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Riga, Latvia

Riga Technical University is the oldest technical university in the Baltics established on October 14, 1862. is located in Riga, Latvia. Wikipedia.

Sadyrbaeva T.Z.,Riga Technical University
Separation and Purification Technology | Year: 2012

The process of gold(III) ions removal from hydrochloric solutions containing 0.01 mol L -1 HAuCl 4 accompanied with metal electrodeposition from cathodic non-cyanide solutions is studied at galvanostatic electrodialysis. A possibility of gold(III) electrodeposition from 0.1 mol L -1 solutions of sulphuric, nitric, hydrochloric, perchloric acids and ammonia is demonstrated. The effects of the main electrodialysis parameters as well as of the composition of the cathodic solution on the gold(III) transport and electrodeposition rate are determined. It is shown that the gold(III) electrodeposition degree increases as the current density (0-6 mA/cm 2) and the process duration (0-3 h) increase. The gold electrodeposits with good adhesion and of bright appearance are obtained in all studied systems. © 2011 Elsevier B.V. All rights reserved. Source

Mierina I.,Riga Technical University
Synlett | Year: 2014

(A) Hydrolysis of Meldrum's acid 2 leads to malonic or acetic acid derivatives.5 Radical reduction of compound 2 with SmI2 and H2O forms 3-hydroxypropanoic acids 4 selectively. The first step is activation of Meldrum's acid via coordination of SmI2 to the carbonyl group, followed by electron transfer.6 (B) Detz et al.7 have reported that dimethyl malonate 5 can be obtained from propargylic derivative 2. The authors propose that the first step of the reaction cascade is copper-mediated addition of Meldrum's acid 2 to the triple bond and sequential methanolysis of the dioxane cycle forming lactone 6, which is further cleaved with methoxide leading to compound 5. (C) -Substituted aldehydes 7 can be synthesized by Lewis base promoted hydrosilylation of Meldrum's acids 1 with phenylsilane, followed by hydrolysis. In situ treatment of aldehyde 7 with an amine and sequential hydrogenation of the formed imine with H2 in the presence of Pd/C or with NaBH(OAc)3 gives γ-substituted amines 8.8 (D) 5-Alkyl Meldrum's acids 1 are used for the rapid synthesis of 2-alkyl acrylates 9 via Mannich-type reactions. The advantage of the method is the clean conversion into products due to the formation of volatile by-products - acetone, carbon dioxide, and dimethylamine.9 (E) Derivatives of 5-(but-3-enyl) Meldrum's acid 10 are suitable for the synthesis of cyclopentanols 11 via radical cyclization upon treatment with SmI2 in H2O.6 Exo-trig/exo-trig radical cyclization cascade occurs, when substituent R2 is an alkene or alkyne; such a transformation gives fused bicyclic system 12.10 (F) Meldrum's acid can act as a carbon-based leaving group. Catalytic hydrogenolysis of Meldrum's acids 2 (R1 = H) is an excellent route for the synthesis of compounds 13 both with secondary and tertiary benzylic stereocenters in 65-96% yield under mild reaction conditions. As the reaction proceeds with inversion at the stereocenter, an SN2 mechanism is proposed.11 Treatment of Meldrum's acid derivatives 2 (R1 = H, Me) with nucleophiles in the presence of Lewis acids furnishes compounds 14 and 15. The yields vary from 51% to quantitative.12 (G) Johnson and co-workers reported the hydroperoxidation of 5- alkyl Meldrum's acids 1 with O2 in the presence of Cu(NO3)2.13 These reaction conditions are compatible with unsaturated bonds in substituent R1. The peroxides 16 are suitable for intramolecular oxidation of unsaturated bonds via electrophilic activation; such an approach was used for the synthesis of lactones 17 and 18. (H) Addition of 5-substituted Meldrum's acid 1 to prop-2-ynal 19 forms 3-(1,3-dioxan-5-yl)-4,4-dimethoxy-but-2-enal. The thermolysis of 2,3-unsaturated aldehyde intermediate provides a synthetic procedure to 2H-pyran-2-one 20. Hydrolysis of acetal moiety results in 4-formyl pyran-2-one 21.14 (I) Copper- and iron-15 or silver-catalyzed16 tandem cyclization- hydrolysis-decarboxylation of 5-propargyl Meldrum's acid 2 is an efficient approach for the synthesis of Z-γ-alkylidene lactones 22. The compatibility of copper(I) and iron(III) is not established yet. The authors15 suggest that copper(I) activates the alkyne moiety, but iron(III) interacts with the oxygen atom. (J) Intermolecular cleavage of Meldrum's acid derivatives with nucleophiles is well known. Sapi et al.17 reported the tandem deprotection- intramolecular cyclization of Meldrum's acids 23 applied for the synthesis of lactones 24a and lactame 24b. © Georg Thieme Verlag Stuttgart · New York. Source

Age-related, diet-related and protein conformational diseases, such as atherosclerosis, diabetes mellitus, cancer, hypercholesterolemia, cardiovascular and neurodegenerative diseases are common in the elderly population. The potential of chitosan, chitooligosaccharides and their derivatives in prevention and treatment of age-related dysfunctions is reviewed and discussed in this paper. The influence of oxidative stress, low density lipoprotein oxidation, increase of tissue stiffness, protein conformational changes, aging-associated chronic inflammation and their pathobiological significance have been considered. The chitosan-based functional food also has been reviewed. © 2015 by the authors; licensee MDPI. Source

Stepins D.,Riga Technical University
IEEE Transactions on Power Electronics | Year: 2016

This paper deals with switching-frequency modulation (SFM) for conducted electromagnetic interference (EMI) suppression in active power factor correctors (PFCs) operating in discontinuous conduction mode (DCM). The effect of SFM on different performance characteristics of DCM PFC is studied in details. It is shown that the use of SFM in PFC with conventional control approach leads to significant increase in total harmonic distortion (THD) and harmonic content of input current. Moreover, SFM leads to noticeable increase in peak power components currents. In order to use SFM for EMI reduction more effectively without significantly worsening THD, power factor, and harmonic content of DCM PFC, an improved control technique and a control circuit are proposed and verified by simulations and experimentally. The technique is based on the use of specific time delay injection to power MOSFET control signal. As show simulations and experiments, SFM PFC with the proposed delay technique in comparison to PFC without SFM has noticeably lower conducted EMI and THD of input current, at the cost of slightly decreased efficiency and moderately increased peak currents. © 1986-2012 IEEE. Source

Sadyrbaeva T.Z.,Riga Technical University
Journal of Electroanalytical Chemistry | Year: 2010

The process of Ag+ ions extraction from nitric solutions by bulk liquid membranes containing di(2-ethylhexyl)phosphoric acid and tri-n-octylamine accompanied with silver(I) electrodeposition from perchloric acid solutions is studied at galvanostatic electrodialysis. The effects of the current density as well as of composition of the liquid membrane and aqueous solutions on the rate of the silver(I) transport are determined. More than 90% of silver ions was extracted from the feed solution containing 0.01 mol l -1 AgNO3 and about 40% of metal was electrodeposited during 1 h of electrodialysis. Sound adherent silver coatings have been deposited on platinum and copper cathodes. © 2010 Elsevier B.V. All rights reserved. Source

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