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A photo-active gellan gum (Gel) derivative was developed by amide bond combination with trans-4-[p-(amino)styryl]pyridine (SP). The SP-Gel was cross-linked by UV curing via the intermolecular 2π + 2π cycloaddition of the inserted SP–CH=CH- moieties. The chemical structure of the obtained photo-crosslinkable biopolymer was investigated before and after the UV curing and the progress of the performed 2π + 2π cycloaddition-based cross-linking was detected via UV–visible light spectra. SP-Gel was evaluated as a polymeric matrix for the immobilization of catalase via entrapment technique. The synthesized biopolymer was mixed with the catalase and molded in the form of membranes that were UV cured to encapsulate the enzyme. The membranes were able to entrap 0.75 mg/cm2 with retained activity reached above 95%. The immobilized catalase displayed higher thermal stability and higher resistance toward the environmental pH disturbances compared to the free enzyme. Also, despite the observed lower catalase-H2O2 affinity upon the entrapment that was indicated from the performed kinetic studies, the reusability and storage stability experiments revealed the economic value of the entire process by preserving around 95% and 83% of the initial catalase activity after the fifth and tenth operation cycles, respectively.

The present review has highlighted the chemistry of pyrimido[1,2-a]pyrimidine compounds as one of the most important classes of heterocyclic systems. The main sections include: (1) The synthesis of pyrimido[1,2-a]pyrimidines, (2) reactivity of the substituents attached to the carbon and nitrogen atoms of the ring and (3) biological applications. A discussion demonstrated that the proposed mechanisms of unexpected synthetic routes were intended. The purpose of this review is to provide an overview of the chemistry of pyrimido[1,2-a]pyrimidines to date, in which the compounds should be widely applied in medicinal and pharmaceutical chemistry based on the significant, variable and potent biological results of pyrimidopyrimidine and pyridopyrimidine analogs. This survey will assist scientists in the organic and medicinal chemistry fields to design and develop procedures for the construction of new standard biological compounds.

Heterocycles incorporating a pyrimidopyrimidine scaffold have aroused great interest from researchers in the field of medical chemistry because of their privileged biological activities; they are used as anti-bacterial, antiviral, anti-tumor, anti-allergic, antihypertensive, anticancer, and hepatoprotective agents. Therefore, the present study aims to investigate the chemistry of heterocycles incorporating pyrimido[1,6-a]pyrimidine and pyrimido[1,6-c]pyrimidine skeletons and their biological characteristics. The main sections discuss (1) the synthetic routes to obtain substituted pyrimidopyrimidines, pyrimido[1,6-a]pyrimidin-diones, pyrimidoquinazolines, tricyclic, tetracyclic, and binary systems; (2) the reactivity of the substituents attached to the pyrimidopyrimidine skeleton, including thione and amide groups, nucleophilic substitutions, condensations, ring transformations, and coordination chemistry; (3) compounds of this class of heterocycles containing a significant characteristic scaffold and possessing a wide range of biological characteristics.

The aim of this work is to discuss the chemistry of pyrido[3,4-d]pyrimidines as one of the most important heterocyclic compounds with remarkable synthetic, biological and medical applications. In this overview, the chemistry of heterocyclic compounds incorporated the pyrido[3,4-d]pyrimidine scaffold as demonstrated by chemical reactions and different preparation processes. The anticipated compounds were synthesized from pyridine or pyrimidine compounds and a description of the reactivity of substituents attached to ring carbon and nitrogen atoms is discussed. On the other hand, the synthesis and reactions of fused heterocycles incorporated pyrido[3,4-d]pyrimidine scaffold is described. The diamine analogs included pyrido[3,4-d]pyrimidine core were reported as tyrosine kinase inhibitors. The chemical reactions of certain unexpected and chemically substantial compounds have been discussed.

The present study describes recent advances in the chemistry of heterocycles incorporated oxazolo[4,5-b]pyridine and oxazolo[5,4-b]pyridine skeletons. The main sections included the synthesis of th...

Pyrazolopyrimidines are a privileged class of 5-6 bicyclic systems with three or four nitrogen atoms, including four possible isomeric structures. The significance of this class of compounds is that they can be applied in medical and pharmaceutical fields due to their unlimited biological aptitude, hence it is the basic skeleton of several synthetic drugs. The current review aimed to highlight all the synthetic routes that have been applied to construct the pyrazolo[1,5-a]pyrimidine ring systems up to date. The sections in this study included the synthesis of pyrazolo[1,5- a]pyrimidines by condensation reactions of 5-aminopyrazoles with each of β-diketones, 1,5-diketones, β- ketoaldehydes, α-cyanoaldehydes, β-enaminones, enamines, enaminonitriles, ethers, with unsaturated ketones, unsaturated thiones, unsaturated esters, unsaturated dienones “1,2-allenic”, unsaturated aldehydes, unsaturated imines, and unsaturated nitriles. The routes adopted to synthesize this class of heterocyclic compounds were extended for ring construction from acyclic reagents and multicomponent reactions under catalytic or catalyst-free conditions.

A newly synthesized ortho-amino thio-phenol modified flax fibers (OATP-MFF) were prepared in two subsequent steps; at first flax fiber was pretreated with sodium periodate. Then the pretreated fibers were condensed with ortho-amino thio-phenol to form the modified flax fiber(OATP-MFF). The OATP-MFF was characterized by (FTIR) Fourier transform infrared spectra, SEM and energy-dispersive X-ray Spectroscopy (EDX). The newly synthesized OATP-MFF chelating fibers were utilized for selective separation of Cu(II) from aqueous solution by adsorption methodology. The effects of pH, the initial concentration of metal ions, the adsorbent dosage, the interfering ions and the contact time on the adsorption capacity of OATP-MFF chelating fibers were investigated. The maximum adsorption capacity of Cu(II) at the optimum conditions was 92 mg/g. The adsorption process fitted well the second-order model kinetic of. The chemical (adsorption) reaction is the rate-limiting step that was proved from the kinetic model.

The present study provides an overview of the chemistry and biological significance of pyrimido[4,5-d]pyrimidine and pyrimido[5,4-d]pyrimidine analogs as types of bicyclic [6 + 6] systems. The main sections include: (1) synthesis methods; (2) the reactivities of the substituents linked to the ring carbon and nitrogen atoms; and (3) biological applications. A discussion demonstrating the proposed mechanisms of unexpected synthetic routes is intended. The aim of this study is to discuss the synthetic significance of the titled compounds and to establish the biological characteristics of this class of compounds as studied to date, where the compounds have been applied on a large scale in the medical and pharmaceutical fields. This survey will help researchers in the fields of synthetic organic and medicinal chemistry to undertake and improve new approaches for the construction of new standard biological components.

This review describes the chemistry of 2,3-dihydroxynaphthalene and the related analogues. The synthetic routes reported to obtain the investigated structures are discussed. In addition, the reacti...

4-Formylphenyl benzoate was utilized as a versatile precursor for the construction of new series of heterocyclic scaffolds that contain thiazole and thiazolidin-5-one rings. The antibacterial activity of these scaffolds against two types of bacteria was screened and most of them exhibited good activity. Among the synthesized thiazole derivatives, compounds 5, 6, and 8a showed antibacterial activity close to the standard chemotherapeutic (Ampicillin).

An enantio-selective l -histidine imprinted polymeric resin was fabricated and evaluated for enantiomeric resolution of histidine racemate. The polymerizable chiral salicyloyl- l -histidine amide was first synthesized and anchored onto a polymeric resin network via condensation polymerization with resorcinol and formaldehyde. l -histidine template molecules were then extracted out of the resin texture via alkaline hydrolysis of the amide bond using sodium hydroxide. The synthetic steps were monitored by means of instrumental techniques including elemental analysis; mass spectra along with both Fourier transform infrared and nuclear magnetic resonance spectroscopy. In addition, the surface morphologies of both imprinted and non-imprinted resins were visualized using scanning electron microscope and the images indicated a relatively rougher surface in case of the imprinted resin. Also, the complete extraction of the template l -histidine molecules was assured using energy-dispersive X-ray spectroscopy, which indicated the absence of nitrogen upon alkaline treatment of the synthesized l -histidine containing resin. Selective adsorption experiments indicated that the maximum adsorption was achieved at pH 8 and followed the pseudo-second-order kinetic model with extracted amounts of 165 ± 1 and 90 ± 1 mg/g with respect to l - and D-histidine, respectively. Moreover, Langmuir model displayed the best fit with the experimentally obtained isotherm data and the maximum adsorption capacities were 195 ± 1 and 102 ± 1 mg/g with respect to l - and d - histidine, respectively. The enantiomeric resolution of d / l -histidine racemate was also carried out utilizing a column backed with the imprinted resin and the outlet collected solution displayed an optical activity related to 36% d -histidine enantiomeric excess.

This review described the synthetic methods, chemical reactivity and biological applications of furan carboxamide compounds. Furan-carboxamides are reported to have important and variable biologica...

The chemical structure of pyrazole-1-carbothioamide derivative 1 was modified through building a thiazole moiety to synthesize derivatives of thiazolyl pyrazolin-5-one. The reaction of 1 with phenacyl bromide furnished the conforming thiazolyl pyrazole derivative 2, which upon base-catalyzed reaction with benzyl isothiocyanate afforded thiazolyl 4-(benzylthiocarbamoyl)-2-pyrazolin-5-one derivative 3. The diazo-coupling reaction of 2 with diazotized 4-aminoacetophenone afforded the conforming thiazolyl 4-(p-acetylphenylhydrazono)-2-pyrazolin-5-one derivative 4. The synthesized thiazolyl pyrazolin-5-one derivatives were utilized as antioxidant additives for lubricating oil. The oxidation stability study revealed that the tested compounds showed good antioxidant activities. The efficiency of the tested antioxidant additives decreases according to the order: compound 2 > compound 4 > compound 3 > compound 1.

The versatile hitherto reported 3-(1H-indol-3-yl)-1H-pyrazol-5-amine () was synthesized by the reaction of 3-cyanoacetylindole () with hydrazine hydrate in refluxing ethanol and used as a key intermediate for the synthesis of novel pyrazolo[1,5-a]pyrimidines via its reactions with appropriate 1,3-biselectrophilic reagents or through three-component condensations with triethyl orthoformate and compounds possessing an activated methylene group. Besides, the applicability and synthetic potency of () to attain polyfunctionally substituted imidazo[1,2-b]pyrazole, pyrazolo[1,5-a][1,3]diazepine and pyrazolo[1,5-c][1,3,5]thiadiazine derivatives of an expected pharmaceutical interest have been investigated. The mechanistic aspects for the formation of the newly synthesized compounds are discussed.

Steroids are components of cell membranes, signaling molecules and are a type of secondary metabolites as a result of their high impact of biological significance. The present review described the literature reports of pentacyclic steroidal pyrimidines as a type of heterocyclic steroids. The main sections included the synthesis of the investigated steroids fused at rings-A or B or D of steroid skeleton, synthesis of binary or linked-type pyrimidines, pyrimidine oxides, macromolecules and mono- or di- or tri-peptides linked-steroidal pyrimidines. Besides, the present research highlighted the biological significance of steroidal pyrimidines, in which the compounds revealed potent anticancer, antioxidant, antibacterial, and anti-Alzheimer agents. In addition, some hetero-steroids were screened for binding DNA assay and gene expression analysis. It was settled that the incorporation of pyrimidine scaffold into steroid basic skeleton is crucial for better biological results.

4-(Thiazol-2-yldiazenyl)-1H-pyrazole-3,5-diamine (2) was prepared by the reaction of 2-thiazolylazomalononitrile (1) with hydrazine hydrate in boiling ethanol and used as key intermediate for the synthesis of polyfunctionally substituted pyrazolo[1,5-a]pyrimidines via its reactions with reagents having 1,3-dielectrophilic centers. Structures of the newly synthesized compounds were established by elemental analysis and spectral data. Representative compounds of the synthesized products were tested and evaluated as antioxidant agents. The results revealed that compounds (3), (4), (5), and (9) displayed promising antioxidant activity compared to the activity of ascorbic acid.

2-Thiazolylhydrazonomalononitrile (1) was used as key intermediate for the synthesis of polyfunctionally substituted heterocycles (e.g. pyridazine, tetrazole, pyrimidines, 1,5-benzo diazepine, benzoimidazo[1,2-a]pyrimidine, triazolo[4,3-a]pyrimidine, pyrazole, pyrazolo[1,5-a]triazines, pyrazolo[1,5-a]pyrimidine and 1,5-benzoxazocine) incorporating thiazole moiety via its reactions with some carbon and nitrogen nucleophiles. Structures of the newly synthesized compounds were established by elemental analysis and spectral data. The mechanistic aspects for the formation of the new compounds were also discussed.

3-(1H-Indol-3-yl)-1H-pyrazol-5-amine 3 was prepared in a quantitative yield by heating 3-(1H-indol-3-yl)-3-oxopropanenitrile 2 in dry ethanol with hydrazine hydrate, and utilized as key intermediate for the synthesis of some new pyrazolo[1,5-a]pyrimidines and pyrazolo[5,1-c]triazines. Structures of the newly synthesized compounds were established by elemental analysis and spectral data and evaluated as antioxidant agents. Most of the tested compounds belonging to the pyrazolo[1,5-a]pyrimidine series exhibited better activities than members of the pyrazolotriazine one.

2-(4-Oxo-1-phenyl-4,5-dihydro-1H-pyrazolo[3,4-d]pyrimidin-6-yl)acetonitrile was used as the key intermediate for the synthesis of polyfunctionally substituted heterocycles (e.g., pyrazoles, pyridine, tetrazole, thiazoles, thiophenes, and chromene) incorporating the pyrazolo[3,4-d]pyrimidin-4-one moiety. Representative compounds of the synthesized series were tested and evaluated as antioxidants. Two of the tested compounds (pyrazole and thiophene derivatives) showed antioxidant activity nearly equal to that of ascorbic acid.

A novel multicomponent synthesis of highly substituted pyrazolo[1,5-a]pyrimidine derivatives starting from simple and readily available reactants is described. Three-component condensation of 4-(thiazol-2-yldiazenyl)-1H-pyrazole-3,5-diamine with triethyl orthoformate and carbonyl compounds or nitriles containing an activated methylene group gave substituted pyrazolo[1,5-a]pyrimidines. An analogous reaction of 4-(thiazol-2-yldiazenyl)-1H-pyrazole-3,5-diamine with ethyl acetoacetate and benzaldehyde in an effective one-pot process furnished ethyl 2-amino-5-methyl-7-phenyl-3-(thiazol-2-yldiazenyl)-4,7-dihydropyrazolo[1,5-a]pyrimidine-6-carboxylate.

4-(4-oxo-4H-benzo[d][1,3]oxazin-2-yl)phenyl-4-methylbenzenesulfonate 2 was prepared and reacted with some nitrogen and sulfur nucleophiles. The prepared products were evaluated as antioxidant and corrosion inhibitors for gasoline lubricating oil in which compounds, 4-(3-amino-4-oxo-3,4-dihydroquinazolin-2-yl)phenyl-4-methylbenzene sulfonate 12, 4-(3-(3-chloro-2-oxo-4-phenylazetidin-1-yl)-4-oxo-3,4-dihydroquinazolin-2-yl)phenyl 4-methylbenzenesulfonate 14, 4-(4-oxo-3-(4-oxo-2-phenylthiazolidin-3-yl)-3,4-dihydroquinazolin-2-yl)phenyl 4-methylbenzenesulfonate 15, and 4-(3-(1,3-dioxoisoindolin-2-yl)-4-oxo-3,4-dihydroquinazolin-2-yl)phenyl 4-methylbenzenesulfonate 18 exhibited the highest antioxidant activity. It was interesting also to study the effect of concentration of additive 15 to find the optimum concentration recommended to be used, 0.1 g for 1 L oil of additive 15 was the more effective concentration to be used. Also, measurements of thermal analyses were carried out.

Benzoxazinone 2 was prepared and reacted with formamide, acetamide, some primary aromatic amines and heterocyclic amines giving the corresponding quinazolone derivatives 3–15 respectively. The reaction of benzoxazinone 2 with hydrazine hydrate and phenyl hydrazine was also studied. Representative compounds of the synthesized products were evaluated as antioxidants and corrosion inhibitors for gasoline engine lubricating oil. The highest antioxidant activities were obtained with compounds 10–15. The optimum concentration recommended for these new additives was found to be 0.1g for 1L of oil for compounds 13–15. In addition, some of the highly effective antioxidant additives, namely 10–15, were thermally analyzed by using thermogravimetric analysis (TGA) and differential thermal gravimetric analysis (DTGA) techniques and the results indicated that compounds are thermally stable and could be used under thermal conditions. Moreover, a comparison of the oxidation stability between the tested oil containing the prepared products and lubricating oil containing commercial additives was also studied.

An enantioselective S-mandelic acid (S-MA) imprinted chitosan (SMIC) was prepared by cross-linking of chitosan using formaldehyde cross-linker, in the presence of S-MA as an imprint template molecule and 0.5% acetic acid solution as a solvent. Non-imprinted cross-linked chitosan (NIC) as control was also prepared by the same procedure in absence of template molecules. The surface morphology of both SMIC and NIC were examined by scanning electron microscope (SEM). SMIC particles were applied to determine the optimum operational condition for S-MA separation from dilute aqueous solution. In adsorption step, optimum pH and retention time were 3.5 and 60 min, while corresponding values in extraction step were 1 and 40 min, respectively. Also, the adsorption isotherms indicated that the maximum adsorption capacities of S- and R-MA on SMIC were 100 ± 0.5 and 64 ± 0.8 mg/g, respectively, while in the case of NIC, both R- and S-MA present the same maximum adsorption.

2-Thiazolylhydrazonomalononitrile (1) was used as key intermediate for the synthesis of polyfunctionally substituted heterocycles (e.g. pyridazine, tetrazole, pyrimidines, 1,5-benzo diazepine, benzoimidazo[1,2-a]pyrimidine, triazolo[4,3-a]pyrimidine, pyrazole, pyrazolo[1,5-a]triazines, pyrazolo[1,5-a]pyrimidine and 1,5-benzoxazocine) incorporating thiazole moiety via its reactions with some carbon and nitrogen nucleophiles. Structures of the newly synthesized compounds were established by elemental analysis and spectral data. The mechanistic aspects for the formation of the new compounds were also discussed.

The starting oxazolone 1 was reacted with p-aminophenol in glacial acetic acid to afford imidazolone 2, which afforded Mannich base 3 via the reaction with piperidine and paraformaldehyde. Moreover, the reaction of 2 with ethylchloroacetate or chloroacetic acid in dry acetone gave the corresponding imidazolones 4 and 5, respectively. Compound 4 reacted with benzylamine or 4-chlorobenzaldehyde to furnish 6 and 7, respectively. On the other hand, oxazolone 1 reacted with hydrazines in glacial acetic acid to afford the 1,2,4-triazines 11–15. Representative compounds of the synthesized products were established and evaluated as antioxidant and corrosion inhibitors for gasoline lube oils.

Oxazolone derivative 2 was utilized as a key intermediate for synthesis of some new oxazolone and imidazolone derivatives. Reaction of oxazolone derivative 2 with diamines under different conditions afforded the corresponding imidazolone derivatives 3–8, respectively. Moreover, oxazolone 2 reacted with some heterocyclic amines in glacial acetic acid giving the corresponding imidazolone derivatives 9–14, respectively. Cyclocondensation of thiosemicarbazide with compound 2 in dry pyridine afforded compound 15. Addition of secondary amines to olefin double bond of compound 2 gave the corresponding addition products 16–19, respectively. Michael addition of compound 2 with some active methylene compounds afforded oxazolone derivatives 20–23, respectively. These prepared products were evaluated as antioxidant and corrosion inhibitors for gasoline lubricating oil and compounds 6a–c, 10 and 15 exhibited the highest antioxidant and anticorrosive activities. The effect of concentration of additives was studied to recommend the optimum concentration to be used. The results showed, for additive 15, 0.1 g for 1 L oil was the more effective concentration. Measurements for thermal analysis and of surface tension of oil after oxidation were also carried out.

Oxazolone derivative 2 was utilised as a key intermediate for the synthesis of some new imidazolone deriva- tives. Reaction of 2 with p-aminoacetophenone in the presence of acetic acid containing catalytic amount of freshly fused sodium acetate gives the corresponding imidazolone derivative 3 which on reaction with diethyl oxalate in the presence of granulated sodium metal afforded the corresponding imidazolone derivative 4. Condensation of compound 3 with some aromatic aldehydes afforded the corresponding cinnamoylphenyl derivatives 5a-c, respectively. Besides, compounds 5a-c reacted with hydrazine hydrate in boiling ethanol and yield the corresponding 4-arylidene-1- (p-(5-aryl-2- pyrazolin-3-y1) phenyl)-2-phenyl-2-imidazolin-5-ones 6a-c, respectively. Moreover, refluxing of compounds 5a-c with both ethylcyanoace- tate and/or malononitrile in ethanol and ammonium acetate afforded 7a-c and 8a-c, respectively. Claisen reaction of 3 with ethylacetate gives the corresponding imidazolone derivative 9 which on reaction with thiourea in ethanol afforded the cor- responding thienopyrimidine derivative 10. On the other hand, reaction of oxazolone 2 with both ethyl p-aminobenzoate and anthranilic acid in glacial acetic acid and freshly fused sodium acetate has been also studied. Structures of the newly synthesized compounds were established by elemental analysis and spectral data. Representative compounds of the synthe- sized products were tested and evaluated as antifungal and antimicrobial agents. Most of compounds exhibited good activi- ties, where 3, 5b, 5c, 6a, 7b, 7c, 8a, 9, 11, 13 and 14 exhibited good activities against Bacillus Thuringenesis and Klebseilla Pneumonia, while compounds 3, 4, 6a, 6b, 6c, 7c, 8a, 8c, 11, 12 and 16 exhibited good activities against Trichoderma

2-(4-Toluenesulphonyloxy phenyl)-3,1-benzoxazine-4-one 2 was prepared and reacted with some nitrogen nucleophiles, e.g., ammonia, o-phenylendiamine, some heterocyclic amines, hydrazine hydrate and hydroxylamine hydrochloride and sulphur nucleophile, e.g., phosphorous pentasulphide. Structures of the newly synthesized compounds were established by elemental analysis and spectral data. All new prepared compounds were subjected to antimicrobial activity evaluation where compounds, 8, 9, 10 and 18 exhibited good activities against Bacillus Thuringenesis and 5, 10, 11, 14, 15, 17 and 18 exhibited good activities against Klebseilla Pneumonia. On the other hand, the results for antifungal activities revealed that, compounds 5, 6, 7, 16 and 17 exhibited good activities against Trichoderma Herzianum and Trichoderma Virdi.

4-(4-Oxo-4H-3,1-benzoxazin-2-yl)phenyl-4-methylbenzenesulfonate (2) was prepared and reacted with some primary aromatic amines, e.g., aniline, p-chloro aniline, p-methoxy aniline, p-amino benzoic acid and p-amino acetophenone. It reacted also with some heterocyclic amines, e.g., 2-aminothiazole, 2-aminobenzothiazole, 5-amino-4-phenylazo-2,4-dihydropyrazol-3-one and 3-amino-2-methylquinazolinone and with diamines; e.g., o-phenylenediamine, p-phenylenediamine, ethylenediamine, semicarbazide hydrochloride and thiosemicarbazide under different conditions. On the other hand, compound (2) reacted with both sodium azide and active methylene compounds, e.g., ethylcyanoacetate and ethylacetoacetate to give (19) and (20), respectively. All new prepared compounds were subjected to antimicrobial activity evaluation.

A novel multicomponent synthesis of highly substituted pyrazolo[1,5-a]pyrimidine derivatives starting from simple and readily available reactants is described. Three-component condensation of 4-(thiazol-2-yldiazenyl)-1H-pyrazole-3,5-diamine with triethyl orthoformate and carbonyl compounds or nitriles containing an activated methylene group gave substituted pyrazolo[1,5-a]pyrimidines. An analogous reaction of 4-(thiazol-2-yldiazenyl)-1H-pyrazole-3,5-diamine with ethyl acetoacetate and benzaldehyde in an effective one-pot process furnished ethyl 2-amino-5-methyl-7-phenyl-3-(thiazol-2-yldiazenyl)-4,7-dihydropyrazolo[1,5-a]pyrimidine-6-carboxylate.

This review highlights the methods used for the synthesis of enaminonitriles of pyrano[2,3-c]pyrazole compounds. Their reactivity and synthetic importance were investigated. In this context, recent progress in the synthesis and use of enaminonitriles of pyrano[2,3-c]pyrazole as precursors for heterocyclic compounds is reviewed. The synthetic routes for preparation of enaminonitriles are based on multicomponent reactions including three- or four-component reactions. Enaminonitriles can be used as building blocks for carbocyclic five-, six-, and seven-membered heterocycles as well as fused heterocyclic compounds. The bibliography includes 94 references.