Thiophene chemistry—IX Preparation and dealkylation of 5-substituted 2-t-butoxythiophenes

Different substituents have been introduced into the 5-position of 2-t-butoxythiophene and by subsequent acid-catalyst dealkylation the corresponding hydroxythiophenes (or their different tautomeric forms) have been isolated in high yields. By NMR, IR and UV-spectroscopy the constitution and the composition of the different tautomeres have been determined. The reaction between carbonyl compounds and 5-t-butoxy-2-thiophenelithium gives carbinols, which upon heating with p-toluenesulphonic acid yielded 5-methylene-3-thiolen-2-ones.     

Studies in polyether synthesis: Spiroacetal formation by directed bisepoxide cyclisation.

The synthesis of tricyclic polyether fragments 15 and 20, which incorporate a spiroacetal subunit, using directed bisepoxide cyclisations is described.     

Stereoselective synthesis of (+)-cryptocarya diacetate by an iterative Prins cyclisation and reductive cleavage sequence

A highly stereoselective synthesis of (+)-cryptocarya diacetate is achieved through our recently developed strategy for the construction of 1,3-diols via Prins cyclisation. The route relies mainly on the reductive cleavage of allylic ethers, ozonolysis and Wittig olefination along with Prins cyclisation.     

Electron-impact studies—XVIII. Fragmentation of some epimeric 7-substituted norbornenes

The fragmentation modes of some epimeric 7-substituted norbornenes have been established from deuteration and high resolution studies. Stereochemical differences were found to have very little influence in directing the fragmentation modes of these epimers. Formation of common intermediates or transition states by ring opening process has been proposed to rationalise the mass spectral behaviour of these compounds.     

Metal-assisted reactions—13 : Rapid, slective reductive cleavage of phenolic hydroxyl groups by catalytic transfer methods

Previous work has shown that, after converting phenols into suitable phenolic ethers, the aromatic C---H bond of the original phenol can be reductively cleaved heterogeneously to give a C---H bond through the use of molecular hydrogen or hydrogen donors together with a transition metal catalyst. The present work provides a method for selectively replacing a phenolic OH group by H in just a few minutes, compared with the 2 to 4 hr required previously using a hydrogen donor and the several hours under pressure required for molecular hydrogen. Various kinds of groups are suitable for preparing the required phenolic ethers from phenols, but the best ones are strongly electro-withdrawing heteroaromatic entities. Solvent appears to play an important role in this heterogeneous reaction, the mechanism of which is discussed.     

On the interaction of silyl triflates with enamines: iminium ion formation versus silylation

The interaction of Me₃SiOTf and (C₆F₅)₃SiOTf with enamines generating α-silyl-substituted iminium ions is investigated. A trimethylsilyl iminium cation is formed as a long-lived species observable by NMR spectroscopy, whilst the tris(pentafluorophenyl)silyl analogue is very labile and prone to the loss of a proton. On the basis of the latter phenomenon, a method for the synthesis of β-silyl enamines is proposed.     

Purine N-oxides—XL The 3-acyloxypurine 8-substitution reaction: Scope: Syntheses of 8-substituted xanthines and guanines

The 3-acyloxypurine 8-substitution reaction is a substitution-elimination reaction involving attack at C-8 by inorganic or organic nucleophiles and departure of an acyloxy group from N-3. It has been studied with 3-acetoxyxanthine, 3-hydroxyguanine and a number of related derivatives and is the method of choice for the preparation of many 8-substituted xanthines or guanines. It proceeds extremely rapidly in neutral aqueous solutions at room temperature. With water alone an 8-hydroxypurine results, and water always competes to some degree with other nucleophiles. The reaction can be carried out in dipolar aprotic solvents, in which it is also possible to prepare the acyloxy derivative in situ and to choose more effective leaving groups such as mesyloxy or tosyloxy. The reaction has been demonstrated with chloride, bromide, nitrite, and azide ions; with the thioether of methionine; a variety of pyridine derivatives, and with primary and secondary alcohols. This reaction is apparently restricted to 3-acyloxypurines which are also substituted at position-2. The behaviour of 3-acetoxy-1-methylxanthine is similar to that of 3-acetoxyxanthine, but 3-acetoxy-7-methylxanthine fails to undergo the reaction.     

The thermal C2-C6/[2 + 2] cyclisation of enyne-allenes: reversible diradical formation

New enyne-allenes, structurally designed toward the thermal C(2)-C(6)/[2 + 2] cyclisation mode, were prepared and characterised, one of them even by X-ray crystallography. The mechanism of their transformation to formal [2 + 2] cycloadducts was interrogated by trapping experiments and DFT computations. The results support a stepwise mechanism that involves the reversible formation of the C(2)-C(6) diradical intermediate.     

Studies on the biosynthesis of cholesterol—7 Rearrangement of methyl groups during enzymic cyclisation of squalene

The present state of knowledge of the biosynthesis of cholesterol is summarised. An experiment is described wherein a special application of the carbon isotope ¹³C permits a decision between possible modes of rearrangement during the enzymic cyclisation of squalene.     

Summary of ISO/TC 201 standard: XVIII,ISO 19318:2004—surface chemical analysis—X‐ray photoelectron spectroscopy—Reporting of methods used for charge control and charge correction

International Standard ISO 19318 specifies the minimum amount of information describing the methods of charge control and charge correction in measurements of core‐level binding energies for insulating specimens by x‐ray photoelectron spectroscopy, which is to be reported with the analytical results. Information is also provided on methods that have been found useful for charge control and charge correction in the measurement of binding energies. Copyright © 2005 John Wiley & Sons, Ltd.     

International journal of quantum chemistry—A journal devoted to quantum theory and computations in chemistry,condensed matter physics,and biology. Editorial—Program and Policies

A comparison is made between the exact classical dynamic theory and the stochastic theory of reaction rates in dense media. The conditions of their equivalence are derived on the basis of a harmonic oscillator model. It is shown in a direct way that in the general case of a multidimensional system the approximate activated complex (transition-state) theory is not identical to the stochastic theory at any value of the viscosity coefficient. © 1992 John Wiley & Sons, Inc.     

Delineating ligand effects in intramolecular aryl amidation reactions: formation of a novel spiro-heterocycle by a tandem cyclisation process

Ligand effects for intramolecular Pd-catalysed aryl amidation reaction were examined for the synthesis of seven-membered benzolactam rings. In an attempt to produce an eight-membered ring, tandem C-N/C-O bond forming reactions occurred to give a novel spiro-benzofuran-lactam structure.     

Enamine formation from anilines and methyl propiolate. The synthesis of 4(1h)-quinolones

The addition of substituted anilines to methyl propiolate produces labile cis-trans mixtures of enamines which can be isomerized by acid, solvent variation, and thermal techniques. Thermal cyclization of these enamines provides a synthesis of 4( lH)-quinolones.     

Exploring the oxidative cyclization of substituted N-aryl enamines: Pd-catalyzed formation of indoles from anilines

The direct Pd-catalyzed oxidative coupling of two C-H-bonds within N-aryl-enamines 1 allows the efficient formation of differently substituted indoles 2. In this cross-dehydrogenative coupling, many different functional groups are tolerated and the starting material N-aryl-enamines 1 can be easily prepared in one step from commercially available anilines. In addition, the whole sequence can also be run in a one-pot fashion. Optimization data, mechanistic insight, substrate scope, and applications are reported in this full paper.     

The investigation of an equilibrium dependent reaction for the formation of enamines in a microchemical system

The paper describes the equilibrium dependant reaction for the formation of enamines in a microchemical system utilising electroosmotic flow (EOF) for fluid mobilisation. The authors have shown that the reaction can be carried out without the presence of a Lewis acid catalyst, in addition the enamine intermediate was synthesised at room temperature using mild solvent conditions. A 42% conversion of cyclohexanone into the enamine has been achieved to date.     

Gold catalysis: 1,3-oxazines by cyclisation of allene amides

A series of allene amides was prepared and their gold-catalysed cyclisation was investigated. The formation of six-membered rings, 1,3-oxazines, was observed. Dihydropyrroles originating from intramolecular hydroamination of the distal C=C double bonds of the allenes were minor side products. Mechanistic studies by in situ (31)P NMR spectroscopy showed only one additional species during the conversion in each case; a computational study of the different allyl gold(I) species involved allowed this to be assigned as the σ-allyl gold species bearing the gold catalyst at the sterically less hindered methylene end. The regiospecific deuterodeauration of this intermediate confirmed a S(E) '-type mechanism for this last step of the catalytic cycle.     

Energy level tuning of polythiophene derivative by click chemistry‐type postfunctionalization of side‐chain alkynes

A polythiophene derivative substituted with electron‐rich alkynes as a side chain was synthesized using the Suzuki polycondensation reaction. The electron‐rich alkynes underwent the “click chemistry”‐type quantitative addition reaction with strong acceptor molecules, such as tetracyanoethylene (TCNE) and 7,7,8,8‐tetracyanoquinodimethane (TCNQ), resulting in the formation of donor–acceptor chromophores. All polymers showed excellent solubilities in the common organic solvents as well as good thermal stabilities with their 5% decomposition temperatures exceeding 230 °C. The TCNE‐/TCNQ‐adducted polymers displayed well‐defined charge‐transfer (CT) bands in the low energy region. The CT energy of the TCNE‐adducted polymer was 2.56 eV (484 nm), which was much greater than that of the TCNQ‐adducted polymer [1.65 eV (750 nm)]. This result was supported by the electrochemical measurements. The electrochemical band gaps of the TCNE‐adducted polymers were much greater than those of the corresponding TCNQ‐adducted polymers. Furthermore, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, determined from the first oxidation and first reduction peak potentials, respectively, decreased with the increasing acceptor addition amount. All these results suggested that the energy levels of the polythiophene derivative can be tuned by varying the species and amount of the acceptor molecules using this postfunctionalization method. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010