Now Is The Time For You To Know The Truth About Phenyl(pyridin-2-yl)methanone

About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Sahin, E or concate me.. COA of Formula: C12H9NO

I found the field of Chemistry very interesting. Saw the article Production of enantiopure chiral aryl heteroaryl carbinols using whole-cell Lactobacillus paracasei biotransformation published in 2020.0. COA of Formula: C12H9NO, Reprint Addresses Sahin, E (corresponding author), Bayburt Univ, Dept Food Engn, Fac Engn, TR-69000 Bayburt, Turkey.. The CAS is 91-02-1. Through research, I have a further understanding and discovery of Phenyl(pyridin-2-yl)methanone

Aryl and heteroaryl chiral carbinols are useful precursors in the synthesis of drugs. Lactobacillus paracasei BD87E6, which is obtained from a cereal based fermented beverage, was investigated as whole cell biocatalyst for the bioreduction of different ketones (including aromatic, hetero-aromatic and fused bicyclic ketone) into chiral carbinols, which can be used as a pharmaceutical intermediate. The study shows that bioreduction of aryl, heteroaryl and fused bicyclic ketone (1-5) to their corresponding chiral carbinols (1a-5a) in excellent enantioselectivity (>99%) with high yields. This study gave the first example for an enantiopure production of (S)-6-chlorochroman-4-ol (3a), which has many antioxidant activity, by a biological method. For asymmetric bioreduction of other prochiral ketones, these results open way to use of L. paracasei BD87E6 as biocatalysts. Also, the present process shows a hopeful and alternative green synthesis for the production of enantiopure carbinols in a mild, inexpensive and environmentally friendly process.

About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Sahin, E or concate me.. COA of Formula: C12H9NO

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

Interesting scientific research on C8H9NO2

Computed Properties of C8H9NO2. About Ethyl nicotinate, If you have any questions, you can contact Fairley, M; Bole, LJ; Mulks, FF; Main, L; Kennedy, AR; O’Hara, CT; Garcia-Alvarez, J; Hevia, E or concate me.

Authors Fairley, M; Bole, LJ; Mulks, FF; Main, L; Kennedy, AR; O’Hara, CT; Garcia-Alvarez, J; Hevia, E in ROYAL SOC CHEMISTRY published article about DEEP EUTECTIC SOLVENTS; POLAR ORGANOMETALLIC COMPOUNDS; EXTERNAL CALIBRATION CURVES; CHEMISTRY RESEARCH AREAS; CHEMOSELECTIVE ADDITION; COUPLING REAGENTS; BOND FORMATION; DOSY-NMR; REACTIVITY; PEPTIDE in [Fairley, Michael; Mulks, Florian F.; Main, Laura; Kennedy, Alan R.; O’Hara, Charles T.; Hevia, Eva] Univ Strathclyde, Dept Pure & Appl Chem, Glasgow G1 1XL, Lanark, Scotland; [Bole, Leonie J.; Mulks, Florian F.; Hevia, Eva] Univ Bern, Dept Chem & Biochem, CH-3012 Bern, Switzerland; [Garcia-Alvarez, Joaquin] Univ Oviedo, Lab Compuestos Organomet & Catalisis, Ctr Innovac Quim Avanzada ORFEO CINQA, Dept Quim Organ & Inorgan IUQOEM,Fac Quim,Unidad, E-33071 Oviedo, Spain in 2020, Cited 97. Computed Properties of C8H9NO2. The Name is Ethyl nicotinate. Through research, I have a further understanding and discovery of 614-18-6

Lithium amides constitute one of the most commonly used classes of reagents in synthetic chemistry. However, despite having many applications, their use is handicapped by the requirement of low temperatures, in order to control their reactivity, as well as the need for dry organic solvents and protective inert atmosphere protocols to prevent their fast decomposition. Advancing the development of air- and moisture-compatible polar organometallic chemistry, the chemoselective and ultrafast amidation of esters mediated by lithium amides is reported. Establishing a novel sustainable access to carboxamides, this has been accomplishedviadirect C-O bond cleavage of a range of esters using glycerol or 2-MeTHF as a solvent, in air. High yields and good selectivity are observed while operating at ambient temperature, without the need for transition-metal mediation, and the protocol extends to transamidation processes. Pre-coordination of the organic substrate to the reactive lithium amide as a key step in the amidation processes has been assessed, enabling the structural elucidation of the coordination adduct [{Li(NPh2)(O=CPh(NMe2))}(2)] (8) when toluene is employed as a solvent. No evidence for formation of a complex of this type has been found when using donor THF as a solvent. Structural and spectroscopic insights into the constitution of selected lithium amides in 2-MeTHF are provided that support the involvement of small kinetically activated aggregates that can react rapidly with the organic substrates, favouring the C-O bond cleavage/C-N bond formation processes over competing hydrolysis/degradation of the lithium amides by moisture or air.

Computed Properties of C8H9NO2. About Ethyl nicotinate, If you have any questions, you can contact Fairley, M; Bole, LJ; Mulks, FF; Main, L; Kennedy, AR; O’Hara, CT; Garcia-Alvarez, J; Hevia, E or concate me.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

An overview of features, applications of compound:91-02-1

Application In Synthesis of Phenyl(pyridin-2-yl)methanone. About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Tan, J; Liu, XB; Chen, WF; Hu, YL or concate me.

An article Synthesis of Magnetically Separable Nanocatalyst CoFe2O4@SiO2@MIL-53(Fe) for Highly Efficient and Selective Oxidation of Alcohols and Benzylic Compounds with Hydrogen Peroxide WOS:000479139200016 published article about METAL-ORGANIC FRAMEWORK; O-IODOXYBENZOIC ACID; TERT-BUTYL HYDROPEROXIDE; LIQUID-PHASE OXIDATION; CATALYZED OXIDATION; SECONDARY ALCOHOLS; ALLYLIC ALCOHOLS; ALDEHYDES; H2O2; COMPOSITE in [Tan, Jin; Chen, Wei Feng; Hu, Yu Lin] China Three Gorges Univ, Coll Mat & Chem Engn, Key Lab Inorgan Nonmetall Crystalline & Energy Co, Yichang 443002, Hubei, Peoples R China; [Liu, Xiao Bing] Jinggangshan Univ, Coll Chem & Chem Engn, Jian 343009, Jiangxi, Peoples R China in 2019.0, Cited 56.0. The Name is Phenyl(pyridin-2-yl)methanone. Through research, I have a further understanding and discovery of 91-02-1. Application In Synthesis of Phenyl(pyridin-2-yl)methanone

A new methodology for the highly efficient and atom-economic oxidation of alcohols and benzylic compounds to the corresponding aldehydes and ketones with environmental-friendly hydrogen peroxide using a magnetic MOF nanocatalyst has been developed. We demonstrated that the resulting magnetic nanocomposite CoFe2O4@SiO2@MIL-53(Fe) can show excellent catalytic performances in the highly selective oxidation at room temperature, based on a synergetic effect between iron sites and CoFe2O4@SiO2. The nanocatalyst offers simple operation for recovery and the recycling test showed that it could be reused for successive runs without significant degradation in catalytic activity.

Application In Synthesis of Phenyl(pyridin-2-yl)methanone. About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Tan, J; Liu, XB; Chen, WF; Hu, YL or concate me.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

A new application aboutC8H9NO2

About Ethyl nicotinate, If you have any questions, you can contact Obradors, C; List, B or concate me.. Recommanded Product: Ethyl nicotinate

Recommanded Product: Ethyl nicotinate. In 2021 J AM CHEM SOC published article about PYRIDINES; SUBSTITUTION; NUCLEOPHILES; HETEROCYCLES; DERIVATIVES in [Obradors, Carla; List, Benjamin] Max Planck Inst Kohlenforsch, D-45470 Mulheim, Germany in 2021, Cited 51. The Name is Ethyl nicotinate. Through research, I have a further understanding and discovery of 614-18-6.

Practical, efficient, and general methods for the diversification of N-heterocycles have been a recurrent goal in chemical synthesis due to the ubiquitous influence of these motifs within bioactive frameworks. Here, we describe a direct, catalytic, and selective functionalization of azines via silylium activation. Our catalyst design enables mild conditions and a remarkable functional group tolerance in a one-pot setup.

About Ethyl nicotinate, If you have any questions, you can contact Obradors, C; List, B or concate me.. Recommanded Product: Ethyl nicotinate

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

Properties and Exciting Facts About C12H9NO

About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Sheng, HC; Hu, YH; Zhou, Y; Fan, SM; Cao, Y; Zhao, XX; Yang, WG or concate me.. Recommanded Product: Phenyl(pyridin-2-yl)methanone

I found the field of Chemistry; Engineering; Materials Science very interesting. Saw the article A highly selective ESIPT-based fluorescent probe with a large Stokes shift for the turn-on detection of cysteine and its application in living cells published in 2019.0. Recommanded Product: Phenyl(pyridin-2-yl)methanone, Reprint Addresses Yang, WG (corresponding author), Nanjing Tech Univ, Sch Pharmaceut Sci, 30 South Puzhu Rd, Nanjing 211816, Jiangsu, Peoples R China.. The CAS is 91-02-1. Through research, I have a further understanding and discovery of Phenyl(pyridin-2-yl)methanone

A new imidazo [1,5-alpha]pyridine-based fluorescent probe, named MZC-AC, is developed for the detection of cysteine (Cys) based on an excited state intramolecular proton transfer (ESIPT) mechanism. The probe MZC-AC uses an acrylate moiety as the ESIPT blocking group as well as the recognition site. Upon the treatment with Cys, MZC-AC exhibits a dramatic fluorescence enhancement (85-fold) and a large Stokes shift (166 nm). The limit of detection obtained from fluorescent titration was as low as 0.07 mu M. MZC-AC displays highly sensitive and selective response to Cys over homocysteine (Hcy) and glutathione (GSH). Significantly, MZC-AC could be used to detect Cys in living cells.

About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Sheng, HC; Hu, YH; Zhou, Y; Fan, SM; Cao, Y; Zhao, XX; Yang, WG or concate me.. Recommanded Product: Phenyl(pyridin-2-yl)methanone

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

A new application about91-02-1

Quality Control of Phenyl(pyridin-2-yl)methanone. About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Dall’Anese, A; Fiorindo, M; Olivieri, D; Carfagna, C; Balducci, G; Alessio, E; Durand, J; Milani, B or concate me.

An article Pd-Catalyzed CO/Vinyl Arene Copolymerization: when the Stereochemistry is Controlled by the Comonomer WOS:000575432700017 published article about ETHYLENE/METHYL ACRYLATE COOLIGOMERIZATION; CARBON-MONOXIDE; ALTERNATING COPOLYMERIZATION; CO/STYRENE COPOLYMERIZATION; PALLADIUM CATALYSTS; PD(II) CATALYSTS; COMPLEXES; DIIMINE; LIGANDS; OLEFINS in [Dall’Anese, Anna; Fiorindo, Mattia; Balducci, Gabriele; Alessio, Enzo; Milani, Barbara] Univ Trieste, Dept Chem & Pharmaceut Sci, I-34127 Trieste, Italy; [Olivieri, Diego; Carfagna, Carla] Univ Bologna, Dept Ind Chem Toso Montana, I-40136 Bologna, Italy; [Durand, Jerome] Univ Toulouse, Lab Chim Coordinat UPR CNRS 8241, Composante ENSIACET, F-31030 Toulouse, France in 2020.0, Cited 55.0. Quality Control of Phenyl(pyridin-2-yl)methanone. The Name is Phenyl(pyridin-2-yl)methanone. Through research, I have a further understanding and discovery of 91-02-1

The control of the stereochemistry of macromolecules is a very important goal, and coordination-insertion polymerization is superior with respect to the other polymerization methods for its achievement. In this contribution, we focus on Pd(II) homogeneous catalysts for the stereocontrolled synthesis of CO/vinyl arene polyketones. We developed a library of aldo- and keto-iminopyridine ligands N-N’ that feature an alpha- or beta-naphthyl or anthracenyl moiety on the imino nitrogen atom (N-imm). With such ligands, the Pd(II) monocationic complexes [Pd(CH3)(CH3CN)(N-N’)][PF6] were synthesized. NMR spectroscopy shows that in solution, each complex exists as an equilibrium mixture of cis and trans stereoisomers, the latter having the CH3 ligand opposite to the Pd-N-imm bond. The isomeric population depends on the N-N’ ligand: an almost 1:1 ratio is found for the ketimine complexes, whereas those with the aldimines show a preference for the trans geometry. These complexes generate very efficient catalysts for the CO/vinyl arene copolymerization. Catalyst performances depend both on the nature of N-N’ and of the vinyl arene comonomer. The ketimine-based catalysts are more stable and more productive than the aldimine counterpart, leading to prevailingly syndiotactic macromolecules of high M-w (up to 280 kDa). The aldimine derivatives produce copolymers with isotactic and syndiotactic stereoblocks of different lengths depending on the vinyl arene. The effect of the prochiral monomer on the copolymer tacticity is steric in nature as demonstrated by the stereochemistry of the obtained CO/4-fluorostyrene polyketone, whose synthesis is reported here for the first time. As a conclusion, we have now demonstrated that when catalysts with nonsymmetric ancillary ligands are used, and stereoisomers are present, the stereochemistry of the copolymerization is driven by both the catalyst isomeric distribution and the prochiral comonomer.

Quality Control of Phenyl(pyridin-2-yl)methanone. About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Dall’Anese, A; Fiorindo, M; Olivieri, D; Carfagna, C; Balducci, G; Alessio, E; Durand, J; Milani, B or concate me.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

New explortion of Phenyl(pyridin-2-yl)methanone

Safety of Phenyl(pyridin-2-yl)methanone. About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Chatterjee, A; Kaur, G; Joshi, M; Choudhury, AR; Ghosh, R or concate me.

Authors Chatterjee, A; Kaur, G; Joshi, M; Choudhury, AR; Ghosh, R in ELSEVIER SCIENCE SA published article about COPPER(II) COMPLEXES; DICOPPER(II) COMPLEXES; PERCHLORATE REDUCTION; MAGNETIC-PROPERTIES; OXIDASE; LIGAND; DINUCLEAR; MODELS; SERIES in [Chatterjee, Arnab; Ghosh, Rajarshi] Univ Burdwan, Dept Chem, Burdwan 713104, W Bengal, India; [Kaur, Gurpreet; Joshi, Mayank; Choudhury, Angshuman Roy] Indian Inst Sci Educ & Res Mohali, Dept Chem Sci, Manauli 140306, India in 2020.0, Cited 29.0. Safety of Phenyl(pyridin-2-yl)methanone. The Name is Phenyl(pyridin-2-yl)methanone. Through research, I have a further understanding and discovery of 91-02-1

Two mononuclear Fe(II) complexes having same ligand framework, coordination geometry but different counter anions [Fe(L)]X-2 [L = N-(phenyl-pyridin-2-yl-methylene)-ethane-1,2-diamine; X = ClO4- (1), PF6- (2)] have been synthesized and crystallographically characterized. Both the complexes catalyzed catechol-quinone oxidation pH dependently. In the pH range 8.3-8.5 the suitable activity of both the complexes were found. Counter anions in the complexes played a significant role in controlling the turn over numbers of the catalytic reactions. In acetonitrile (MeCN), the turn over number for 1 was 4.99 h(-1) and for 2, it was 42.75 h(-1).

Safety of Phenyl(pyridin-2-yl)methanone. About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Chatterjee, A; Kaur, G; Joshi, M; Choudhury, AR; Ghosh, R or concate me.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

Discovery of C12H9NO

HPLC of Formula: C12H9NO. About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Araujo, VO; Tirloni, B; Streit, L; Schwade, VD or concate me.

HPLC of Formula: C12H9NO. In 2019.0 ACTA CRYSTALLOGR C published article about SUPEROXIDE-DISMUTASE ACTIVITY; DOT-N INTERACTIONS; COPPER(II) COMPLEXES; CRYSTAL-STRUCTURE in [Araujo, Vinicius Oliveira; Schwade, Vania Denise] Fundacao Univ Fed Grande Dourados, Fac Ciencias Exatas & Tecnol, Rodovia Dourados Itahum,Km 12, BR-79804970 Dourados, MS, Brazil; [Tirloni, Brbara] Univ Fed Santa Maria, Dept Quim, Ave Roraima 1000, BR-97105900 Santa Maria, RS, Brazil; [Streit, Livia] Univ North Georgia, Dept Chem & Biochem, 3820 Mundy Mill Rd, Oakwood, GA 30566 USA in 2019.0, Cited 22.0. The Name is Phenyl(pyridin-2-yl)methanone. Through research, I have a further understanding and discovery of 91-02-1.

Semicarbazones can exist in two tautomeric forms. In the solid state, they are found in the keto form. This work presents the synthesis, structures and spectroscopic characterization (IR and NMR spectroscopy) of four such compounds, namely the neutral molecule 4-phenyl-1-[phenyl(pyridin-2-yl)methylidene] semicarbazide, C19H16N4O, (I), abbreviated as HBzPyS, and three different hydrated salts, namely the chloride dihydrate, C19H17N4O+center dot Cl-center dot 2H(2)O, (II), the nitrate dihydrate, C19H17N4O+center dot NO3-center dot 2H(2)O, (III), and the thiocyanate 2.5-hydrate, C19H17N4O+center dot SCN-center dot 2.5H(2)O, (IV), of 2-[phenyl({[(phenylcarbamoyl) amino]imino})methyl]pyridinium, abbreviated as [H(2)BzPyS](+)center dot X-center dot nH(2)O, with X = Cl- and n = 2 for (II), X= NO3- and n = 2 for (III), and X = SCN- and n = 2.5 for (IV), showing the influence of the anionic form in the intermolecular interactions. Water molecules and counter-ions (chloride or nitrate) are involved in the formation of a two-dimensional arrangement by the establishment of hydrogen bonds with the N-H groups of the cation, stabilizing the E isomers in the solid state. The neutral HBzPyS molecule crystallized as the E isomer due to the existence of weak pi-pi interactions between pairs of molecules. The calculated IR spectrum of the hydrated [H(2)BzPyS](+) cation is in good agreement with the experimental results.

HPLC of Formula: C12H9NO. About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Araujo, VO; Tirloni, B; Streit, L; Schwade, VD or concate me.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

Some scientific research about Ethyl nicotinate

About Ethyl nicotinate, If you have any questions, you can contact Li, FZ; Geng, JS; Hu, KQ; Yu, JP; Liu, N; Chai, ZF; Mei, L; Shi, WQ or concate me.. HPLC of Formula: C8H9NO2

An article Proximity Effect in Uranyl Coordination of the Cucurbit[6]uril-Bipyridinium Pseudorotaxane Ligand for Promoting Host-Guest Synergistic Chelating WOS:000675430900047 published article about METAL-ORGANIC FRAMEWORK; POLYROTAXANES; INCLUSION; DESIGN in [Li, Fei-Ze; Liu, Ning] Sichuan Univ, Inst Nucl Sci & Technol, Minist Educ, Key Lab Radiat Phys & Technol, Chengdu 610064, Peoples R China; [Li, Fei-Ze; Geng, Jun-Shan; Hu, Kong-Qiu; Yu, Ji-Pan; Chai, Zhi-Fang; Mei, Lei; Shi, Wei-Qun] Chinese Acad Sci, Inst High Energy Phys, Lab Nucl Energy Chem, Beijing 100049, Peoples R China; [Chai, Zhi-Fang] Chinese Acad Sci, Ningbo Inst Ind Technol, Engn Lab Adv Energy Mat, Ningbo 315201, Zhejiang, Peoples R China in 2021, Cited 43. The Name is Ethyl nicotinate. Through research, I have a further understanding and discovery of 614-18-6. HPLC of Formula: C8H9NO2

behavior of cucurbituril-bipyridinium pseudorotaxane ligand by utilizing meta-functionalized bipyridinium dicarboxylate guest. A tailored pseudorotaxane precursor involving 1,1′-(hexane-1,6-diyl)bis(3-cyanopyridin-1-ium) bromide (C6BPCN3) and cucurbit[6]uril (CB[6]) has designed and synthesized. Through in situ hydrolysis of the pseudorotaxane ligands and their coordination assembly with uranyl cations, seven new uranyl-rotaxane coordination polymers URCP1-URCP7 have been obtained under hydrothermal conditions in the presence of different anions. It is demonstrated that the variation of carboxylate groups from para- to meta-position greatly affected the coordination behaviors of the metafunctionalized pseudorotaxane linkers, which are enriched from simple guest-only binding to host-guest simultaneous coordination and synergistic chelating. This effective regulation on uranyl coordination of supramolecular pseudorotaxane can be attributed to the proximity effect, which refers to the meta-position carboxyl group being spatially closer to the portal carbonyl group of CB[6]. Moreover, by combining other regulation methods such as introducing competing counterions and modulating solution acidity, the nuclearity of the uranyl center and the coordination patterns of the pseudorotaxane ligand can be diversely tuned, which subsequently exert great influence on the final dimensionality of resultant uranyl compounds. This work presents a large diversity of uranyl-based coordination polyrotaxane compounds with fascinating mechanically interlocked components and, most importantly, provides a feasible approach to adjust and control the metal coordination behavior of the pseudorotaxane ligand that might expand the scope of application of such supramolecular ligands.

About Ethyl nicotinate, If you have any questions, you can contact Li, FZ; Geng, JS; Hu, KQ; Yu, JP; Liu, N; Chai, ZF; Mei, L; Shi, WQ or concate me.. HPLC of Formula: C8H9NO2

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

An overview of features, applications of compound:Phenyl(pyridin-2-yl)methanone

About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Hosseini, A; Schreiner, PR or concate me.. Quality Control of Phenyl(pyridin-2-yl)methanone

Quality Control of Phenyl(pyridin-2-yl)methanone. Recently I am researching about ONE-POT SYNTHESIS; ARYL HALIDES; DERIVATIVES; ALCOHOLS; FLUORIDE; MECHANOCHEMISTRY; NANOPARTICLES; HYDROCARBONS; ACTIVATION; VINYLATION, Saw an article supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering DivisionUnited States Department of Energy (DOE) [DE-AC02-76SF00515]. Published in WILEY-V C H VERLAG GMBH in WEINHEIM ,Authors: Hosseini, A; Schreiner, PR. The CAS is 91-02-1. Through research, I have a further understanding and discovery of Phenyl(pyridin-2-yl)methanone

Ball milling of calcium carbide (CaC2) enables the reaction of its ethynyl moiety with organic electrophiles. This was realized simply by co-milling CaC(2)with organic substrates in a sealed jar without the need for an additive or a catalyst. Various ketones including those bearing alpha-hydrogens were ethynylated in good yields at short reaction times. Aryl halides are also amenable substrates for this protocol as they furnish aryl ethynes through a benzyne intermediate. This method offers a practical and cheap alternative to the established procedures for introducing ethynyl functionalities.

About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Hosseini, A; Schreiner, PR or concate me.. Quality Control of Phenyl(pyridin-2-yl)methanone

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem