When did you first realize you had a special interest and talent in91-02-1

Formula: C12H9NO. About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Li, G; Jia, CQ; Cai, XF; Zhong, L; Zou, L; Cui, XL or concate me.

An article Ruthenium-catalyzed meta-C-H bond alkylation of aryl 2-pyridyl ketones WOS:000503765300022 published article about FUNCTIONALIZATION; ARENES; DECARBONYLATION; SULFONATION; ACTIVATION; AMIDATION; MILD in [Li, Gang; Cai, Xiaofeng; Zhong, Lei; Zou, Lei] Anyang Normal Univ, Coll Chem & Chem Engn, Anyang 455000, Peoples R China; [Li, Gang; Cai, Xiaofeng; Zhong, Lei; Zou, Lei] Anyang Normal Univ, Henan Prov Key Lab New Optoelect Funct Mat, Anyang 455002, Peoples R China; [Jia, Chunqi; Cui, Xiuling] Huaqiao Univ, Key Lab Fujian Mol Med, Key Lab Xiamen Marine & Gene Drugs, Sch Biomed Sci,Engn Res Ctr Mol Med,Minist Educ, Xiamen 361021, Fujian, Peoples R China in 2020.0, Cited 41.0. Formula: C12H9NO. The Name is Phenyl(pyridin-2-yl)methanone. Through research, I have a further understanding and discovery of 91-02-1

The first example of meta-selective C-Ar-H bond functionalization of aryl 2-pyridyl ketones has been developed using [Ru(p-cymene)Cl-2](2) as the catalyst and alkyl bromide as the coupling reagent. This development provides an efficient strategy for modifying the meta-position of aryl 2-pyridyl ketone skeletons, which are found in various functional molecules.

Formula: C12H9NO. About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Li, G; Jia, CQ; Cai, XF; Zhong, L; Zou, L; Cui, XL or concate me.

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

Search for chemical structures by a sketch :500-22-1

About 3-Pyridinecarboxaldehyde, If you have any questions, you can contact Yalcin, E; Duyar, H; Cakmaz, D; Sahin, E; Seferoglu, Z or concate me.. Safety of 3-Pyridinecarboxaldehyde

Recently I am researching about SELECTIVE HYDROGENATION; EFFICIENT SYNTHESIS; FLUORENE; ARYLIDENEMALONODINITRILES; NANOPARTICLES; REDUCTION; CATALYSTS; ALCOHOLS; NITRO, Saw an article supported by the Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [113Z704]. Published in PERGAMON-ELSEVIER SCIENCE LTD in OXFORD ,Authors: Yalcin, E; Duyar, H; Cakmaz, D; Sahin, E; Seferoglu, Z. The CAS is 500-22-1. Through research, I have a further understanding and discovery of 3-Pyridinecarboxaldehyde. Safety of 3-Pyridinecarboxaldehyde

Novel 3-Amino-1-hetarylfluorene derivatives have medicinal uses and challenging transformation in organic synthesis have been synthesized via decyanation process. Surprisingly, the alkylated compounds derived from 3-Amino-1-hetarylfluorenes were obtained via further nucleophilic substitution to the 9-C and 3-N positions of the fluorene ring because of the harsh reaction condition employed. The synthesized compounds were characterized by spectroscopic techniques as well as X-ray single crystal diffraction analysis. This new family of blue emitting fluorene derivatives have low to moderate quantum yields (the largest value of Phi(F) = 0.52 for compound 15). (C) 2019 Elsevier Ltd. All rights reserved.

About 3-Pyridinecarboxaldehyde, If you have any questions, you can contact Yalcin, E; Duyar, H; Cakmaz, D; Sahin, E; Seferoglu, Z or concate me.. Safety of 3-Pyridinecarboxaldehyde

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

How did you first get involved in researching 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride

Formula: C8H10ClNO3. About 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride, If you have any questions, you can contact Bagautdinova, RH; Kibardina, LK; Pudovik, EM; Burilov, AR; Pudovik, MA or concate me.

An article Pyridoxal Azomethine Salts WOS:000511198000003 published article about AMINO-ACIDS in [Bagautdinova, R. H.; Kibardina, L. K.; Burilov, A. R.] Russian Acad Sci, Kazan Sci Ctr, Fed Res Ctr, AE Arbuzov Inst Organ & Phys Chem, Kazan 420088, Russia; [Pudovik, E. M.; Pudovik, M. A.] Kazan Volga Fed Univ, Kazan 420008, Russia in 2019.0, Cited 9.0. Formula: C8H10ClNO3. The Name is 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride. Through research, I have a further understanding and discovery of 65-22-5

The reactions of 4-methylpiperazin-1-amine, 2-amino- and 4-aminomethylpiperidines with pyridoxal afforded the corresponding azomethines. Their reactions with organic and inorganic acids lead to the formation of salt derivatives of pyridoxal azomethines.

Formula: C8H10ClNO3. About 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride, If you have any questions, you can contact Bagautdinova, RH; Kibardina, LK; Pudovik, EM; Burilov, AR; Pudovik, MA or concate me.

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

Brief introduction of Phenyl(pyridin-2-yl)methanone

About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Murayama, H; Heike, Y; Higashida, K; Shimizu, Y; Yodsin, N; Wongnongwa, Y; Jungsuttiwong, S; Mori, S; Sawamura, M or concate me.. HPLC of Formula: C12H9NO

Recently I am researching about ASYMMETRIC TRANSFER HYDROGENATION; LIGAND BIFUNCTIONAL CATALYSIS; RUTHENIUM COMPLEX; AROMATIC KETONES; BASIS-SETS; METAL; REDUCTION; ALDEHYDES; DESIGN; ESTERS, Saw an article supported by the MEXT KAKENHIMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of ScienceGrants-in-Aid for Scientific Research (KAKENHI) [JP15H05801, JP18H04233]; JSPS KAKENHIMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of ScienceGrants-in-Aid for Scientific Research (KAKENHI) [JP18H03906]; Sakura Science Plan through Collaborative research activity course from JST [S2018F0228129]. Published in WILEY-V C H VERLAG GMBH in WEINHEIM ,Authors: Murayama, H; Heike, Y; Higashida, K; Shimizu, Y; Yodsin, N; Wongnongwa, Y; Jungsuttiwong, S; Mori, S; Sawamura, M. The CAS is 91-02-1. Through research, I have a further understanding and discovery of Phenyl(pyridin-2-yl)methanone. HPLC of Formula: C12H9NO

Iridium-catalyzed enantioselective transfer hydrogenation of ketones with formic acid was developed using a prolinol-phosphine chiral ligand. Cooperative action of the iridium atom and the ligand through alcohol-alkoxide interconversion is crucial to facilitate the transfer hydrogenation. Various ketones including alkyl aryl ketones, ketoesters, and an aryl heteroaryl ketone were competent substrates. An attractive feature of this catalysis is efficient discrimination between the alkyl and aryl substituents of the ketones, promoting hydrogenation with the identical sense of enantioselection regardless of steric demand of the alkyl substituent and thus resulting in a rare case of highly enantioselective transfer hydrogenation of tert-alkyl aryl ketones. Quantum chemical calculations revealed that the sp(3)-C-H/pi interaction between an sp(3)-C-H bond of the prolinol-phosphine ligand and the aryl substituent of the ketone is crucial for the enantioselection in combination with O-H center dot center dot center dot O/sp(3)-C-H center dot center dot center dot O two-point hydrogen-bonding between the chiral ligand and carbonyl group.

About Phenyl(pyridin-2-yl)methanone, If you have any questions, you can contact Murayama, H; Heike, Y; Higashida, K; Shimizu, Y; Yodsin, N; Wongnongwa, Y; Jungsuttiwong, S; Mori, S; Sawamura, M or concate me.. HPLC of Formula: C12H9NO

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

New learning discoveries about 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride

About 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride, If you have any questions, you can contact Bachmann, T; Schnurr, C; Zainer, L; Rychlik, M or concate me.. Recommanded Product: 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride

An article Chemical synthesis of 5 ‘-beta-glycoconjugates of vitamin B-6 WOS:000519529400008 published article about PYRIDOXINE-BETA-GLUCOSIDE; PARTICULATE GLUCOSYLTRANSFERASE; PROTEIN GLYCOSYLATION; GROWING CULTURE; N-GLYCOSYLATION; FUSARIUM TOXINS; RICE BRAN; SEEDLINGS; GLUCURONIDATION; BIOAVAILABILITY in [Bachmann, Thomas; Schnurr, Christian; Zainer, Laura; Rychlik, Michael] Tech Univ Munich, Chair Analyt Food Chem, Maximus von Imhof Forum 2, D-85354 Freising Weihenstephan, Germany in 2020.0, Cited 107.0. The Name is 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride. Through research, I have a further understanding and discovery of 65-22-5. Recommanded Product: 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride

Various 5′-beta-saccharides of pyridoxine, namely the mannoside, galactoside, arabinoside, maltoside, cellobioside and glucuronide, were synthesized chemically according to KOENIGS-KNORR conditions using alpha 4,3-O-iso-propylidene pyridoxine and the respective acetobromo glycosyl donors with AgOTf (3.0 eq.) and NIS (3.0 eq.) as promoters at 0 degrees C. Furthermore, 5′-beta-[C-13(6)]-labeled pyridoxine glucoside (PNG) was prepared starting from [C-1(3)6]-glucose and pyridoxine. Additionally, two strategies were examined for the synthesis of 5′-beta-pyridoxal glucoside (PLG).

About 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride, If you have any questions, you can contact Bachmann, T; Schnurr, C; Zainer, L; Rychlik, M or concate me.. Recommanded Product: 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride

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

New learning discoveries about Ethyl nicotinate

About Ethyl nicotinate, If you have any questions, you can contact Huang, C; Qiao, J; Ci, RN; Wang, XZ; Wang, Y; Wang, JH; Chen, B; Tung, CH; Wu, LZ or concate me.. Application In Synthesis of Ethyl nicotinate

An article Quantum dots enable direct alkylation and arylation of allylic C(sp(3))-H bonds with hydrogen evolution by solar energy WOS:000652330400010 published article about SP(3) C-H; LIGHT PHOTOREDOX CATALYSIS; TRANSITION-METAL; FUNCTIONALIZATION; ALLYLATION; CARBONYL; PHOTOCATALYSIS; SELECTIVITY; ACTIVATION; ALDEHYDES in [Huang, Cheng; Qiao, Jia; Ci, Rui-Nan; Wang, Xu-Zhe; Wang, Yang; Wang, Jing-Hao; Chen, Bin; Tung, Chen-Ho; Wu, Li-Zhu] Chinese Acad Sci, Tech Inst Phys & Chem, Key Lab Photochem Convers & Optoelect Mat, Beijing 100190, Peoples R China; [Huang, Cheng; Qiao, Jia; Ci, Rui-Nan; Wang, Xu-Zhe; Wang, Yang; Wang, Jing-Hao; Chen, Bin; Tung, Chen-Ho; Wu, Li-Zhu] Chinese Acad Sci, Univ Chinese Acad Sci, Beijing 100190, Peoples R China; [Huang, Cheng; Qiao, Jia; Ci, Rui-Nan; Wang, Xu-Zhe; Wang, Yang; Wang, Jing-Hao; Chen, Bin; Tung, Chen-Ho; Wu, Li-Zhu] Univ Chinese Acad Sci, Sch Future Technol, Beijing 100049, Peoples R China in 2021, Cited 67. The Name is Ethyl nicotinate. Through research, I have a further understanding and discovery of 614-18-6. Application In Synthesis of Ethyl nicotinate

Direct alkylation and arylation of allylic C(sp3)-H bonds are straight-forward approaches for accessing allylic C-C products. However, the engagement of allylic C-C bond- forming reaction without introducing extraneous functional groups and stoichiometric oxidant or reductant has proved difficult. Here, we report a general and mild strategy using semiconductor quantum dots (QDs) as photocatalysts for coupling a broad range of available allylic C(sp(3))-H bonds with alpha- amino C-H bonds or heteroarenes, respectively, under sunlight irradiation (> 85 examples). The protocol bypasses stoichiometric oxidant or reductant and pre-functionalization of both the coupling partners and produces hydrogen (H-2) as the byproduct. The outstanding efficiency and selectivity and step- and atom-economy represents the first direct alkylation and arylation of allylic C(sp(3))-H bonds with hydrogen evolution powered by solar energy.

About Ethyl nicotinate, If you have any questions, you can contact Huang, C; Qiao, J; Ci, RN; Wang, XZ; Wang, Y; Wang, JH; Chen, B; Tung, CH; Wu, LZ or concate me.. Application In Synthesis of Ethyl nicotinate

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

The Best Chemistry compound:C8H9NO2

SDS of cas: 614-18-6. 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.

In 2021 INORG CHEM 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. SDS of cas: 614-18-6

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.

SDS of cas: 614-18-6. 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.

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

Extended knowledge of 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride

About 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride, If you have any questions, you can contact Zubenko, AA; Divaeva, LN; Morkovnik, AS; Fetisov, LN; Sochnev, VS; Kononenko, KN; Bodryakov, AN; Klimenko, AI or concate me.. HPLC of Formula: C8H10ClNO3

HPLC of Formula: C8H10ClNO3. Authors Zubenko, AA; Divaeva, LN; Morkovnik, AS; Fetisov, LN; Sochnev, VS; Kononenko, KN; Bodryakov, AN; Klimenko, AI in MAIK NAUKA/INTERPERIODICA/SPRINGER published article about in [Zubenko, A. A.; Fetisov, L. N.; Kononenko, K. N.; Bodryakov, A. N.; Klimenko, A. I.] Fed Rostov Agr Sci Ctr, North Caucasian Zonal Vet Res Inst, Novocherkassk 346406, Russia; [Divaeva, L. N.; Morkovnik, A. S.; Sochnev, V. S.] Southern Fed Univ, Inst Phys & Organ Chem, Rostov Na Donu 344090, Russia in 2020.0, Cited 29.0. The Name is 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride. Through research, I have a further understanding and discovery of 65-22-5

4-Hydroxymethyl-2-hetaryl(hetaroyl)furo[2,3-c]pyridines, the products of furan cyclization of pyridoxal with acylmethyl- and heteroarylmethyl halides, easily react with thionyl chloride in DMF to form new series of 4-chloromethyl-2-heteroaryl[2,3-c]pyridines. Further action of primary or secondary amines on these chloromethyl derivatives leads to the nucleophilic substitution of chlorine atoms with the formation of 4-aminomethyl-2-heteroaryl[2,3-c]pyridines. The study of anti-infective activity of the 4-RCH2-furo[2,3-c]pyridines (R = OH, Cl, (NRR2)-R-1) showed significant protistocidal and moderate antibacterial activity of some of representatives of these compounds.

About 3-Hydroxy-5-(hydroxymethyl)-2-methylisonicotinaldehyde hydrochloride, If you have any questions, you can contact Zubenko, AA; Divaeva, LN; Morkovnik, AS; Fetisov, LN; Sochnev, VS; Kononenko, KN; Bodryakov, AN; Klimenko, AI or concate me.. HPLC of Formula: C8H10ClNO3

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

Chemistry Milestones Of 500-22-1

Formula: C6H5NO. About 3-Pyridinecarboxaldehyde, If you have any questions, you can contact Shestopalov, AM; Rodinovskaya, LA; Zubarev, AA; Nesterov, VN; Ugrak, BI; Dutova, TY or concate me.

Shestopalov, AM; Rodinovskaya, LA; Zubarev, AA; Nesterov, VN; Ugrak, BI; Dutova, TY in [Shestopalov, Anatoliy M.; Rodinovskaya, Lyudmila A.; Zubarev, Andrey A.; Ugrak, Bogdan I.; Dutova, Tatyana Ya.] Russian Acad Sci, ND Zelinsky Inst Organ Chem, Leninsky Prospekt 47, Moscow 119991, Russia; [Nesterov, Vladimir N.] Univ North Texas, Dept Chem, Denton, TX 76203 USA published Synthesis and domino reactions of polymethylene-3-cyanopyridine-2(1H)-thiones in 2020.0, Cited 30.0. Formula: C6H5NO. The Name is 3-Pyridinecarboxaldehyde. Through research, I have a further understanding and discovery of 500-22-1.

A new and facile three-component method was developed for the synthesis of polymethylene-3-cyanopyridine-2(1H)-thiones by the domino Knoevenagel -> Michael -> heterocyclization -> dehydrogenation reaction from cycloalkanones, cyanothioacetamide, and polymethoxy-substituted benzaldehydes. The final dehydrogenation step was found to involve arylidenecyanothioacetamide. The resulting pyridinethiones and 4-chloroacetoacetic ester were introduced into the domino S(N)2 -> Thorpe-Ziegler -> Guareschi-Thorpe reaction to synthesize tetracyclic dipyridothiophenes. Starting from these compounds, 8,9-polymethylenepyranothienodipyridines were also synthesized by the domino Knoevenagel -> Michael -> hetero-Thorpe-Ziegler reaction. Furthermore, a method for the synthesis of thienodipyridine annulated to the steroid skeleton is proposed.

Formula: C6H5NO. About 3-Pyridinecarboxaldehyde, If you have any questions, you can contact Shestopalov, AM; Rodinovskaya, LA; Zubarev, AA; Nesterov, VN; Ugrak, BI; Dutova, TY or concate me.

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

An update on the compound challenge: 3-Pyridinecarboxaldehyde

Recommanded Product: 500-22-1. About 3-Pyridinecarboxaldehyde, If you have any questions, you can contact Wang, LC; Xu, L; Xu, XQ; Su, BM; Lin, J or concate me.

I found the field of Engineering very interesting. Saw the article An L-threonine aldolase for asymmetric synthesis of beta-hydroxy-alpha-amino acids published in 2020.0. Recommanded Product: 500-22-1, Reprint Addresses Lin, J (corresponding author), Fuzhou Univ, Coll Chem Engn, Fuzhou 350116, Peoples R China.. The CAS is 500-22-1. Through research, I have a further understanding and discovery of 3-Pyridinecarboxaldehyde

L-threonine aldolase (LTA) is a PLP-dependent enzyme that can reversibly catalyze aldol reaction of glycine and acetaldehyde to produce beta-hydroxy-alpha-amino acids. In the present work, a putative Ita gene from Actinocorallia herbida (AhLTA) was mined and over-expressed in Escherichia coli BL21 (DE3). The substrate spectrum assay indicated that AhLTA only used glycine as donor substrate and tolerated a wild range of aromatic aldehydes as acceptor substrates. It was found that the type and position of substituents in the aromatic aldehydes exerted a significant impact on the activity and stereoselectivity at beta-carbon of AhLTA. Among those substrates, AhLTA could catalyze glycine and 4-methylsulphonyl benzaldehyde (14a) to produce L-threo-4-methylsulfonylphenylserine ((2S,3R)-14b) with high conversion (94.4%) and moderate stereoselectivity (19% de). By conditional optimization, the de value of (2S, 3R)-14b was improved to 61% and the conversion was 75%. Taken together, our study suggested that AhLTA might be a promising catalyst for producing chiral beta-hydroxy-alpha-amino acids. (C) 2020 Elsevier Ltd. All rights reserved.

Recommanded Product: 500-22-1. About 3-Pyridinecarboxaldehyde, If you have any questions, you can contact Wang, LC; Xu, L; Xu, XQ; Su, BM; Lin, J or concate me.

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