pentane are plotted as experimental values of [best fit to Eq. (12)1 versus experimental values of 0c. The data for isopropanol (solid circles) are displaced to the right... 

The treatment of an ester (or lactone) with a base and a silyl halide or trillate gives rise to a particular type of sUyl enol ether normally referred to as a silyl ketene acetal. The extent of O- versus C-silylation depends on the structure of the ester and the reaction conditions. The less-bulky methyl or ethyl (or 5-tert-butyl) esters are normally good substrates for O-silylation using LDA as the base. Acyclic esters can give rise to two geometrical isomers of the silyl ketene acetal. Good control of the ratio of these isomers is often possible by careful choice of the conditions. The f-isomer is favoured with LDA in THF, whereas the Z-isomer is formed exclusively by using THF/HMPA (1.24). Methods to effect stereoselective silyl enol ether formation from acyclic ketones are less well documented. ... 

Abidi and Mounts [338] studied a-, P-, y, and 5-tocopherol and 5,7-dimethyltocol retention on j5- and y-cyclodextrin columns (A = 298 nm, ex 345 nm, em) using both cyclohexane and hexane mobile phases modified with alcohols (ethanol, IPA, n-propyl alcohol, 1-butanol, and 2-methyl-2-propanol), ethers (dioxane, THE, diisopropyl ether, MrBE, or tetrahydropyran) or ethyl acetate. A k versus percent hexane and percent cyclohexane plot was shown for each modifier, Selected chromatograms and extensive tables of k and a values are presented. Most elutions were complete in less than 45 min. In general, peak shapes were excellent with the notable exception of when MtBE was used as the modifier. In this instance, very broad peaks were generated. Why this occurred for MrBE and not for diisopropyl ether is not explained, nor is it readily explainable. Also observed was a significant decrease in fluorescence intensity when ediyl acetate was the mobile phase modifier. The authors ascribed this result to the decreased solubility of the analytes in the solvent, since the effect was not observed with any other solvent system. Anotho-possibility is that ethyl acetate may effectively quench the fluorescence (it is the only carbonyl-containing solvent used in the study). 

The solubility of ionic substances in relatively nonpolar aprotic solvents can be greatly enhanced by using catalytic quantities of macrocyclic polyethers, such as 18-crown-6, the structure of which is shown in Fig. 5.5. These macrocyclic ethers selectively solvate the cation, both enhancing solubility and also leaving the anion in a very weakly solvated state. The anions behave under these conditions as highly reactive species, sometimes termed naked anions. A study of the relative rates of nucleophilic substitution on benzyl tosylate by potassium salts in acetonitrile in the presence of 18-crown-6 revealed a pronounced leveling effect. " All the potassium halides (fluoride, chloride, bromide, and iodide) were approximately equal in their reactivity. Potassium acetate was observed to be almost ten times more reactive than potassium iodide under these conditions—a reversal of the normal reactivity of acetate ion versus iodide ion in nucleophilic substitution reactions. As measured by cHji values in Table 5.5, iodide is 3 log units, i.e., 10 times, more reactive than acetate ion in the protic solvent methanol. 

A 5-g sample of a-olefm sulfonate is mixed with 50 g of basic alumina, activity grade I, which has been activated at 450 C for 3 hr. The mixture is then extracted four times with 50-mL aliquots of ethyl ether. The extracts are combined and evaporated to dryness, and the residue is dissolved in the HPLC mobile phase. The analysis is performed on a CPS-Hypersil normal phase HPLC column, 4.6 x 20 cm, with a mobile phase of 90 10 hexane/ethyl acetate and differential RI detection. Peak identification and quantification is made versus authentic sultone standards. The method was demonstrated for determination of hexadecane-l,4-suItone, hexadecane-l,3-sultone, hexadec-l-ene-l,3-suItone, and 2-alkylalkane-l,4-sultone, as well as 2-hexadecylhexadec-l-ene-l-sulfonate. 

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