Polarographic analysis of corticosteroids: Part 6. Mechanism of polarographic electroreduction of some Δ4-3-ketosteroids and Δ1,4-3-ketosteroids

Prednisolone and dexamethasone give waves in d.c. and normal pulse polarography and peaks in differential pulse polarography which correspond to a one-electron uptake. The transfer of the first electron is reversible and at pH < 7 (i₁) is preceded or at pH >7 (i₃) is followed by a proton transfer. Protonation occurs on the carbonyl group and pinacol is formed. At pH >7, wave i₃ is followed by wave i₄ which involves transfer of another proton and electron on the carbonyl group and yields the unsaturated alcohol. At pH <3, the radical formed in the first electron uptake is further protonated and is reduced in wave (i₁₊₂). In tetramethylammonium hydroxide solution, reduction of the side chains on C-17 occurs at more negative potentials. Reduction of testosterone and hydrocortisone follows a similar mechanism (A)—(G), but the second electron uptake in wave i₄ at pH >7 is not observed.     

Potential Bile Acid Metabolites. 5.1 12B-Hydroxy Acids by Stereoselective Reduction

According to previous research the practical route² to 12β-hydroxycholanic³ acids was by Raney nickel catalytic hydrogenation of the corresponding keto acids, but the method suffers from several disadvantages: (1) the required ratio of catalyst to substrate is inordinately high, (2) the reaction is usually slow, with complete reduction time ranging from 6 hours (12-oxocholanic acid) to 48 hours (certain dihydroxy keto acids), (3) recovery from the adsorbed product from the catalyst is incomplete, and (4) with long reaction times side reactions can occur, as was recently reported^2c that during Raney nickel reduction of methyl 3β, 7α-dihydroxy-12-oxocholanate (1), requiring 48 hours for completion, partial inversion at C-3 took place.     

Enantioselective Synthesis of α‐Hydroxy Amides and β‐Amino Alcohols from α‐Keto Amides

Synthesis of enantiomerically enriched α‐hydroxy amides and β‐amino alcohols has been accomplished by enantioselective reduction of α‐keto amides with hydrosilanes. A series of α‐keto amides were reduced in the presence of chiral Cu^II/(S)‐DTBM‐SEGPHOS catalyst to give the corresponding optically active α‐hydroxy amides with excellent enantioselectivities by using (EtO)₃SiH as a reducing agent. Furthermore, a one‐pot complete reduction of both ketone and amide groups of α‐keto amides has been achieved using the same chiral copper catalyst followed by tetra‐n‐butylammonium fluoride (TBAF) catalyst in presence of (EtO)₃SiH to afford the corresponding chiral β‐amino alcohol derivatives.     

Electrochemical determination of paraquat using a DNA-modified carbon ionic liquid electrode

Deoxyribonucleic acid (DNA) was electrochemically deposited on a carbon ionic liquid electrode to give a biosensor with excellent redox activity towards paraquat as shown by cyclic voltammetry and differential pulse voltammetry. Experimental conditions were optimized with respect to sensing paraquat by varying the electrochemical parameters, solution pH, and accumulation time of DNA. Under the optimized conditions, a linear relation exists between the reduction peak current and the concentration of paraquat in the range from 5?×?10^?8 mol L^?1 to 7?×?10^?5 mol L^?1, with a detection limit of 3.6?×?10^?9 mol L^?1. The utility of the method is illustrated by successful analysis of paraquat in spiked real water samples.

Hemin Modified Carbon Fat Electrode for Analyzing Antimalarial Endoperoxide Artemisinin in Artemisia annua

A hemin bulk modified carbon electrode with Adeps neutralis (solid fat) as binder was developed for the determination of antimalarial endoperoxide artemisinin in plant matrix. The hemin modified electrode showed significant catalytic activity for the electrochemical reduction of artemisinin at about ?380 mV vs. Ag/AgCl in phosphate buffer solution of pH 7 by using cyclic and differential pulse voltammetry. Under optimized conditions strict linearity between artemisinin concentration and height of the cathodic catalytic current peak was observed in 4.8×10^?6–7.8×10^?5 M concentration range (R=0.9991) when using differential pulse voltammetry. The detection limit was calculated as 1.4×10^?6 M of artemisinin. The developed electroanalytical device is suitable for the determination of artemisinin in Artemisia annua extracts.     

Electrochemical Study of Gliclazide and Its Complexation with β‐Cyclodextrin

The electrochemical oxidation of gliclazide has been investigated at glassy carbon electrode in phosphate buffer solutions over the pH range 2.7–11.8 using cyclic and differential pulse voltammetry (DPV). Gliclazide exhibited one anodic peak in the pH range of 2.7–6.3 and a second peak was produced above pH 6.3. The oxidation processes have been shown to be irreversible and diffusion controlled. The formation of an inclusion complex of gliclazide with β‐cyclodextrin (β‐CD) has been investigated by cyclic and differential pulse voltammetry. A phase solubility study with spectrophotometric detection has been also applied. The stability constant of the complex was determined to be 839 and 360 M^?1 using the differential pulse voltammetric method and the phase solubility method, respectively.     

克拉霉素的电化学反应机理研究与应用

应用线性扫描伏安法、循环伏安法、常规脉冲伏安法等电化学手段并结合紫外吸收光谱研究了药物克拉霉素(clarithromycin, CAM)在pH 1.8～9.2 Britton-Robinson缓冲溶液和0.05 mol•L^－1 NaOH溶液中的电化学行为. 在所研究的pH范围, CAM分别产生P₁, P₂, P₃, P₄四个还原波, 其中P₁, P₂, P₄三个波均为其药效活性基团C-9位羰基的还原所产生. 实验结果表明: 在pH 1.8～5.7的B-R缓冲溶液条件下所获得的P₁波为两电子不可逆弱吸附还原波; 在6.0＜pH＜9.2的B-R缓冲溶液中, CAM产生P₂和P₃两个波, 其中P₂为两电子不可逆还原波, P₃为催化氢波. 在0.05 mol• L^－1 NaOH溶液中, CAM产生的P₄波是一个单电子的不可逆吸附还原波. 根据P₄波的峰电流i_P与CAM浓度的线性关系, 建立了CAM含量测定的新方法.     

Voltammetric determination of agriculturally-significant benzenearsonic acids

Direct current and differential pulse polarographic measurements are reported on a series of substituted benzenearsonic acid compounds that are important in agricultural applications. Compounds studied were o-aminobenzenearsonic acid, p-aminobenzene-arsonic acid, p-nitrobenzenearsonic acid, p-ureidobenzenearsonic acid, and 3-nitro-4-hydroxybenzenearsonic acid. Polarographic reduction potentials varied with pH for all compounds, shifting to more negative values as the pH was increased. Although diffusion-controlled reduction waves were observed in all cases, some compounds exhibited a dependence of E$\text{1}\text{2}$ on concentration, especially at relatively high concentrations. Differential pulse polarographic peak currents were proportional to concentration from 10^-4 M to 10^-6 M.     

Differential pulse polarographic behaviour of thiazopyr herbicide and application to its determination in fruit juice and soil samples

A differential pulse polarographic method for the determination of the herbicide thiazopyr has been developed. The polarographic study of thiazopyr exhibited two well-defined cathodic peaks within the pH range of 1.0 to 8.0. The variation of pH and polarographic parameters indicated that the optimum conditions under which thiazopyr could be reduced were a pH 7.0 BR buffer solution, a reduction peak potential of ?1270 mV (vs. SCE), scan rate of 5 mV s^?1, pulse amplitude of 50 mV with pulse duration of 50 ms at an ambient temperature of 25 ± 3°C. The main reduction peak was characterised by cyclic voltammetry as being irreversible and diffusion-controlled. A linear relationship between the peak current and the concentration of thiazopyr was obtained in the range of 0.43–38.6 µg mL^?1, with a detection limit of 0.127 µg mL^?1. The proposed method was successfully applied to the determination of thiazopyr in spiked fruit juice and soil samples. The mean recoveries of the 19.8 µg g^?1 and 3.96 µg mL^?1 thiazopyr spiked to soil and orange juice were 20.2 ± 1.0 µg g^?1 and 3.84 ± 0.12 µg mL^?1, at 95% confidence level, respectively. The sufficiently good recoveries and low relative standard deviation (RSD) data confirm the high accuracy and precision of the proposed method. The interferences effects of several commonly used pesticides and inorganic species were also studied. Interfering effects were eliminated either by providing selectivity with pH, or using EDTA as complexing agent.     

Polarographic and liquid chromatographic methods for the determination of sulfur residues on grapes and wheat

Sampled d.c. and differential pulse polarography are used, in batch mode, to determine sulfur in methanol/0.1 M ammonium acetate (pH 5.0). A two-electron reaction (reduction of sulfur to sulfide) is shown to be involved. Differential pulse polarography is sensitive for the determination of sulfur in relatively clean solutions; the detection limit is 7.2 μg l^?1. The interference of heavy metals (Pb and Cd) is avoided by addition of EDTA. For complex matrices, such as extracts of wheat and grapes, matrix effects are serious. For such samples, reversed-phase liquid chromatography with amperometric detection (dropping mercury electrode) gives excellent results. A relatively simple procedure is described for the determination of sulfur residues in wheat and grapes at levels ? 0.5 mg kg ^?1; linear response is obtained up to ca. 7 mg kg^?1.     

Rapid Differential Pulse Polarographic Determination of Thiamethoxam in Commercial Formulations and some Real Samples

The polarographic behavior of thiamethoxam (a neonicotinoid insecticide) was studied by direct current and differential pulse polarography. Depending on the pH thiamethoxam exhibited one or two well-defined cathodic polarographic waves. The characteristics of the electrode reaction were investigated and it was found that at pH > 5.0 the target molecule captures four electrons in the first step, and two in the second. Based on the reduction behavior of the target molecule on the mercury electrode, a differential pulse polarographic method was elaborated for the rapid determination of thiamethoxam at pH 8.0. With the optimized method, a linear response for thiamethoxam was found in the concentration range of 31.1 − 470 ng cm⁻³, the relative standard deviation did not exceed 1.6%, and the detection and quantitation limits were found to be 9.3 ng cm⁻³ and 31.1 ng cm⁻³, respectively. The method was applied to the determination of thiamethoxam in commercial formulations and real samples (potato and maize). The procedure is simple, fast, sensitive, and compares well with comparative spectrophotometric and chromatographic (HPLC/DAD) methods.     

Pulse polarograpmc study of the electrochemical reduction of lucigenin in aqueous medium

The electrochemical reduction of lucigenin (bis-N-methylacridinium nitrate) in aqueous solution was studied by normal pulse polarography, normal pulse polarography with differential detection of the current, and differential pulse polarography with cathodic and anodic pulses at several pulse amplitudes. The effects of pH and lucigenin concentration were studied. In confirmation of an earlier d.c. polarographic study, lucigenin is shown to be reduced in two separate one-electron steps. An adsorption peak accompanies the first step, while the second, below pH 3.5, is catalytic owing to chemical regeneration of the intermediate reduction product at the electrode surface.     

Microbiological transformations of steroids

Summary A culture ofActinomyces albus 3006 can effect the selective and stereospecific reduction of the C-20 keto group of corticosteroids and their ¹-dehydro analogs with formation of the corresponding 20-hydroxy compounds.     

Electrochemical Redox Behavior of Omeprazole Using a Glassy Carbon Electrode

The electrochemical redox behavior of omeprazole (OMZ), a gastric acid pump inhibitor, was investigated at a glassy carbon electrode using cyclic, differential pulse and square‐wave voltammetry over a wide pH range. The pH‐dependent oxidation occurs in two irreversible consecutive charge transfer reactions. Adsorption of the nonelectroactive product was also observed. The first oxidation involves removal of one electron, followed by deprotonation and leads to the formation of a hydroxylated species. The second oxidation process is related to the hydroxyl and amino groups in the benzimidazole moiety. The reduction is irreversible, also pH‐dependent, and occurs in a single step at the sulfoxide group in a diffusion‐controlled mechanism. The diffusion coefficient of omeprazole was calculated to be D_OMZ=2.31×10^?6 cm² s^?1.     

Extraction of PCDD/PCDF from soil with supercritical CO2: Optimization by a three-level factorial design approach

A highly sensitive and selective voltammetric procedure is described for the simultaneous determination of eleven elements (Cd, Pb, Cu, Sb, Bi, Se, Zn, Mn, Ni, Co and Fe) in water samples. Firstly, differential pulse anodic stripping voltammetry (DPASV) with a hanging mercury drop electrode (HMDE) is used for the direct simultaneous determination of Cd, Pb, Cu, Sb and Bi in 0.1 M HCl solution (pH = 1) containing 2 M NaCl. Then, differential pulse cathodic stripping voltammetry (DPCSV) is used for the determination of Se in the same solution. Zn is subsequently determined by DPASV after raising the pH of the same solution to pH 4. Next, the pH of the medium is raised to pH 8.5 by adding NH₃/NH₄Cl buffer solution for the determination of Mn by DPASV. Ni and Co are determined in the same solution by differential pulse adsorptive stripping voltammetry (DPAdSV) after adding DMG (1 × 10^–4 M). Finally, 1 × 10^–5 M 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) is added to the solution for the determination of Fe by DPAdSV. The optimal conditions are described. Relative standard deviations and relative errors are calculated for the eleven elements at three different concentration levels. The lower detection limits for the investigated elements range from 1.11 × 10^–10 to 1.05 × 10^–9 M, depending on the element determined. The proposed analysis scheme was applied for the determination of these eleven elements in some ground water samples.     

Convenient and Short Route to a Key Intermediate in the Synthesis of Forskolin

The intermediate 2, previously used in several synthetic sequences toward forskolin (1), has been synthesized from enone 3. The key step in the sequence is the 1,3-oxidative rearrangement of the tertiary alcohol 5, which allowed the easy introduction of the keto group at C-6 of 2.     

Selectivite de la reduction de l'androsten-4 dione-3,17 et de la progesterone par divers borohydrures: role de l'assistance electrophile par le cation ou un solvant hydroxyle

The reduction of Δ⁴-androsten-3, 17 dione 1 and of progesterone 2 by nBu₄NBH₄ is highly chemioselective: in THF only the a-enone moiety is reduced, the saturated C₁₇ or C₂₀ keto group being kept unchanged. When TMEDA is added, saturated alcohols are obtained, without any allylic alcohol when the reaction goes to completion. However this reduction is poorly stereoselective as 70:30 mixtures of A/B cis and trans ring junction compounds are obtained. In MeOH, the saturated keto group is more than 90% selectively reduced. However, the reduction of 1 and 2 by LiBH₄ and Zn(BH₄)₂ is poorly chemioselective. These results are interpreted in terms of competition between electrophilic assistance and steric effects.     

Electrooxidation of the antiviral drug valacyclovir and its square-wave and differential pulse voltammetric determination in pharmaceuticals and human biological fluids

The electrochemical properties of valacyclovir, an antiviral drug, were investigated in pH range 1.8-12.0 by cyclic, differential pulse and square-wave voltammetry. The drug was irreversibly oxidized at a glassy carbon electrode in one or two oxidation steps, which are pH-dependent. For analytical purposes, a very resolved diffusion controlled voltammetric peak was obtained in Britton-Robinson buffer at pH 10.0 using differential pulse and square-wave modes. Limits of detection were 1.04 × 10⁻⁷ and 4.60 × 10⁻⁸ M for differential pulse and square-wave voltammetry, respectively. The applicability to direct assays of tablets, spiked human serum and simulated gastric fluid, was described.     

Electrochemical study and polarographic determination of ranitidine

In the polarographic reduction of ranitidine, an H₂-antagonist of histamine, three waves are observed; their half-wave potentials and limiting currents depend strongly on the pH of the solution. The first and second waves are due to reduction of teh protonated, CHNO₂H⁺, and unprotonated, CHNO₂, nitroethene group of ranitidine, respectively; the origin of the third wave is unknown. The characteristics of the second and third waves are studied in acetic acid/acetate buffer at pH 5.5; the first wave does not appear at this pH. The second wave (E_{$\text{1}\text{2}$} = ?0.90 V, vs. Ag/AgCl) is useful for determining ranitidine in the range 2.4–4.9 × 10^?4 M by direct current polarography and in the range 2.5 × 10^?7?2.05 × 10^?5 M by differential pulse polarography.     

Graphene Paste Electrode: Analytical Applications for the Quantification of Dopamine,Phenolic Compounds and Ethanol

This work reports the analytical applications of a graphene paste electrode (GrPE) for the quantification of dopamine, ethanol and phenolic compounds. Dopamine was detected by differential pulse voltammetry‐adsorptive stripping with medium exchange at submicromolar levels even in the presence of high excess of ascorbic acid and serotonin. The electrocatalytic activity of graphene towards the oxidation of NADH and the reduction of quinones allowed the sensitive amperometric determination of ethanol and phenols using GrPE modified with alcohol dehydrogenase/NAD⁺ or polyphenol oxidase, respectively, with successful applications in real samples like alcoholic beverages and tea.