Tautomerism of anthraquinones: IV. 1-Hydroxy-9,10-anthraquinone and its substituted derivatives

1-Hydroxyanthraquinone and its substituted derivatives exist as equilibrium mixtures of four tautomers and rotational isomers. Their anions have 9,10-and 1,10-quinoid structures. Each tautomer or conformer is characterized by a single π₁,π* band in the electronic absorption spectrum.     

Tautomerism of anthraquinones: VIII. Tautomerism and conformations of 1,4-diamino-9,10-anthraquinone

The compound widely known as 1,4-diamino-9,10-anthraquinone is in fact an equilibrium mixture of 4,9-diamino-1,10-anthraquinone and tautomeric imino forms, 10-amino-9-hydroxy-1,4-anthraquinone 1-imine and its conformer, and 4-amino-1-hydroxy-9,10-anthraquinone 9-imine or 4,9-dihydroxy-1,10-anthraquinone diimine. Amino-imino tautomerism and rotational isomerism are responsible for fine structure of the π_l,π*-absorption of the title compound.     

Tautomerism of anthraquinones: IX. Protonated 1,5- and 1,8-dihydroxyanthraquinones

Fine structure of π_l,π* absorption of mono- and dications derived from 1,5- and 1,8-dihydroxyanthraquinones originates from their existence as dynamic equilibrium mixtures of isomers differing by position of the positive charge, double bond distribution, and number of intramolecular hydrogen bonds. Protonation is accompanied by displacement of isomeric equilibria. Isomeric transformations of protonated dihydroxyanthraquinones involve mainly excited states of their molecules.     

Tautomerism of the Natural 1,8-Dihydroxy-9,10-anthraquinones Chrysophanol,Aloe-emodin,and Rhein

Existence of chrysophanol, aloe-emodin, and rhein in solutions as primarily the 9,10-anthraquinoid forms was established by quantum-chemical and correlation methods. However, the 1,10-anthraquinoid tautomers are observed in certain media. As a rule, these compounds in alkaline solutions form 1,10-anthraquinoid anions that are stabilized by intramolecular H-bonds and have a single α-hydroxy group. Tautomers exist in both the ground and excited states of the molecule. Quantitative rules that control changes in the ionization and tautomerization parameters of chrysophanol were found.__________Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 119–124, March–April, 2005.     

Separation of 9,10-anthraquinone derivatives: evaluation of C18 stationary phases

Chromatographic study of natural products helps to determine their molecular composition and to identify their sources (biological, geographical, etc.). However, identifying anthraquinoids is still a challenge because this chemical family is composed of more than half a thousand molecules. In the present work, a series of C18 stationary phases were systematically studied to evaluate their separation potential for these compounds in high performance reversed phase liquid chromatography (RPLC). The stationary phases with various physicochemical properties were evaluated with respect to the separation of 30 standards. Selectivity of the studied stationary phases and symmetry of the obtained peaks were chosen as the evaluation criteria. Calculations of symmetry and separation capacity parameters have been proposed. These parameters are not dependent on the dimensions of the columns and were used for the classifications of phases. In conclusion, this study is intended to be a guide as to which stationary phases analysts can use for resolving separation of a complex mixture of anthraquinoids.     

Synthetic transformations of higher terpenoids: XVIII. Synthesis of optically active 9,10-anthraquinone derivatives

Retro-Diels-Alder decomposition of dodecahydro-endo-4b,12-ethenochrysene-1,4-diones obtained from a tricyclic diterpenoid, levopimaric acid, gave optically active 5-[2-(6-vinyl-2,6-dimethyl-2-carboxycyclohexyl) ethyl]-7-isopropyl-1,4-naphthoquinones which reacted with silyloxybutadienes to produce the corresponding 6- and 7-hydroxyanthraquinones, 5-furyl-7-hydroxytetrahydroanthraquinones, or 5-furyl-7-oxohexahydroanthraquinones. Condensation of the naphthoquinone derivatives with 5-isopropenyl-2,3-dihydrothiophene 1,1-dioxide resulted in the formation of 6,11-dioxodihydro- and 6,11-dioxohexahydroanthra[2,1-b]thiophene 3,3-dioxides. 6- and 7-Hydroxyanthraquinones were also obtained by reaction of dodecahydro-endo-4b,12-ethenochrysene-1,4-diones with Danishevsky diene, followed by cleavage of the polycyclic adducts. The cycloaddition of 5-[2-(-2-carboxy-2,6-dimethyl-6-vinylcyclohexyl)ethyl]-7-isopropyl-1,4-naphthoquinones in the presence of Lewis acids was characterized by increased regioselectivity.     

Tautomerism of anthraquinones: X. Quinizarin boron complex

Products of reactions of hydroxyanthraquinones with boric acid exist as equilibrium mixtures of tautomeric boron complexes and boric acid esters in which one or two boron atoms are not coordinated to carbonyl groups. Tautomerism is responsible for the appearance of several π_l,π bands in the electronic absorption spectra and considerable differences in the data obtained by different authors. Boron-containing quinizarin derivatives have mostly 1,10-quinoid structures. The use of quinizarin as an analytical reagent for the determination of boron is based on replacement of tautomeric equilibria due to complex formation rather than on coordination-induced red shift of the absorption maximum.     

Tautomerism of anthraquinones: XI. 1-amino-4-hydroxyanthraquinone

The disperse dye red 2C regarded as 1-amino-4-hydroxy-9,10-anthraquinone is not a 9,10-anthraquinone derivative. It is characterized by a keto-enol and amino-imine tautomerism, and it consists of equilibrium mixtures of tautomers and conformers containing 4-amino-9-hydroxy-1,10-, 9 amino-4-hydroxy-1,10-, 9 amino-10-hydroxy-1,4-anthraquinones, 9 hydroxy-1,10-anthraquinone 10-imine, and also their conformers with a single intramolecular hydrogen bond. The significant differences in the absorption spectra known for this dye originate from the dissimilar isomeric composition of the samples of this dye prepared or purified by various procedures     

Quantum-chemical and correlation study on the tautomerism and ionization of 1,4,5,8-tetrahydroxy-9,10-anthraquinone and its alkyl-substituted derivatives

1,4,5,8-Tetrahydroxy-9,10-anthraquinone and its alkyl derivatives exist as equilibrium mixtures of prototropic tautomers and rotational isomers differing in the mode of intramolecular hydrogen bonding. Their electronic absorption spectra contain π_l,π* bands corresponding to 9,10-, 1,10-, 1,4-, and (more rarely) 1,5-anthraquinoid structures. Introduction of substituents, solvation, ionization, and complex formation lead to displacement of tautomeric and conformational equilibria, which are responsible for the observed diversity of their absorption spectra.     

Separation of 9,10-anthraquinone derivatives: evaluation of functionalised stationary phases in reversed phase mode

A series of reversed phases bonded with several functional groups was investigated for separation of anthraquinone derivatives, following the previous work, dedicated to the selectivity of octadecyl silica bonded phases. Considering wide diversity of substitutions in hydrophobic anthraquinone skeleton, interactions like dipole-dipole, π-π or H-bond acceptor/donor, as well as inclusion complexes formation can be employed to improve separation. In this study, several phases with grafts like cyano, nitro, aromatic, PEG, diol, calixarene and cyclodextrin were used with water-acetonitrile gradient for separation of thirty anthraquinoids' standards. The evaluation of performances was measured using the symmetry parameter and the number of critical pairs of peaks formed. The results point out the aromatic and calixarene bonded silica as the most interesting in terms of symmetry and critical pairs number. Finally we tested the performance of Caltrex Resorcinaren, Pursuit XRs DP and Luna Phenyl-Hexyl on real samples of anthraquinone natural dye extracted from a red thread taken from a 15th C. tapestry. We observed and compared the retention behaviour of some new anthraquinoids additional to our standards set and showing behaviour particular to substituted anthraquinone carboxylic acids.     

Spectrofluorometric study of complexation of some amino derivatives of 9,10-anthraquinone with beta-cyclodextrin

Complexation reactions between 1-amino-9,10-anthraquinone (AA1), 1-amino-2-methyl-9,10-anthraquinone (AA2), 1-amino-2,4-dimethyl-9,10-anthraquinone (AA3) and 1-amino-2-ethyl-9,10-anthraquinone (AA4) and beta-cyclodextrin were studied spectrofluorometrically, under optimized experimental conditions. The formation constants of the resulting 1:1 beta-cyclodextrin complexes were evaluated and found to decrease in the order AA4>AA1>AA3>AA2. Possible reasons for the observed stability sequence are discussed based on the structures proposed for the resulting inclusion complexes.     

Metal Complexes with 1,5- and 1,8-Dihydroxy-9,10-Anthraquinones: Electronic Absorption Spectra and Structure of Ligands

Metal complexes with 1,5-dihydroxy-9,10-anthraquinone are studied by the spectrophotometric, quantum-chemical, and correlation methods. It was established that the ligand in these complexes can occur in seven excited states that differ not only in the ionization degree but also in the prevailing contribution of the tautomeric 9,10-, 1,10-, and 1,5-anthraquinoid structures. In all known complexes with 1,8-dihydroxy-9,10-anthraquinone and singly ionized ligand, this ligand has the 1,10-anthraquinoid structure; in complexes with the doubly ionized ligand, the latter ligand most often has the 9,10-anthraquinoid structure. The known complexes are classified according to the ligand structures.     

Tautomerism of azine derivatives. 6. Tautomerism of 2-pyrimidinylmethane derivatives

The existence of a pyrimidinyl-pyrimidinylidene tautomeric equilibrium in solutions of 2-pyrimidinylcyanoacetic acid esters in CDCl₃ was observed. Unsymmetrically substituted compounds form two types of ylidene tautomers that differ with respect to the position of the NH proton, the ratio between which is controlled by the substituents in the 4 (6) position. The introduction of both donor and acceptor substituents into the 5 position of the pyrimidine ring increases the amount of the pyrimidine form. The same thing occurs when the polarity of the solvent is decreased. The addition of DMSO or DMF to CDCl₃ leads to conversion of the intrachelate ylidene tautomers to unchelated tautomers. Protonation (CF₃COOH) shifts the equilibrium to favor the ylidene tautomer that has higher basicity.See [1] for communication 5.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 827–831, June, 1984.     

Photoreduction of 9,10-anthraquinone derivatives: transient spectroscopy and effects of alcohols and amines on reactivity in solution

The photoreduction of 9,10-anthraquinone (AQ), the 2-methyl, 2-ethyl, 2,3-dimethyl, 1,4-difluoro, 1-chloro and 1,8-dichloro derivatives as well as 1,4,4a,9a-tetrahydroanthraquinone, 1,2-benzanthraquinone and 6,13-pentacenequinone in nonaqueous solution at room temperature was studied by time-resolved UV-visible spectroscopy. Upon 308 nm excitation of AQ the triplet state reacts with alcohols and triethylamine (TEA). The rate constant of triplet quenching by amines is close to the diffusion-controlled limit. The semiquinone radical *QH/ Q*- is the main intermediate, and the half-life of the second-order decay kinetics depends significantly on the donor and the medium. Photoinduced charge separation after electron transfer from amines to the triplet state of AQ in acetonitrile and the subsequent charge recombination or neutralization also were measured by transient conductivity. The maximum quantum yield, lambdairr = 254 nm, of photoconversion into the strongly fluorescing 9,10-dihydroxyanthracenes is close to unity. The fluorescence with maximum at 460-480 nm and a lifetime of 20-30 ns disappears as a result of a complete recovery into AQ, when the dihydroxyanthracenes are exposed to oxygen. The mechanisms of photoreduction of parent AQ in acetonitrile by 2-propanol and in benzene and acetonitrile by TEA are discussed. The effects of AQ follow essentially the same pattern. The various functions of oxygen, e.g. (1) quenching of the triplet state; (2) quenching of the semiquinone radical, thereby forming HO2*/O2*- radicals; and (3) trapping of the dihydroxyanthracenes are outlined.     

Laser flash photolysis and magnetic field effect studies on the interaction of uracil and its derivatives with menadione and 9,10-anthraquinone

Laser flash photolysis and an external magnetic field have been used to study the interaction of two quinone molecules, namely, 9,10-anthraquinone (AQ) and 2-methyl-1,4-naphthoquinone, commonly known as menadione (MQ), with the RNA base uracil (U) and two of its derivatives, 1,3-dimethyluracil (dmU) and uridine (dU). We have conducted our studies in homogeneous organic and heterogeneous micellar media in order to investigate the effect of media on the molecules and any change in reactivity on account of substitution. In organic homogeneous medium, both the quinones have behaved similarly with the bases. Here U has undergone both electron transfer (ET) and hydrogen (H) transfer, while dU and dmU have failed to exhibit any ET. Failure to support ET has been attributed to keto-enol tautomerism, which has been found to have a significant role in determining the occurrence of ET from these pyrimidine bases. However, in SDS micelles some variations regarding the reactivity of these molecules have been discerned. The variations are 2-fold. Here ET from U has been found to get completely eclipsed by a dominant H abstraction with both the quinones, and AQ reveals a difference in the extent of H abstraction with the bases in SDS. With U and dU, the prevailing H abstraction with AQ has succeeded in formation of only AQH(*), while dmU has produced both AQH(*) and AQH(2), the latter being formed by two successive H abstraction. Explanations of this intriguing behavior with U and its derivatives with quinone molecules have been the main concern in this work.     

Electrochemical properties of modified carbon paste electrodes containing some amino derivatives of 9,10-anthraquinone

The preparation and aqueous electrochemistry of carbon paste electrodes modified by some amino-substituted 9,10-anthraquinones are reported. In all cases, the reduction processes studied by cyclic voltammetry reveal a quasi-reversible behavior. The half-wave potentials were calculated as a function of the solution pH and from the resulting potential-pH plots the formal potentials and pK _a ^′ values of different redox and acid-base couples involved at various pH ranges were evaluated. The diffusion coefficients of different anthraquinone derivatives used in paraffin oil were calculated by chronoamperometry. Received: 3 May 1999 / Accepted: 15 November 1999     

Solubilities of some recently synthesized 1,8-dihydroxy-9,10-anthraquinone derivatives in supercritical carbon dioxide

The solubility of four recently synthesized 1,8-dihydroxy-9,10-anthraquinone derivatives, as potential complexing agents in some extraction and membrane transport experiments, have been measured in supercritical carbon dioxide. The measurements were carried out in the pressure range 120-400 atm at temperatures 35, 45, 55, 65, and 75 degrees C. The measured solubilities were correlated using the model proposed by Chrastil. The calculated results show good agreement with the experimental data.     

Tautomerism of derivatives of azines. 16. Tautomerism of acylmethylpyrazines and -quinoxalines

The tautomeric equilibria of acylmethylpyrazines and -quinoxalines in chloroform were studied by ¹H, ¹⁴N, and ¹⁷O NMR spectroscopy. It was shown that keto-enol tautomerism is realized in the acylmethylpyrazine series. Annelation leads to the development of an ylidene tautomer in the acylmethylquinoxaline series. A marked dependence of the position of the intrachelate equilibrium on the character of the solvent was observed.See [1] for Communication 15.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1663–1667, December, 1987.