Nicotinic Acid Esters Research Articles

Attraction of thrips by salt forms of alkyl nicotinates and isonicotinates

Methyl and ethyl esters of nicotinic and isonicotinic acids are effective attractants for catching thrips. The salt forms of nicotinic and isonicotinic acid esters dissociate in air and gradually secrete an attractant. The use of salt forms of alkyl nicotinates or isonicotinates as thrips attractants has not been previously described. The efficiency of catching Western flower thrips in the closed ground on cucumber crops and aster flowers on blue sticky traps with dispensers containing salts of hydrochloric, phosphoric, citric and trifluoroacetic acids with methyl nicotinate was investigated. It was found that the use of dispensers with salt forms of methylnicotinate increased the catch of Western flower thrips by 1.8–2.3 times during 1 month. Dispensers with citrates of ethyl and methyl esters of nicotinic and isonicotinic acids increased the catch of thrips by 1.7–2.2 times.

Toxicity of tobacco Nicotiana tabacum Linnaeus (Solanaceae) leaf extracts to the rice weevil Sitophilus oryzae Linnaeus 1763 (Coleoptera: Curculionidae)

BackgroundRice is the agricultural commodity with the third highest worldwide production and losses in rice storage due to insect pests may drastically affect the food availability for a large number of people. One of its serious pests in tropics is the “rice weevil”, Sitophilus oryzae. Although few effective fumigants are available, there is a global concern about environmental pollution, toxicity to non-target organisms, and pesticide residues. Botanicals are a promising source of pest control compounds as the pool of plants possessing insecticidal substances are enormous and have generated extraordinary interest in recent years as potential sources of natural insect control agents. The present study was conducted to determine the toxicity of tobacco (Nicotiana tabacum) leaf extracts as an alternative to synthetic insecticides in the management of rice weevils. The crude solvent leaf extracts of N. tabacum were analyzed for its phytochemical compounds via GC-MS and tested for its toxicity to the adults of S. oryzae at concentrations of 0.625, 1.25, 2.50, 5.00, 10.00, and 20.00 mg/L by Petri dish bioassay method. Adult mortality was calculated 24, 48, and 72 h after exposure.ResultsThe constituent compounds in the chloroform and acetone revealed 13 compounds represented by benzene carboxylic acid ester, alkaloids, and steroids. The major compounds found in GC-MS analysis are nicotine, nicotinonitrile, nornicotine, nicotinic acid, neonicotine, cotinine, indole, farnesol, sclareol, 9,12-octadecadienoic acid, squalene, palmitic acid, and 15-tetracosenoic acid methyl ester. Overall assessment indicates that the chloroform and acetone extracts of N. tabacum leaves exhibited the highest adulticidal activity against S. oryzae. Their LD50 values were 1.62, 0.64, and 0.48; 1.54, 0.83, and 0.48 mg/L after 24, 48, and 72 h of exposure respectively. In addition, the petroleum ether extract also indicated high adult mortality with LD50 value of 0.53 mg/L only at 72 h.ConclusionsThe present research has documented its first report for the management of rice weevil using tobacco leaf extracts, and therefore on the basis of the results of the present study, it is highly encouraging to note that the use of tobacco leaf extracts with its excellent insecticidal activity owing to the action of nicotine would be economically feasible to alleviate the rice weevil problem.

Synthesis of functionally substituted esters of nicotinic and isonicotinic acid

Nicotinic (3-pyridinecarboxylic) acid is one of the most important vitamins as well as substance with versatile physiological activity, since it plays significant role in the human organism. Nicotinic acid (niacin, vitamin PP, vitamin B3) is widely used in medicine as drug which has a vasodilating effect, prevents the accumulation of cholesterol and normalizes the heart function. Isonicotinic (4-pyridinecarboxylic) acid is used for the production of valuable anti-tuberculosis drugs, antidepressants, etc. However, nicotinic and isonicotinic acids have various side effects: skin hyperemia, pruritus, formation of stomach peptic ulcers, liver dysfunction and hyperglycemia. To reduce the severity of side effects, their slightly soluble salts, esters or amides are obtained. The synthesis of nicotinic and isonicotinic acid esters with vanillin benzaldehydes, cholesterol, 8-hydroxyquinoline, quinine and (4,5-dichloroisothiazol-3-yl)methanol is described. Esters were obtained by acylation of hydroxybenzaldehydes and alcohols with nicotinic and isonicotinic acid hydrochlorides in the presence of triethylamine in anhydrous methylene chloride. The IR- UV- and NMR spectra of the compounds obtained are presented.

The Rise and Fall "ing" of the HDL Hypothesis.

Earlier epidemiological studies have shown an inverse correlation between high-density lipoprotein cholesterol (HDLc) and coronary heart disease (CHD). This observation along with the finding that reverse cholesterol transport is mediated by HDL, supported the hypothesis that the HDL molecule has a cardioprotective role. More recently, epidemiological data suggest a U-shaped curve correlating HDLc and CHD. In addition, randomized clinical trials of drugs that significantly increase plasma HDLc levels, such as nicotinic acid and cholesterol ester transfer protein (CETP) inhibitors failed to show a reduction in major adverse cardiovascular events. These observations challenge the hypothesis that HDL has a cardioprotective role. It is possible that HDL quality and function is optimal only when de novo synthesis of apo A-I occurs. Inhibition of turnover of HDL with currently available agents yields HDL molecules that are ineffective in reverse cholesterol transport. To test this hypothesis, newer therapeutic drugs that increase de novo production of HDL and apo A-I should be tested in clinical trials.

Formal [3 + 3] Cycloaddition Reactions between Electron-Deficient Cyclopropenes and Hydrazones: A Route to Alkyl 1,4,5,6-Tetrahydropyridazine-3-carboxylates.

A formal [3 + 3] cycloaddition reaction between alkyl 2-aroyl-1-chlorocyclopropanecarboxylates 1 and common hydrazones 2 was developed. This process readily proceeded in diastereo- and regioselective fashion and gave alkyl 1,4,5,6-tetrahydropyridazine-3-carboxylates in high yields under mild basic conditions. Treatment of the annulation products with DDQ leads to the formation of functionalized pyridazine-3-carboxylates, analogues of nicotinic acid ester.

Ru-Catalyzed Regioselective Cascade Annulation of Acrylamides with 2-Alkynoates for the Synthesis of Various 6-Oxo Nicotinic Acid Esters.

Ru-catalyzed regioselective cascade annulation of acrylamides with 2-alkynoates via aza-Michael/C-H activation sequence for the synthesis of various 6-oxo nicotinic acid esters is described. The regioselectivity of the protocol has been confirmed by performing silver mediated protodecarboxylation of the corresponding 6-oxo nicotinic acid to furnish 2-pyridone. The developed protocol is copper or silver salt-free and uses inexpensive, safe, and environmentally benign peroxide-based "oxone" as the sole oxidant. A redox-neutral version of the protocol is also demonstrated.

Synthesis of Some New 1,3,4-Thiadiazole, Thiazole and Pyridine Derivatives Containing 1,2,3-Triazole Moiety.

In this study, 1-(5-Methyl-1-(p-tolyl)-1H-1,2,3-triazol-4-yl)ethan-1-one, was reacted with Thiosemicarbazide, alkyl carbodithioate and benzaldehyde to give thiosemicarbazone, alkylidenehydrazinecarbodithioate and 3-phenylprop-2-en-1-one-1,2,3-triazole derivatives. The 1,3,4-thiadiazole derivatives containing the 1,2,3-triazole moiety were obtained via reaction of alkylidenecarbodithioate with hydrazonoyl halides. Also, hydrazonoyl halides were reacted with thiosemicarbazone and pyrazolylthioamide to give 1,3-thiazoles derivatives. Subsequently, 3-phenyl-2-en-1-one was used to synthesize substituted pyridines and substituted nicotinic acid ester. The latter was converted to its azide compound which was reacted with aromatic amines and phenol to give substituted urea and phenylcarbamate containing 1,2,3-triazole moiety. The newly synthesized compounds were established by elemental analysis, spectral data and alternative synthesis whenever possible.

Permeability of novel 4′-N-substituted (aminomethyl) benzoate-7-substituted nicotinic acid ester derivatives of scutellarein in Caco-2 cells and in an in vitro model of the blood-brain barrier

A series of 4′-N-substituted (aminomethyl) benzoate-7-substituted nicotinic acid ester derivatives of scutellarein was designed and synthesized. Evaluation of physiochemical properties showed that the newly designed compounds had greater chemical stability and aqueous solubility than scutellarin or scutellarein. The permeabilities (P app AP to BL) of compounds 7b and 7e in Caco-2 cells were 5.9-fold and 3.7-fold higher than that of scutellarin, and 3.7-fold and 2.4-fold higher than that of scutellarein. The permeabilities (P app AP to BL) of compounds 7b and 7e in an in vitro model of the blood–brain barrier were 9.7-fold and 5.9-fold higher than that of scutellarin, and 9.2-fold and 5.6-fold higher than that of scutellarein.

The effect of lipid and aqueous solubilities on flux of nicotinic acid esters from water through silicone membrane

Objective: The maximum fluxes (JM) of nicotinic acid esters (NAE) across silicone membranes from water (JMPAQ) have been measured to determine how well they correlate with JM of NAE across human skin from water in vitro (JMHAQ) and in vivo (JMHAQ1) and with JM of NAE across hairless mouse skin from water (JMMAQ).Materials and methods: The NAE were all commercially available. Solubilities in water (SAQ), isopropyl myristate (SIPM) and octanol (SOCT) were obtained from literature sources. JMPAQ were measured at saturation for all the esters except the methyl ester. In that case, flux was measured at a concentration (C) less than saturation (JPAQ) and converted to JMPAQ = (JPAQ)(SAQ/CAQ).Results and discussion: JMPAQ values predicted from the previously reported coefficients to the parameters in the Roberts-Sloan (RS) equation (PRE JMPAQ) were substantially lower than the experimental JMPAQ values (EXP JMPAQ) values obtained here. The EXP JMPAQ were incorporated into the previous JMPAQ database and new coefficients were obtained: x = −1.837; y = 0.742; z = 0.00435; r2 = 0.86. Correlation of JMPAQ values with JMHAQ, JMHAQ1, and JMMAQ values show the same trend as the JMPAQ values.Conclusions: The inclusion of the NAE n = 6 data into the previous n = 32 database for the permeation of the prodrugs through a silicone membrane from water (JMPAQ) greatly improved the fit of the n = 38 database to the RS equation: r2 = 0.86 vs r2 = 0.77. The correlation between log JMHAQ and log JMPAQ gave r2 = 0.98. This suggests that JMPAQ values are good predictors of JMHAQ values.

Percutaneous absorption: in vivo experiments.

The percutaneous absorption of esters of nicotinic acid has been studied in vivo in man. The time for erythema to be produced has been measured both when the ester is applied continuously and in 'pulse' experiments when the ester is removed before the erythema develops. The results show that the erythema is produced long before steady state diffusion across the epidermis is established and the penetration of methyl nicotinate is characterized by D/l2 = 2.3 X 10(-4)s-1 where D is the diffusion coefficient and l the thickness of the barrier. Results using glycerol water mixtures in the external phase show that the route of penetration for methyl nicotinate is through the interstitial channels and not through the keratinized cells. Data for absorption from various creams and ointments (Barrett et al 1964) show that the route is independent of the nature of the external phase. Steady state data for the absorption of salicylic acid and carbinoxamine through the abdominal skin of guinea-pigs (Arita et al 1970) show that the route of penetration does not change as the experiment proceeds. Data for the absorption of other substances (Michaels et al 1975) also fit the interstitial route.

In-vitro permeability of the human nail and of a keratin membrane from bovine hooves: influence of the partition coefficient octanol/water and the water solubility of drugs on their permeability and maximum flux.

Penetration of homologous nicotinic acid esters through the human nail and a keratin membrane from bovine hooves was investigated by modified Franz diffusion cells in-vitro to study the transport mechanism. The partition coefficient octanol/water PCOct/W of the esters was over the range 7 to > 51,000. The permeability coefficient P of the nail plate as well as the hoof membrane did not increase with increasing partition coefficient or lipophilicity of the penetrating substance. This indicates that both barriers behave like hydrophilic gel membranes rather than lipophilic partition membranes as in the case of the stratum corneum. Penetration studies with the model compounds paracetamol and phenacetin showed that the maximum flux was first a function of the drug solubility in water or in the swollen keratin matrix. Dissociation hindered the diffusion of benzoic acid and pyridine through the hoof membrane. Since keratin, a protein with an isoelectric point of about 5, is also charged, this reduction can be attributed to an exclusion of the dissociating substance due to the Donnan equilibrium. Nevertheless, the simultaneous enhancement of the water solubility makes a distinct increase of the maximum flux possible. In order to screen drugs for potential topical application to the nail plate, attention has to be paid mainly to the water solubility of the compound. The bovine hoof membrane may serve as an appropriate model for the nail.

Fluorophilicity of alkyl and polyfluoroalkyl nicotinic acid ester prodrugs

The fluorophilicity of a series of hydrocarbon and fluorocarbon-functionalized nicotinic acid esters (nicotinates) is measured from their partitioning behavior (log K p) in the biphasic solvent system of perfluoro(methylcyclohexane) (PFMC) and toluene. The chain length of the hydrocarbon or fluorocarbon alkyl group of the ester ranges from one to twelve carbon atoms. Knowledge of the fluorophilicity of these solutes is relevant to the design of these prodrugs for fluorocarbon-based drug delivery. The experimental log K p values range from −1.72 to −3.40 for the hydrocarbon nicotinates and −1.64 to 0.13 for the fluorinated nicotinates, where only the prodrug with the longest fluorinated chain (2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl nicotinic acid ester) partitions preferentially into the fluorinated phase (log K p = 0.13). Predictions of the partition coefficients using solubility parameters calculated from group contribution techniques or molecular dynamics simulation are in reasonable agreement for the perhydrocarbon nicotinates and short chained perfluorinated nicotinates (≈0.3–39% deviation). Significant deviations from experimental partition coefficients (greater than 100%) are observed for the longest chain perfluoroalkyl nicotinates.

Partitioning of homologous nicotinic acid ester prodrugs (nicotinates) into dipalmitoylphosphatidylcholine (DPPC) membrane bilayers

The partitioning behavior of a series of perhydrocarbon nicotinic acid esters (nicotinates) between aqueous solution and dipalmitoylphosphatidylcholine (DPPC) membrane bilayers is investigated as a function of increasing alkyl chain length. The hydrocarbon nicotinates represent putative prodrugs, derivatives of the polar drug nicotinic acid, whose functionalization provides the hydrophobic character necessary for pulmonary delivery in a hydrophobic, fluorocarbon solvent, such as perfluorooctyl bromide. Independent techniques of differential scanning calorimetry and 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence anisotropy measurements are used to analyze the thermotropic phase behavior and lipid bilayer fluidity as a function of nicotinate concentration. At increasing concentrations of nicotinates over the DPPC mole fraction range examined ( X DPPC = 0.6–1.0), all the nicotinates (ethyl (C2H5); butyl (C4H9); hexyl (C6H13); and octyl (C8H17)) partition into the lipid bilayer at sufficient levels to eliminate the pretransition, and decrease and broaden the gel to fluid phase transition temperature. The concentration at which these effects occur is chain length-dependent; the shortest chain nicotinate, C2H5, elicits the least dramatic response. Similarly, the DPH anisotropy results demonstrate an alteration of the bilayer organization in the liposomes as a consequence of the chain length-dependent partitioning of the nicotinates into DPPC bilayers. The membrane partition coefficients (logarithm values), determined from the depressed bilayer phase transition temperatures, increase from 2.18 for C2H5 to 5.25 for C8H17. The DPPC membrane/water partitioning of the perhydrocarbon nicotinate series correlates with trends in the octanol/water partitioning of these solutes, suggesting that their incorporation into the bilayer is driven by increasing hydrophobicity.

Regioselective addition of organomagnesium reagents to N-silyl activated nicotinic acid esters—a convenient method for the synthesis of 4,4-disubstituted 1,4-dihydronicotinates

The addition of RMgX or R 2Mg to nicotinic acid esters, activated with triisopropylsilyl triflate, was investigated. The regioselectivity of this reaction, where 4-unsubstituted and 4-substituted pyridine derivatives were employed as starting materials, was examined. Depending on the structure of the organomagnesium reagent varying ratios of 1,2-, 1,4-, and 1,6-regioisomers were obtained but in any case the 1,4-addition products were clearly predominating. Desilylation of the isomeric pure addition products with LiOH provided stable 4,4-disubstituted dihydropyridines, which could be easily N-alkylated with alkylhalides in high yield. Furthermore, it was shown that the silyl protecting group of the obtained dihydropyridines can be easily replaced by an acyl group.

Functional sulfur-containing compounds

The Thorpe-Ziegler intramolecular cyclization of 2-RCH2S-, 2-RCH2S(O)-, and 2-RCH2SO2-substituted nicotinic acid esters and nitriles (R is alkyl, aryl, and 2-thienyl) upon the action of potassium tert-butoxide has been studied. The reaction results in the formation of the corresponding 2-R-substituted 3-aminothieno[2,3-b]pyridines, 3-aminothieno[2,3-b]pyridine 1-oxides, and 3-aminothieno[2,3-b]pyridine 1,1-dioxides with the reaction taking place only in the case if R is aryl or 2-thienyl. Methyl esters of 2-RCH2S-, 2-RCH2S(O)-, and 2-RCH2SO2-substituted nicotinic acids also undergo the intramolecular cyclization of the Dieckmann type to form the corresponding 2-R-substituted 3-hydroxythieno[2,3-b]pyridines, thieno[2,3-b]pyridin-3(2H)-one 1-oxides, and thieno[2,3-b]pyridin-3(2H)-one 1,1-dioxides. Such a reaction takes place for all the R groups except when R = AlkCH2S and AlkCH2S(O).

Influence of Impregnation of a Mixture of Silica Gel and Kieselguhr with Copper (II) Sulphate (VI) on Profile Change of the Spectrodensitograms and the RF Values of Nicotinic Acid and its Derivatives

Nicotinic acid and its derivatives were separated using adsorption TLC on aluminum plates precoated with a mixture of silica gel 60 and kieselguhr F254 non impregnated and impregnated with 2.5% and 5% aqueous solutions of CuSO4. The mixture of an acetone‐n‐hexane in different volume compositions was used as mobile phase. The obtained results of the investigations concerning the evaluation of chromatographic separations were considered in three groups, namely, 1st group: nicotinic acid (1), methyl nicotinate (2), ethyl nicotinate (3), isopropyl nicotinate (4), butyl nicotinate (5), hexyl nicotinate (6), benzyl nicotinate (7); 2nd group: nicotinic acid (1), nicotinamide (8), N‐methylnicotinamide (9), N,N‐diethylnicotinamide (10); and 3rd group: nicotinic acid (1), nicotinamide (8), 3‐pyridinecarbaldehyde (11), 3‐pyridinecarbonitrile (12), 3‐pyridylmethanol (13), methyl 3‐pyridyl ketone (14). Impregnation of the mixture of silica gel and kieselguhr with 2.5% and 5% aqueous solutions of CuSO4 causes a decrease of the RF values of the 1st, 2nd, and 3rd groups of the compounds in relation to the RF values of these substances obtained on non impregnated solid support. Impregnation with a water solution of copper (II) sulphate (VI) of the mixture of silica gel and kieselguhr worsens the separation of the 2nd and 3rd groups of the investigated compounds. Improvement of the chromatographic separations is observed only in the case of esters of nicotinic acid (1st group of compounds). It was found that the spectrodensitograms of the studied compounds on impregnated plates and by use of Rhodamine B as visualizing reagent were different than the spectrodensitograms obtained on the plates without use of a visualizing reagent. Spectra of the investigated compounds on non impregnated plates and impregnated with aqueous solution of CuSO4 and without the use of a visualizing reagent have two or three absorption bands. Investigated compounds on the plates impregnated with the aqueous solution of CuSO4 and by use of Rhodamine B as visualizing reagent have one additional absorption band at λ≈560 nm. This fact has analytical significance in the identification of the investigated substances.

Synthesis, physicochemical properties and in vitro cytotoxicity of nicotinic acid ester prodrugs intended for pulmonary delivery using perfluorooctyl bromide as vehicle

This study explores perfluorooctyl bromide (PFOB) as a potential vehicle for the pulmonary delivery of a series of prodrugs of nicotinic acid using cell culture studies. The prodrugs investigated have PFOB-water (logK(p)=0.78 to >2.2), perfluoromethylcyclohexane-toluene (logK(p)=-2.62 to 0.13) and octanol-water (logK(p)=0.90-10.2) partition coefficients spanning several orders of magnitude. In confluent NCI-H358 human lung cancer cells, the toxicity of prodrugs administered in culture medium or PFOB depends on the medium of administration, with EC20's above 8 mM and 2.5 mM for culture medium and PFOB, respectively. Short-chain nicotinates administered both in PFOB and medium increase cellular NAD/NADP levels at 1mM nicotinate concentrations. Long-chain nicotinates, which could not be administered in medium due to their poor aqueous solubility, increased NAD/NADP levels if administered in PFOB at concentrations > or =10 mM. These findings suggest that even highly lipophilic prodrugs can partition out of the PFOB phase into cells, where nicotinic acid is released and converted to NAD. Thus, PFOB may be a novel and biocompatible vehicle for the delivery of lipophilic prodrugs of nicotinic acid and other drugs directly to the lung of laboratory animals and humans.

Pyridine-2(1H)-thione in Heterocyclic Synthesis: Synthesis of Some New Nicotinic Acid Ester, Thieno[2, 3-b]pyridine, Pyrido[3′, 2′: 4, 5]thieno [3, 2-d]pyrimidine, and Thiazolylpyrazolo[3, 4-b]pyridine Derivatives

Nicotinic acid esters 3a–c were prepared by the reaction of pyridine-2(1H)-thione derivative 1 with α-halo-reagents 2a–c. Compounds 3a–c underwent cyclization to the corresponding thieno[2, 3-b]pyridines 4a–c via boiling in ethanol/piperidine solution. Compounds 4a–c condensed with dimethylformamide-dimethylacetal (DMF-DMA) to afford 3-{[(N,N-dimethylamino)methylene]amino}thieno[2, 3-b]- pyridine derivatives 6a–c. Moreover, compounds 4a–c and 6a–c reacted with different reagents and afforded the pyrido[3′,2′:4, 5]thieno[3, 2-d]pyrimidine derivatives 10a–d, 11a–c, 12a,b, 14a,b, 17, and 19. In addition, pyrazolo[3, 4-b]pyridine derivative 20 (formed via the reaction of 1 with hydrazine hydrate) reacted with ethylisothiocyanate yielded the thiourea derivative 21. Compound 21 reacted with α-halocarbonyl compounds to give the 3-[(3H-thiazol-2-ylidene)amino]-1H-pyrazolo[3, 4-b]pyridine derivatives 23a–c, 25, and 27a,b.

Interaction of long-chain nicotinates with dipalmitoylphosphatidylcholine

The interaction of four long-chain nicotinates, compounds that are of interest as potential chemopreventive agents, with dipalmitoylphosphatidylcholine (DPPC) was investigated in monolayers at the air-water interface and in fully hydrated bilayers. For the monolayer studies, the compression isotherms of mixtures of the respective nicotinate with DPPC were recorded at various compositions on a hydrochloric acid subphase (pH 1.9-2.1, 37 +/- 2 degrees C). The headgroup of the nicotinates (24-29 A2/molecule) is larger than that of the hydrophobic tail (20 A2/molecule). The pure nicotinates exhibit a temperature- and chain length-dependent transition from an expanded to a condensed phase. Analysis of the concentration dependence of the average molecular area at constant film pressure and the concentration dependence of the breakpoint of the phase transition from the expanded to the condensed state suggests that all four DPPC-nicotinate mixtures are partially miscible at the air-water interface. Although a complex phase behavior with several phase transitions was observed, differential scanning calorimetry studies of the four mixtures are also indicative of the partial miscibility of DPPC and the respective nicotinate. Overall, the complex phase behavior most likely results from the head-tail mismatch of the nicotinates and the geometric packing constraints in the two-component lipid bilayer.

Mass spectrometric decompositions of copper complexes with esters and amides of nicotinic acid

Mass spectrometric decompositions of complexes of nicotinic acid esters and amides with copper are discussed. Liquid secondary ion mass spectrometry (LSIMS) was used as an ionisation technique and metastable ion spectra were recorded by using B/E linked scans. The fragmentation pathways consist mainly of the loss of one ligand molecule and also the cleavage of amide or ester bonds. It may be possible, on the basis of cleaved neutral fragments, to determine the site of coordination in the gas phase. It was also found that the presence of a chlorine atom affects the fragmentation pathways of complexes.