Showing papers in "Journal of The Chemical Society-perkin Transactions 1 in 1977"
TL;DR: In this paper, the use of these intermediates for the synthesis of model procyanidins and for the biogenetically patterned synthesis of natural procyaneidins is discussed.
Abstract: Means to generate the C-4 carbocations (3) and (4) corresponding to (+)-catechin (1) and (–)-epicatechin (2), respectively, are outlined, and the use of these intermediates for the synthesis of model procyanidins and for the biogenetically patterned synthesis of natural procyanidins is discussed. 13C N.m.r. data for model flavan systems and natural procyanidins are reported and analysed and the information is used to assign the 4R-configuration to four natural procyanidin dimers. The phenomenon of conformational isomerism is demonstrated for the natural procyanidin dimers, and two different forms of restricted rotation about the interflavan bond are proposed. The information is used to clarify many earlier structural anomalies, to predict preferred conformations, and to specify a C(4)–C(8) link for the four principal dimers (B-1–4). The properties of some procyanidin polymers are noted, and structures of opposite helicity are proposed for two of the major types found in nature.
149 citations
TL;DR: The base-induced reaction of diazomethyltrimethylsilane (1) or of the dialkyl DMDs with aromatic carbonyl compounds leads directly to the corresponding homologous alkynes.
Abstract: The base-induced reaction of diazomethyltrimethylsilane (1) or of the dialkyl diazomethylphosphonates (4) and (5) with aromatic carbonyl compounds leads directly to the corresponding homologous alkynes. The mechanism, scope, and limitations of this synthetically useful process have been investigated.
113 citations
TL;DR: In this article, the authors evaluated theoretical reaction heats, evaluated by using total molecular energies from ten basis sets as reported in the literature, and compared with experiment for nine acyclic C4 hydrocarbons, seven cyclic C3 and C4 polycyclic C5 and five benzenoid Hydrocarbons.
Abstract: Ab initio theoretical reaction heats, evaluated by using total molecular energies from ten basis sets as reported in the literature, are compared with experiment for nine acyclic C4 hydrocarbons, seven cyclic C3 and C4 hydrocarbons, and five benzenoid hydrocarbons. The closeness of the agreement is examined to assess the effect of the greater matching of structural features in the homodesmotic reactions, and correlated with the type of basis set employed. Extended basis sets are essential for good agreement, other than fortuitous, if one of the reactant or product species is markedly destabilized or stabilized with repect to the rest, as is the case with the small ring structures and the benzenoid hydrocarbons, respectively.
109 citations
TL;DR: Radical-initiated reduction of five- or six-membered ring thiocarbonates, followed by alkaline hydrolysis, affords a convenient synthesis of 2, 3, 4, and 5-deoxy-sugars, and regiospecificity of the reaction complements the recently developed synthesis of 6- deoxy-Sugars by ring opening of the same thiOCarbonates with methyl iodide.
Abstract: Radical-initiated reduction of five- or six-membered ring thiocarbonates, followed by alkaline hydrolysis, affords a convenient synthesis of 2-, 3-, 4-, and 5-deoxy-sugars. For 5,6- and 4,6-thiocarbonates the regiospecificity of the reaction complements the recently developed synthesis of 6-deoxy-sugars by ring opening of the same thiocarbonates with methyl iodide. Applications in the synthesis of 2-deoxy-D-ribose and 2′-deoxyadenosine are described.
83 citations
TL;DR: In this article, conditions were established for the preparation of 2,5-dilithio-furan and -thiophen and the effects of changes in reaction conditions on the rates of hydrogen-lithium exchange for furan and thiophen have been investigated and discussed in detail.
Abstract: Conditions are established for the preparation in yields approaching quantitative of 2,5-dilithio-furan and -thiophen. The effects of changes in reaction conditions on the rates of hydrogen–lithium exchange for furan and thiophen have been investigated and are discussed in detail. The appearance of a 2,4-dilithio-intermediate, peculiar to N-methyl-pyrrole, has been explored, and optimum conditions are presented for the syntheses of both 2,4- and 2,5-intermediates. 3-Methylfuran and 3-melhylthiophen are similar to the parent heterocycles in their hydrogen–lithium exchange properties. The results of applying the procedure found to produce high yields of dilithio-products to a further eight heterocycles are described.
81 citations
TL;DR: The structure (I) of pseudomonic acid A, 9-{4-[5-(2,3-epoxy-5-hydroxy-4-methylhexyl)-3,4-dihydroxytetra-hydropyran-2-yl]-3-methylbut- 2-enoyloxy}nonanoic acid, has been established from mass spectral, and degradative studies of its methyl ester and various derivatives.
Abstract: The structure (I) of pseudomonic acid A, 9-{4-[5-(2,3-epoxy-5-hydroxy-4-methylhexyl)-3,4-dihydroxytetra-hydropyran-2-yl]-3-methylbut-2-enoyloxy}nonanoic acid, has been established from n.m.r., mass spectral, and degradative studies of its methyl ester and various derivatives.
78 citations
TL;DR: In this article, it was shown that the major product from these fragmentation reactions is (12)(precursor radical at C-5) from a van de Graaff generator.
Abstract: D-Glucose (5 × 10–3M) has been γ-irradiated in N2O–O2(80 : 20 v/v) saturated aqueous solutions (dose rate 11 × 1015 eV g–1 s–1), and the G values of 22 products determined Major products (G values in parentheses) are : D-gluconic acid (1)(09) D-arabino-hexosulose (2)(09) D-ribo-hexos-3-ulose (3)(057), D-xylo-hexos-4-ulose (4)(050), D-xylo-hexos-5-ulose (5)(060) D-gluco-hexodialdose (6)(155), and L-threo-tetrodialdose (12)(020) Products (1)–(6) are thought to be formed by OH attack at C-1–C-6, followed by oxygen addition and and HO2 elimination In competition, reactions between the glucose peroxyl radicals and HO2·(O2–·) give rise to fragmentation of C–C bonds The major product from these fragmentation reactions is (12)(precursor radical at C-5) From a dose rate study using electron pulses from a van de Graaff generator it has been shown that the rates of HO2· elimination are in the order: C-1 C-2, C-3, C-4 > C-6 C-5 Furthermore, it has been calculated that OH radicals abstract a hydrogen atom from C-1 and C-2 with a probability of ca 20% from C-3 and C-4 of ca 10% from C-5 of ca 15%, and from C-6 ca 30%
75 citations
TL;DR: Caldariellaquinone has been isolated from cultures of Caldariella acidophila, an extremely thermophilic and acidophilic bacterium.
Abstract: Caldariellaquinone {6-(3,7,11,15,19,23-hexamethyltetracosyl)-5-methylthiobenzo[b]thiophen-4,7-quinone (7)} has been isolated from cultures of Caldariella acidophila, an extremely thermophilic and acidophilic bacterium. The structure was determined by degradation, spectroscopic, and 13C biosynthetic studies.
74 citations
TL;DR: In this article, Nitro-compounds were reduced to amines in high yields by using palladium, platinum, or rhodium metal catalyst with formic, phosphinic, or phosphorous acid.
Abstract: Nitro-compounds were reduced to amines in high yields by using palladium, platinum, or rhodium metal catalyst with formic, phosphinic, or phosphorous acid. With formic acid, nitro-compounds containing fluorine were reduced but not those containing chlorine, bromine, or iodine. With the other acids, nitro-compounds containing any of the halogens were reduced with retention of the halogen.
73 citations
TL;DR: In this article, self-sensitised and Methylene Blue Sensitised photo-oxidation of 4-arylazo-1-and 1- Darylazo-2-naphthols gives 1,4 and 1,2 naphthoquinone, respectively, via the reaction of singles oxygen with the hydrazone tautomeric forms.
Abstract: Self-sensitised and Methylene Blue-sensitised photo-oxidation of 4-arylazo-1- and 1-arylazo-2-naphthols gives 1,4- and 1,2-naphthoquinone, respectively, via the reaction of singles oxygen with the hydrazone tautomeric forms. The reactivity in both cases decreases with increasing electron-withdrawing strength of substituents in the aryl ring.
71 citations
TL;DR: The versatile rhodium trichloride trihydrate has been employed in catalytic amounts to promote double-bond migration and the rearrangement of ergosterol was found to be more complex.
Abstract: The versatile rhodium trichloride trihydrate has been employed in catalytic amounts to promote double-bond migration. This method allows good yields in otherwise difficult, or impossible, exocyclic–endocyclic isomeris-ations. The rearrangement of ergosterol was found to be more complex. Besides known isomers a new cis-AB-ergosterol isomer has been isolated.
TL;DR: In this paper, a new force field was developed for the calculation of geometric and thermodynamic properties of alkanes and non-conjugated alkenes, which is a significant improvement on those of its kind in current usage and reproduces the heats of formation and hydrogenation of 60 diverse alkanes with an avarage absolute difference of 0.55 kcal mol 1 between observed and calculated enthalpies.
Abstract: A new force field has been developed for the calculation of geometric and thermodynamic properties of alkanes and non-conjugated alkenes. The force field is a significant improvement on those of its kind in current usage and reproduces the heats of formation and hydrogenation of 60 diverse alkanes and alkenes with an avarage absolute difference of 0.55 kcal mol–1 between observed and calculated enthalpies. The corresponding figures for geometric properties are 0.009 A. 0.6° and 0.9°. in bond lengths, bond angles, and torsion angles, respectively over a set of 93 observations. Particular attention has been paid to the structural and thermodynamic properties of molecules which are not adequately reproduced by previous force fields.
TL;DR: In this article, the configurations of 2-substituted 1,3,2-oxazaphospholidin(e)-2-ones, -2-thiones, and 2-selones derived from (−)-ephedrine have been established by spectroscopic and chemical methods.
Abstract: The configurations of the cis- and trans-isomers of 2-substituted 1,3,2-oxazaphospholidin(e)-2-ones, -2-thiones, and 2-selones derived from (–)-ephedrine have been established by spectroscopic and chemical methods. Displacements of the exocyclic 2-substituents occur with retention of configuration at phosphorus. With sodium alkoxides, the 1,3,2-oxazaphospholidine ring is opened by P–N rather than P–O bond cleavage, and inversion of configuration at phosphorus is observed. Under basic conditions the 2-methylamino-1-phenylpropyl phosphates derived from (–)-ephedrine rearrange to afford aziridines and phosphoric acid derivatives; in the cases of the sulphur- and selenium-containing derivatives optically active phosphorus thioacids and phosphorus selenoacids are formed and can be isolated as SMe and SeMe derivatives. These latter derivatives are converted into O-alkyl derivatives on treatment with alcohols in the presence of alkoxides or bromine or silver nitrate, and the stereoselectivities of these reactions have been determined. The optical purities of the title compounds have been measured by an n.m.r. method using chiral shift reagents.
TL;DR: N-Benzyloxycarbonyl and benzyl ester groups in peptides can be conveniently removed by transfer hydrogenation with cyclohexene and 10% palladium–carbon catalyst.
Abstract: N-Benzyloxycarbonyl and benzyl ester groups in peptides can be conveniently removed by transfer hydrogenation with cyclohexene and 10% palladium–carbon catalyst. If freshly prepared palladium black catalyst is used, other commonly used protecting groups like Nτ-benzyl (histidine), benzyl ether (tyrosine, serine), and nitro (arginine) can also be removed much more rapidly than in the case of the usually employed catalytic hydrogenation.
TL;DR: In this paper, an intermediate charge transfer complex between an iminium iodide and the perfluoroalkyl iodide has been detected, which can be extended to α-alkoxy-enamines and pyrroles.
Abstract: Condensation of enamines with perfluoroalkyl iodides yields α-perfluoroalkyl ketones. An intermediate charge-transfer complex between an iminium iodide and the perfluoroalkyl iodide has been detected. The reaction has been extended to α-alkoxy-enamines, ynamines, and pyrroles. It is rapid and uncatalysed for many simple enamines; others require u.v. light.
TL;DR: Five new neoechinulin-type metabolites have been isolated from the mycelium of Aspergillus amstelodami, grown on molasses beet cultures, and are characterised by a didehydrotryptophyl structure with a reversed isoprenic chain at the 2-position of the indole nucleus.
Abstract: Five new neoechinulin-type metabolites have been isolated from the mycelium of Aspergillus amstelodami, grown on molasses beet cultures. They are all characterised by a didehydrotryptophyl structure with a reversed isoprenic chain at the 2-position of the indole nucleus, and have been named neoechinulins A–E. On the basis of chemical and spectral evidence they have been shown to be, respectively, cyclo-alanyl-2-(1,1-dimethylprop-2-enyl)-didehydrotryptophyl (II), cyclo-didehydroalanyl-2-(1,1-dimethylprop-2-enyl)didehydrotryptophyl (III), cyclo-didehydroalanyl-2-(1,1-dimethylprop-2-enyl)-6-(3-methylbut-2-enyl)didehydrotryptophyl (IV), cyclo-alanyl-2-(1,1-dimethylprop-2-enyl)-6-(3-methylbut-2-enyl)didehydrotryptophyl (V), and cyclo-oxamyl-2-(1,1-dimethylprop-2-enyl)didehydrotryptophyl (VI).
TL;DR: In this article, a wide range of thiols have been exposed to 60Co γ-rays at 77 K both in the pure state and in dilute methanolic or aqueous glasses.
Abstract: A wide range of thiols have been exposed to 60Co γ-rays at 77 K both in the pure state and in dilute methanolic or aqueous glasses. Spectra assigned to RS· radicals had gz values (z is along the C–S bond) which varied with the environment, but which were all close to 2.158 in methanol. Dissociative electron capture at sulphur (RSH + e–→ R·+ SH–) was only observed in methanolic or aqueous glasses, and it is postulated that solvation of the ejected SH– ion plays an important role. The structure of a species (X) containing two sulphur atoms and having g values close to 2.058, 2.025, and 2.001 is discussed. Two rival structures, RS·–SR2 and RSS· are considered in depth and it is concluded that the former is most probable unless both species have very similar properties. Electron loss to give RSH+ followed by reaction to give R(H)S·–S(H)R+ cations is postulated to explain certain features in the e.s.r. spectra, as an alternative to proton loss to give RS· radicals.
TL;DR: The isolation, structures, and absolute configurations of sirodesmins A, B, C, and G, epipolythiopiperazine-2,5-dione antibiotics produced by Sirodesmium diversum, are reported.
Abstract: The isolation, structures, and absolute configurations of sirodesmins A, B, C, and G, epipolythiopiperazine-2,5-dione antibiotics produced by Sirodesmium diversum, are reported. The structure of sirodesmin A (1), an epidithiopiperazinedione, was determined by X-ray analysis of the derived diacetate (6). The structures of sirodesmins B (3), C (2), and G (7), epi-tetra-,-tri-, and -di-thiopiperazinediones, respectively, were determined by comparison of their chemical and spectroscopic properties with those of sirodesmin A. Sirodesmins B and C differ from sirodesmin A only in the size of the polysulphide bridge; sirodesmins A and G are epimers.
TL;DR: 4-Amino-2-[2-(piperidin-1-ylazo)phenyl]quinazoline behaves as a masked diazonium compound and decomposes in mineral acids, in acetic acid containing copper-bronze, in hot ethylene glycol, on photolysis in methanol or ethanol, or on reduction.
Abstract: The reactions of anthranilonitrile and 5-bromoanthranilonitrile with sodium hydride in dimethyl sulphoxide yield 4-amino-2-(2-aminophenyl)quinazoline (7) and its dibromo-analogue (8), respectively. Nitrosation of the diamines affords unstable diazonium salts which cyclise to either quinazolino[3,2-c]- or quinazolino[1,2-c]-[1,2,3]benzotriazines. These tetracyclic triazines readily undergo ring opening in the presence of secondary-amines to form the title compounds. 1,3-Bis-(2-cyano-4-bromophenyl)triazene (12) is smoothly transformed into dibromoquinazolines (18) in boiling secondary amines.4-Amino-2-[2-(piperidin-1-ylazo)phenyl]quinazoline (17a) behaves as a masked diazonium compound and decomposes in mineral acids, in acetic acid containing copper-bronze, in hot ethylene glycol, on photolysis in methanol or ethanol, or on reduction. The triazene linkage of (17a) is resistant to alcoholic potassium hydroxide but the 4-aminoquinazoline nucleus is hydrolysed to the corresponding quinazolin-4(3H)-one system. Methylation of (17a) with methyl iodide in tetrahydrofuran affords an N(1) methiodide (29) which is hydrolysed to the corresponding 1-methylquinazolin-4(1H)-one (30) in aqueous alkali. The unusual properties of this and other 1-methylquinazolin-4(1H)-ones can be attributed to their dipolar character. This renders the 1-methyl group liable to removal in acidic conditions.
TL;DR: Six new geranyl-hydroquinone-derived compounds have been isolated from the acetone extract of Cordia alliodora heartwood and characterized and the structural character of these compounds allows the proposal of a detailed bio-genetic pathway.
Abstract: Six new geranyl-hydroquinone-derived compounds have been isolated from the acetone extract of Cordia alliodora heartwood and characterized. The structural character of these compounds allows the proposal of a detailed bio-genetic pathway to the geranyl-hydroquinone and geranyl-benzoquinone constituents of Cordia sp.
TL;DR: In this paper, cyclopenta, cyclohexa, cyclosulphonyl isocyanate and cyclo-octa-1,3-dienes were shown to undergo stepwise thermal rearrangements with varying degrees of ease to thermodynamically more stable products of 1,4-addition or substitution products.
Abstract: Cyclopenta-, cyclohexa-, cyclohepta- and cyclo-octa-1,3-dienes react with chlorosulphonyl isocyanate to yield, in each case, a single N-chlorosulphonyl β-lactam as the primary product. In the first three examples, these 1,2-adducts undergo stepwise thermal rearrangements with varying degrees of ease to thermodynamically more stable products of 1,4-addition (cyclisation occurring through O or N in the ambident anion) or substitution products; the β-lactam from cyclo-octadiene could not be induced to rearrange constructively. The reactivity patterns in acyclic dienes are compared and a dipolar reaction mechanism is discussed. Selected n.m.r. spectra are analysed in detail and examples of conversion of the 1,4-adducts into 2-azabicyclo[2.2.1]-heptanes, -heptenes, and -heptadienes and into 2-oxa- or 2-aza-bicyclo[2.2.2]-octanes and -octenes and -bicyclo[3.3.2]nonenes are recorded.
TL;DR: Vilsmeier formylation of the nickel dimer yields mono- and di-substitution products.
Abstract: Treatment of the nickel(II) or copper(II) derivative of octaethyl-meso-hydroxymethylporphyrin with sulphuric acid in dimethyl-formamide or -acetamide gives mainly the meso,meso′-ethylenebis(porphyrinatometal) derivative by reactions involving electron transfer from the metal, In the absence of metal, only disproportionation reactions occur. Vilsmeier formylation of the nickel dimer yields mono- and di-substitution products.
TL;DR: In this article, the spin adduct obtained in the oxidation of propan-2-ol in the presence of phenyl N-t-butyl nitrone (PBN) was found to agree well with those observed in the cyclohexa-1.4-diene.
Abstract: The autoxidation of several organic compounds has been studied by the spin trapping technique. The chaincarrying peroxyl radicals were trapped by phenyl N-t-butyl nitrone (PBN) under the conditions of oxidation and hyperfine splitting constants were measured for several peroxyl radicals. Peroxyl radicals generated from other sources such as induced decomposition of hydroperoxides and thermal decomposition of azo-compounds under oxygen also gave similar results. The rate constants for the reaction of cumyl- and tetralinyl-peroxyl radicals with PBN were found to be much lower than those for the reactions of corresponding alkyl radicals with PBN. The hyperfine splitting constants of the spin adduct obtained in the oxidation of propan-2-ol in the presence of PBN were found to agree well with those observed in the oxidation of cyclohexa-1.4-diene, suggesting that the chain-carrying radical in the oxidation of propan-2-ol is not 1 -hydroxy-1 -methylethylperoxyl radical but hydroperoxyl radical.
TL;DR: Pseudomonic acid B, a minor antibiotic produced by Pseudomonas fluorescens, has been identified as 9-{4-[5-(2,3-epoxy-5-hydroxy-4-methylhexyl)-3,4,5-trihydroxytetrahydropyran-2-yl]-3-methylbut- 2-enoyloxy}nonanoic acid from a comparison of spectral data of its derivatives.
Abstract: Pseudomonic acid B, a minor antibiotic produced by Pseudomonas fluorescens, has been identified as 9-{4-[5-(2,3-epoxy-5-hydroxy-4-methylhexyl)-3,4,5-trihydroxytetrahydropyran-2-yl]-3-methylbut-2-enoyloxy}nonanoic acid from a comparison of spectral data of its derivatives with those of derivatives of pseudomonic acid A.
TL;DR: The bark and timber extractives of C. cuneifolium Thw. and C. soulattri Burm. f. have been studied as discussed by the authors, and they yielded calabaxanthone, trapezifolixanthone (2), taraxerol, simiarenol, friedelin, and a solid bark acid identified as isoapetalic acid.
Abstract: The bark and timber extractives of C. cuneifolium Thw. and C. soulattri Burm. f. have been studied. The bark extract of C. cuneifolium Thw. yielded calabaxanthone (1), trapezifolixanthone (2), taraxerol, simiarenol, friedelin, and a solid bark acid identified as isoapetalic acid (4a). The timber extract yielded 1,3,5-trihydroxy-2-(3-methylbut-2-enyl)xanthone (6a), six known xanthones, and β-sitosterol. The bark extract of C. soulattri Burm f. gave a new coumarin alcohol, soulattrolide (13a), taraxerol, taraxerone, and β-sitosterol. The timber extract afforded four known xanthones and the trihydroxyxanthone (6a).
TL;DR: The biosynthetic precursors of pseudomonic acid A (la) have been defined by using radioisotopically labelled substrates using 1JCC couplings to determine the route to (la), which proceeds through C12,C9, and C5 units.
Abstract: The biosynthetic precursors of pseudomonic acid A (la) have been defined by using radioisotopically labelled substrates. Positions of labelled atoms and values of 1JCC couplings in (la) biosynthesised from [1-13C]-, [2-13C]-, and [1,2-13C2]-acetate, [1-13C]- and [3-13C]-propionate, and L-[Me-13C]methionine have been determined by 13C n.m.r. spectroscopy. A biosynthetic route to (la) is suggested which proceeds through C12,C9, and C5 units.
TL;DR: In this paper, the results of feeding experiments with labelled (3H, 14C) cinnamic acids are presented to show that the various procyanidin dimers are biosynthesised from two metabolically distinct units.
Abstract: Evidence based on the results of feeding experiments with labelled (3H,14C) cinnamic acids is presented to show that the various procyanidin dimers are biosynthesised from two metabolically distinct units. One is the flavan-3-ol [(+)-catechin (10) or (–)-epicatechin (9)] and the other is the C-4 carbocation (7) or (8). It is postulated that the carbocations are derived by protonation of the flav-3-en-3-ol (6) and are intermediates in the reduction to the flavan-3-ols (9) and (10). The relationship of procyanidin biosynthesis to anthocyanidin formation in certain plants is briefly discussed.
TL;DR: In this article, the use of chiral diols each containing a two-fold axis of symmetry to achieve asymmetric reductions of some ketones with metal hydrides is described.
Abstract: The use of chiral diols each containing a two-fold axis of symmetry to achieve asymmetric reductions of some ketones with metal hydrides is described. The symmetry of these complexes simplifies interpretation of the observed enantioselectivity.
TL;DR: The synthesis of azamethine dyes containing the pyrazolo[3,2-c]-s-triazole nucleus is described in this paper, and the absorption properties of these dyes are discussed.
Abstract: The condensation of ethyl 5-hydrazino-3-methylpyrazole-4-carboxylate with various aldehydes and acid chlorides is outlined. The cyclisation of the resulting hydrazones and hydrazides, under oxidative and dehydrative conditions, respectively, gives ethyl 1H-pyrazolo[3,2-c]-s-triazole-7-carboxylates. Hydrolysis of these esters followed by decarboxylation of the corresponding acids, gives 1H-pyrazolo[3,2-c]-s-triazoles. Another method of synthesis of 1H-pyrazolo[3,2-c]-s-triazoles, involves the condensation of S-methylisothiocarbohydrazide hydroiodide with β-oxo-esters. The synthesis of a range of azamethine dyes containing the pyrazolo[3,2-c]-s-triazole nucleus is described, and the absorption properties of these dyes are discussed.
TL;DR: In this paper, photoelectron spectra of the title compounds demonstrate that 3 and 4-hydroxy- and 3-and 4-mercapto-pyridine exist in the vapour phase in the hydroxy-and mercapto-forms with less than 5% of the oxo- or thioxo-tautomers.
Abstract: Photoelectron spectra of the title compounds demonstrate that 3- and 4-hydroxy- and 3- and 4-mercapto-pyridine exist in the vapour phase in the hydroxy- and mercapto-forms with less than 5% of the oxo- or thioxo-tautomers However, although the hydroxy- and mercapto-tautomers are also predominant for the 2-series, significant quantities of 2-pyridone and pyridine-2-thione exist in equilibrium