Showing papers in "Pure and Applied Chemistry in 1999"

Electrochemical Biosensors: Recommended Definitions and Classification

Abstract: Republication or reproduction of this report or its storage and/or dissemination by electronic means is permittedwithout the need for formal IUPAC permission on condition that an acknowledgement, with full reference to thesource along with use of the copyright symbol q, the name IUPAC and the year of publication are prominentlyvisible. Publication of a translation into another language is subject to the additional condition of prior approvalfrom the relevant IUPAC National Adhering Organization.

Passivity : the key to our metals-based civilization

Abstract: Humankind has been able to develop a metals-based civilization primarily because the reactive metals (Fe, Ni, Cr, Al, Ti, Zr, . . .) exhibit extraordinary kinetic stabilities in oxidizing environments. From the time of Schonbein and Faraday (1830s), the reason for this stability has been attributed to the existence of a thin reaction product ®lm on the metal (or alloy) surface. This ®lm effectively isolates the metal from the corrosive environment. However, attempts to elucidate the mechanisms of the formation of passive oxide ®lms, which generally comprise bilayer structures consisting of a defective oxide that grows directly into the metal and an outer, precipitated hydroxide (or oxyhydroxide on even oxide) layer, have yielded only a rudimentary understanding of the chemistry and physics of the growth and breakdown processes. In this paper, selected aspects of passivity and passivity breakdown are reviewed, with emphasis on the physical models that have been proposed to account for the experimental observations. One such model, the Point Defect Model, is shown to account for most, if not all, experimental observations, and to provide a robust basis for predicting the occurrence of passivity breakdown in any given system. By combining the Point Defect Model with deterministic models for pit growth and crack growth, it is now possible to predict the evolution of localized corrosion damage in a wide range of systems.

Phosphorescent materials for application to organic light emitting devices

Abstract: Organic phosphors have demanded the attention of the organic electroluminescence community because they enable efficiencies quadruple that of fluorescent materials. In this work, we review the categories of organic phosphors: lanthanide complexes, organic phosphors and metal-organic complexes. The characteristics necessary for efficient phosphor- escence are considered and conclusions are drawn as to the most promising materials.

From ozone generators to flat television screens: history and future potential of dielectric-barrier discharges

Abstract: Dielectric-barrier discharges (silent discharges) combine the ease of atmospheric pressure operation with nonequilibrium plasma conditions suited for many plasma chemical processes. In most gases at this pressure the discharge consists of a large number of randomly distributed short-lived microdischarges. Their properties are discussed in detail. Traditionally mainly used for industrial ozone production, dielectric-barrier discharges have found addi- tional large volume applications in surface treatment, high-power CO2 lasers, excimer ultraviolet lamps, pollution control and, most recently, also in large-area flat plasma display panels. Future applications may include their use in greenhouse gas control technologies. Historical aspects, properties and applications of dielectric-barrier discharges are discussed.

Critical compilation of scales of solvent parameters. Part I. Pure, non-hydrogen bond donor solvents

Abstract: Introduction and Fundamental Concepts It has long been known that solvents often affect chemical reactivity, this involving, e.g., the shift of the position of chemical equilibria (thermodynamic aspect) as well as significant changes in reaction rate constants (kinetic aspect). Physical properties, particularly the frequencies and intensities of transitions in IR, UV-visible, fluorescence, NMR and ESR spectroscopies are also known to be affected by solvents. These phenomena are consequences of differences in the solvation of reagents and products (thermodynamic effects) or reagents and activated complexes (kinetic effects). Differential solvation of species in the ground and excited states accounts for the spectral phenomenology indicated above. Differences in solvation of a given solute in two different solvents determine the size of the corresponding partition coefficient. It is customary to state that these effects reflect the influence of \"solvent polarity\". According to Reichardt, \"solvent polarity\" is \"the overall solvation capability (or solvation power) of solvents, which in turn depends on the action of all possible, nonspecific and specific intermolecular interactions between solute ions or molecules and solvent molecules, excluding, however, those interactions leading to definite chemical alterations of the ions or molecules of the solute (such as protonation, oxidation, reduction, chemical complex formation, etc.)\". This definition underscores the extreme complexity of \"solvation effects\" at the molecular level. This notwithstanding, solvent effects (SE) often display some remarkable regularities that allow in many cases an \"empirical treatment\" that sheds light on their origin and main contributors. Consider a solute S and two different properties P1 and P2, taking the values {P10, P11, ..., P1i, ...} and {P20, P21, ..., P2i, ...} respectively in solvents S0, S1, ..., Si,... A scale of SEs is simply constructed by taking for each solvent, Si, the difference P1i - P10, S0 being chosen as a reference solvent. If the property P1 is a \"good descriptor\" of SEs on P2, equation (1) holds for solvent Si : P 2i - P20 = k (P1i - P10) (1) or P 2i - P20 = kpi (2) wherein k is a constant independent of the solvents and determined solely by P2. pi is the \"solvent parameter\" characteristic of solvent Si. More generally, the properties being compared might belong to two different solutes. For any property Pm of any solute, and if P1 is a \"good descriptor\", equation (3) holds: Pmi - Pm0 = kmpi (3) We draw attention to the (frequently overlooked) fact that if equations such as (3) were truly general, then, all SEs would be linearly related to an extremely high degree of precision and a single, universal scale of SEs would exist. This is against all the available experimental evidence. Excellent correlations of narrower scope do exist however, that successfully link a very large amount of experimental data for a substantial variety of solvents and solutes and a relatively small number of empirical scales. In this compilation, scales are selected on the basis of criteria to be discussed below. For each of them, the most reliable values of solvent parameters are given. The physical foundations and the scope of the scales are discussed. Some suggestions regarding their use are made. For the purpose of facilitating future work in the field, some indications are given regarding the experimental determination of the various parameters. Solvent-solute interactions always involve dispersion or London's forces and, very often, dipolar and/or multipolar interactions. Current theoretical models including London's, Hildebrand's and/or reaction field (RF) theories allow to express these \"non specific\" parts of SEs as functions of physical properties such as the refractive index, relative permittivity and thermodynamic properties of the solvent. Quantitative rankings of solvents (\"solvent scales\") can thus be constructed on the basis of such properties as the refractive index, n; electric permittivity (formerly known as dielectric constant), er; Hildebrand's solubility parameter, dH; the modulus of the molecular dipole moment, μ, and various functions thereof. These scales shall be termed \"model-independent\". Quite generally, it is assumed that dispersive and electrostatic interactions are independent and additive. Following Palm and Koppel, it is further assumed that other contributions to solvent-solute interactions, notably hydrogen bonding are also independent from and additive to, the \"non specific\" contributions. Use is often made of \"model-dependent\" scales. They are based on the similarity principle: the ranking of the efficiency of solvents on a given property is quantitatively compared to their influence on a reference physical or chemical property of a referencesolute (\"molecular probe\"). The associated formalism is quite simple and has been outlined above. These scales can be divided into two different categories, depending on whether they quantify the overall \"polarity effect\" of the solvent, in Reichardt's sense or else, they are intended to measure one or various components of the overall solvation power of the solvent. Some scales have been built on the basis of a statistical treatment of SEs on large sets of experimental data of various origins. They are absolutely \"empirical\" in that they try to quantify \"average\" SEs without specifically seeking a formal link with current theoretical concepts on solvent-solute interactions. Last we mention that the case of self-associated solvents and mixtures thereof involves a number of conceptual and experimental difficulties that, in our opinion, justify a separate treatment. Here we deal with non-hydrogen bond donor solvents, although a number of weak hydrogen bond donor solvents are also included. Part I of this compilation is organized as follows: Description of the Tables. Physical properties and model-independent scales. Table I. Model-dependent scales: a) \"Overall solvation\" scales. b) Scales of dipolarity/polarizability. c) Scales of hydrogen bonding accepting power (hydrogen bonding basicity). d) Scales of \"hard\" and \"soft\" Lewis acidity and basicity. Statistical scales. Table II. General comments on the scales.

Harmonized guidelines for the use of recovery information in analytical measurement

Abstract: ISO, IUPAC and AOAC International have co-operated to produce agreed protocols or guidelines on the ‘Design, Conduct and Interpretation of Method Performance Studies’ [1] on the ‘Proficiency Testing of (Chemical) Analytical Laboratories’ [2] and on ‘Internal Quality Control in Analytical Chemistry Laboratories’ [3]. The Working Group that produced these protocols/guidelines was asked to prepare guidelines on the use of recovery information in analytical measurement. Such guidelines would have to outline minimum recommendations to laboratories producing analytical data on the internal quality control procedures to be employed., A draft of the guidelines was discussed at the Seventh International Symposium on the Harmonisation of Quality Assurance Systems in Chemical Laboratories, sponsored by IUPAC/ISO/AOAC International, held in Orlando, USA, 4–5 September 1996. Proceedings from that Symposium are available [4]. The purpose of these guidelines is to outline the conceptual framework needed for considering those types of analysis where loss of analyte during the analytical procedure is inevitable. Certain questions cannot be satisfactorily addressed, and hence remain irreducibly complex, unless such a conceptual framework is established. The questions at issue involve: (a) the validity of methods for estimating the recovery of the analyte from the matrix of the test material, and (b) whether the recovery estimate should be used to correct the raw data to produce the test result. The types of chemical analysis most affected by these considerations are those where an organic analyte is present at very low concentrations in a complex matrix.

Glossary of terms used in theoretical organic chemistry

Abstract: Abstract The glossary contains definitions and explanatory notes for more than 450 terms used in the context of multidisciplinary research and publications related to applications of modern theoretical concepts, computational and graph-theoretical methods to investigation into structure, reactivity, spectroscopic and other physical and physicochemical properties of organic, organometallic and metal coordination compounds. The aim of the glossary is to provide guidance on terminology used in theoretical organic chemistry and to contribute to the elimination of inconsistencies and ambiguities in the meanings of terms in the area.

Mechanochemistry: an overview

Abstract: The field of mechanochemistry is reviewed. A large number of mechanochemical reactions are described as well as industrial applications.

Terminology, relative photonic efficiencies and quantum yields in heterogeneous photocatalysis. Part I: Suggested protocol

Abstract: The term photocatalysis is one amongst several in a quagmire of labels used to describe a photon-driven catalytic process; a simple description of photocatalysis is proposed herein. Other labels such as quantum yield and/or quantum efficiency used in solid/liquid and solid/gas heterogeneous photocatalytic systems to express process efficiencies have come to refer (incorrectly) to the ratio of the rate of a given event to the rate of incident photons impinging on the reactor walls and typically for broadband radiation. There is no accord on the expression for process efficiency. At times quantum yield is defined; often, it is ill-defined and more frequently how it was assessed is not described. This has led to much confusion in the literature, not only because of its different meaning from homogeneous photochemistry, but also because the description of photon efficiency precludes comparison of results from different laboratories owing to variations in light sources, reactor geometries, and overall experimental conditions. The previously reported quantum yields are in fact apparent quantum yields, i.e. lower limits of the true quantum yields. We address this issue and argue that any reference to quantum yields or quantum efficiencies in a heterogeneous medium is inadvisable until the number of photons absorbed by the light harvester (the photocatalyst) is known. A practical and simple alternative is proposed for general use and in particular for processes employing complex reactor geometries: the concept of relative photonic efficiency (jr) is useful to compare process efficiencies using a given photocatalyst material and a given standard test molecule. A quantum yield can subsequently be calculated since F 1⁄4 jr Fphenol, where Fphenol denotes the quantum yield for the photocatalyzed oxidative transformation of phenol used as the standard secondary actinometer and Degussa P-25 TiO2 as the standard photocatalyst. For heterogeneous suspensions (only), an additional method to determine quantum yields F is also proposed.

Geodesic polyarenes with exposed concave surfaces

Abstract: Polycyclic aromatic hydrocarbons (PAHs) in which five-membered rings have been interspersed among the six-membered rings generally display curved p systems that can be either fully closed (fullerenes) or partially open (bowl-shaped fullerene fragments), depending on the number of five-membered rings. Both classes of compounds belong to the larger family we call 'GEODESIC POLYARENES.' Herein, we highlight the utility of high temperature aryl radical cyclizations as a general strategy for constructing strained geodesic polyarenes. We also report the first examples of reactions at the 'interior' carbon atoms of neutral PAHs that are not fullerenes, e.g. carbene additions, 1,3-dipolar cycloadditions, osmylation, nucleophilic addition of MeLi, and electrophilic addition of L

Chemistry of carotenoid oxidation and free radical reactions

Abstract: When oxygenic photosynthesis evolved, one of the key functions of carotenoids was to protect aerobic photosynthetic organisms against destruction by photodynamic sensitization. Aerobic photosynthesis would not exist without the coevolution of carotenoids alongside the chlorophylls. As carotenoids are abundant in nature, in many fruits and vegetables, they are able to react with excited states of appropriate energy and quench them, and they can react with free radicals according to their reactivity, redox potentials, and X—H bond energies. This report concerns the bimolecular reactions of carotenoids with oxygen species, such as 3 O2, 1 O2, HO · , HOO · ,O ·ˇ 2 , etc.

Pillared Clays and Pillared Layered Solids

Abstract: Pillaring is a commonly used procedure to transform a layered crystalline inorganic compound into a material with microporosity and mesoporosity. First, the terms layered compound, pillaring, pillaring agent and interlayer region are de®ned, followed by the terms pillared layered solid or pillared compound. In the second part, the standard characterization methods and procedures and a systematic classi®cation scheme are proposed.

Terminology, relative photonic efficiencies and quantum yields in heterogeneous photocatalysis. part ii: experimental determination of quantum yields†

Abstract: In the preceding article [Pure Appl. Chem. 71, 303–320 (1999)] we examined two principal features of heterogeneous photocatalysis that demanded scrutiny: (i) description of photocatalysis and (ii) description of process efficiencies. For the latter we proposed a protocol relative photonic efficiency which could subsequently be converted to quantum yields. A difficulty in expressing a quantum yield in heterogeneous photochemistry is the very nature of the system, either solid/liquid or solid/gas, which places severe restrictions on measurement of the photon flow absorbed by the light harvesting component, herein the photocatalyst TiO2, owing to non-negligible scattering by the particulates. It was imperative therefore to examine the extent of this problem. Extinction and absorption spectra of TiO2 dispersions were determined at low titania loadings by normal absorption spectroscopy and by an integrated sphere method, respectively, to assess the extent of light scattering. The method is compared to the one reported by Grela et al. [J. Phys. Chem. 100, 16 940 (1996)] who used a polynomial extrapolation of the light scattered in the visible region into the UV region where TiO2 absorbs significantly. This extrapolation underestimates the scattering component present in the extinction spectra, and will no doubt affect the accuracy of the quantum yield data. Further, we report additional details in assessing limiting photonic efficiencies and quantum yields in heterogeneous photocatalysis.

Approaches to catalyst discovery. New carbon–heteroatom and carbon–carbon bond formation

Abstract: Studies on the palladium-catalyzed formation of aryl amines, aryl ethers and a-aryl carbonyl compounds from aryl halides are reported. These studies range from synthetic methodology, to detailed mechanistic analysis, to new methods one can use to screen for catalytic covalent bond formation. Improved methods for formation of aryl ethers and room temperature amination chemistry have resulted from a mechanistic understanding of the reaction.

Electron tunneling in biological molecules

Abstract: Electron transfers in photosynthesis and respiration commonly occur between protein-bound prosthetic groups that are separated by large molecular distances (often greater than 10A). Although the electron donors and acceptors are expected to be weakly coupled, the reactions are remarkably fast and proceed with high specificity. Tunneling timetables based on analyses of Fe^(2+)/Cu^+ to Ru^(3+) electron-transfer rates for Ru-modified heme and copper proteins reveal that the structure of the intervening polypeptide can control these distant donor-acceptor couplings. Multistep tunneling can account for the relatively rapid Cu^+ to Re^(2+) electron transfer observed in Re-modified azurin.

Environmental applications of low-temperature plasmas

Abstract: Treatment of NOx in diesel engine exhaust represents a big opportunity for the environmental application of low-temperature plasmas. This paper discusses the effect of gas composition on the NOx conversion chemistry in a plasma. It is shown that the plasma by itself cannot chemically reduce NOx to N2 in the highly oxidizing environment of a diesel engine exhaust. To implement the reduction of NOx to N2, it is necessary to combine the plasma with a heterogeneous process that can chemically reduce NO2 to N2. Data is presented that demonstrates how the selective partial oxidation of NO to NO2 in a plasma can be utilized to enhance the selective reduction of NOx to N2 by a catalyst.

Enaminones: versatile intermediates for natural product synthesis*

Abstract: Efforts in our laboratories to devise a general approach to the synthesis of alkaloids focus on the versatile reactivity of enaminones and related compounds containing the structural unit N-CaC-Z (Za COR, CO2R, CN, NO2 ,S O 2Ar, etc.). This lecture presents an overview of our research with these useful building blocks. Themes to be elaborated include chemo- selectivity and diastereoselectivity in reactions of enaminones, and the challenge of controlling absolute stereochemistry.

Mammalian carotenoid absorption and metabolism

Abstract: Carotenoids are purported to provide widespread function in the biology and health of humans and other mammalian species. Provitamin A carotenoids, such as b-carotene, are valued in the diet of many mammals for their contribution as precursors of vitamin A and retinoids. Carotenoids may also function in the prevention of some chronic diseases by improving intercellular communication. enhancing immune response, and operating as antioxidants in vivo. It is widely known that humans and other mammalian species absorb and accumulate carotenoids in body tissues. However, the potential use of carotenoids as modulators of disease and in the prevention of vitamin A deficiency has been hindered by the limited progress in understanding carotenoid absorption and metabolism. In fact, major gaps in knowledge still exist in the fundamental pathways beginning with release from the food matrix and ending with distribution in body tissues and excretion. Continued development of assessment methods for humans, appropriate animal models for mechanistic studies, and analytical techniques for quantification and identification of compounds is needed to advance our understanding of these critical pathways. This review will discuss the current knowledge involving the fundamental pathways of absorption and metabolism of carotenoids in mamma- lian species. When applicable, emphasis will be placed on the human.

In situ burning

Abstract: Although there have been numerous incidents of vessel oil spills that inadvertently caught fire, the intentional ignition of oil slicks on open water has only been seriously considered since the development of fire-resistant oil containment boom beginning in the early 1980s. The development of these booms offered the possibility of conducting controlled burns in open water conditions. In situ burning operations using these booms have been conducted at three spills in the last decade: a major offshore tanker spill, a burning blowout in an inshore environment, and a pipeline spill into a river.

Oil spill dispersants

Abstract: When oil is spilled at sea, a small proportion will be naturally dispersed by the mixing action caused by waves. This process can be slow and proceed to only a limited extent for most situations. Dispersants are used to accelerate the removal of oil from the surface of the sea by greatly enhancing the rate of natural dispersion of oil and thus prevent it from coming ashore. Dispersed oil will also be more rapidly biodegraded by naturally occurring microorganisms. The rationale for dispersant use is that dispersed oil is likely to have less overall environmental impact than oil that persists on the surface of the sea, drifts and eventually contaminates the shoreline.

Chemistry by microwaves

Abstract: The pioneers of the application of microwaves into more canonic systems thought that it could produce similar effects in assisting the reactions carried out in research laboratories. In most cases it was soon evident that reactions were more rapid, cleaner and with an easier workup of the final material, saving a great deal of time [1]. These findings stimulated the diffusion of microwave ovens in many laboratories to take advantage of this instrument for their reactions.

Phosphorus Speciation in Water and Sediments

Abstract: Environmentally significant phosphorus species in water and phosphorus fractions in sediments are briefly discussed and the methods for their determination are described. One of the most critical analytical steps is the separation of the different forms which, after conversion into orthophosphates, may be determined by a multitude of various techniques. Spectrophotometric methods are often preferred for routine analysis. Several rapid automatic methods for the separation and determination of orthophosphate, linear polyphosphates, cyclic condensed phosphates and lower oxidation state anions of phosphorus, which may exist in natural and waste waters, have been developed. They are mainly based on the use of flow-injection analysis, high-performance liquid chromatography including ion chromatography, capillary electrophoresis and a few of other techniques. These methods have been described and critically evaluated.

Transferring iodine: more than a simple functional group exchange in organic synthesis

Abstract: Some synthetic features of the IPy2BF4 reagent are presented. Among others, its utility to promote unusual ‘carbon-carbon’ coupling processes will be discussed. Furthermore, the unique iodinating ability of this reagent towards aromatic and other unsaturated systems will be summarized. The rich chemistry of the carbon-iodine bond has made it a particularly rewarding synthetic tool of routine use for the purpose of functional group interconversion. Iodinated compounds are also credited as helpful diagnostic aids when the radioactive isotopes of the element are used [1]. Moreover, the growing presence and impact of hypervalent iodine compounds represents an additional source of interest in iodine containing reagents [2]. The bis(pyridinium) iodonium (I) tetrafluoroborate (IPy2BF4) is a stable and solid reagent, that acts as a mild source of iodonium ions towards different types of unsaturated compounds, as it will be shown in this article. Initial efforts were devoted to the development of a methodology that can be useful to accomplish vicinal iodofunctionalization of an alkene. Thus, when the reagent is mixed in CH2Cl2 with an alkene and a nucleophile the corresponding products derived from the addition of an iodine atom and the nucleophile across the double bond are formed, for instance MeOH for 1a, in Scheme 1 [3]. Usually, an acid is required to neutralize the supply of pyridine molecules from the iodinating reagent, avoiding the formation of adducts, such as 1b, that incorporate pyridine as a nucleophile through a competition process for the capture of the intermediate iodonium ion. In this regard, tetrafluoroboric acid is very useful due to the low nucleophilic character of the BF4 1 counteranion. When simply the reagent, the acid and the alkene are mixed at low temperature, a clean, regioand stereoselective iodofluorination takes place [3,4], as also depicted in Scheme 1 for the case of 1c. *Lecture presented at the 5th International Conference on Heteroatom Chemistry (ICHAC-5), London, Ontario, Canada, 5–10 July 1998, pp. 369–512. Scheme 1 Addition is the usual outcome of the reaction of IPy2BF4 with alkenes. However, starting from vinylsilanes an easy substitution process occurs, resulting in a very efficient and simple procedure for the iodo-silicon exchange from E 2a, and Z 2b monosubstituted trimethylsilylalkenes, giving rise stereospecifically to the formation of compounds 3a and 3b, respectively (Scheme 2) [5]. Otherwise, this is known to be an elusive transformation. Interestingly, the reagent is also an efficient iodinating agent promoting addition reactions of alcohols to the same alkenylsilanes furnishing compounds 4 (Scheme 3), by simply running the transformation in the presence of the corresponding alcohol as cosolvent [6]. The resulting adducts can be dehydroiodinated with the tertiary base 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU), giving the stereochemically well-defined trisubstituted enol ethers, in a stereospecific manner. The same sequence, but requiring t-butyldimethylsilyl derivatives, gives aryl-substituted enol ethers. Adducts 6a and 6b are stereospecifically formed [6], now by a syn addition (Scheme 4). It was reported that the IPy2BF4 reagent promotes electrophilic addition of iodine and nucleophiles to alkynes in a similar way, a process where the stereochemistry of the resulting alkenyliodides relies on the structure of both the alkyne and the nucleophile [7]. These functionalization reactions can be equally achieved using heteroatom-substituted alkynes as iodonium acceptors. The regiochemistry of the products formed upon addition to these substrates nicely illustrates the electronic tuning of the process (Fig. 1) [8]. Thus, 1-iodoalkynes are transformed into 2-functionalized-1,1-diiodoalkenes, whereas for alkynylsulfides the regiochemistry switches, yielding the corresponding 2-iodovinylsulfides [8b]. 432 J. BARLUENGA q 1999 IUPAC, Pure Appl. Chem. 71, 431–436 Scheme 2

Cancer prevention by carotenoids

Abstract: Various natural carotenoids have been proven to have anticarcinogenic activity. Epidemiological investigations have shown that cancer risk is inversely related to the consumption of green and yellow vegetables and fruits. As b-carotene is present in abundance in these vegetables and fruits, it has been investigated extensively as a possible cancer preventive agent. However, various carotenoids which coexist with b-carotene in vegetables and fruits also have anticarcinogenic activity, and some of these, such as a-carotene, lutein and lycopene, show a higher potency than b-carotene in suppressing experimental carcinogenesis. Thus, we have carried out more extensive studies on cancer preventive activities of natural carotenoids in foods. For example, we found that b-cryptoxanthin showed antitumor initiating activity, as well as antitumor promoting activity. It is of interest that not only carotenoids distributed in vegetables and fruits, but also animal carotenoids, such as astaxanthin, are promising as cancer preventive agents. In the present study, the cancer preventive potential of phytoene was also con®rmed. The establishment of NIH3T3 cells that produce phytoene by introducing the crtB gene provides evidence that resistance against transformation, imposed by transfection of activated H-ras oncogene, was acquired by phytoene production. Analysis of the action mechanism of these natural carotenoids is now in progress, and some interesting results have already been obtained; for example, various carotenoids were suggested to stimulate the expression of RB gene, an antioncogene.

Palladium-catalyzed annulation

Abstract: The palladium-catalyzed annulation of cyclic and bicyclic alkenes; unsaturated cyclopropanes and cyclobutanes; 1,2-, 1,3- and 1,4-dienes; as well as internal alkynes, by functionally substituted aryl or vinylic halides and triflates provides a very versatile route to a wide variety of heterocycles and carbocycles.

Rapid, productive and stereoselective hydrogenation of ketones

Abstract: RuCl2(phosphine)2(1,2-diamine) complexes act as excellent precatalysts for homo- geneous hydrogenation of simple ketones which lack any functionality capable of interacting with the metallic center. This newly devised catalytic system allows preferential saturation of a CaO function over a coexisting CaC linkage in 2-propanol containing an alkaline base. Furthermore, the stereoselectivity of the reaction is easily controlled by the electronic and steric properties (bulkiness and chirality) of the ligands as well as the reaction conditions. Selective reduction of carbonyl compounds to alcohols has relied heavily on the use of metal hydride reagents. Our goal is to replace such stoichiometric reductions by catalytic hydrogenation, because the latter is obviously more beneficial, particularly for large-scale reactions. Here we describe a very practical homogeneous hydrogenation of ketones catalyzed by RuCl2(phosphine)2(1,2-diamine) and an inorganic base. The precatalysts can be preformed or generated in situ by mixing the phosphine-RuCl2 complex and an appropriate 1,2-diamine in 2-propanol. The reaction is normally conducted at room temperature and at < 8 atm of hydrogen. The new method shows promise for the synthesis of a wide range of achiral and chiral alcohols from ketonic substrates. This chemistry is entirely different from the BINAP-Ru catalyzed asymmetric hydrogenation of functionalized ketones developed earlier in our laboratories (1). RAPID HYDROGENATION OF SIMPLE KETONES

Glossary of Terms Used in Combinatorial Chemistry

Abstract: Prepared for publication by D. MACLEAN, J. J. BALDWIN, V. T. IVANOV, Y. KATO, A. SHAW, P. SCHNEIDER AND E. M. GORDON Affymax Research Institute, Palo Alto, California, USA Pharmacopeia, Inc., Cranbury, New Jersey, USA Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, Moscow, Russia Teijin Institute for Bio-Medical Research, Tokyo, Japan Astra-Zeneca Ltd., Maccles®eld, Cheshire, UK Novartis, Inc., Basle, Switzerland Sunesis, Inc., Redwood City, California, USA

Finding the needle in the haystack. The dereplication of natural product extracts

Abstract: Dereplication is an integral part of the early stages of the drug discovery process, where there is a focus on the rapid identification of novel, biologically active metabolites from natural sources. We describe a strategy and a system for dereplication and offer several examples of its successful appplication.

The bite angle makes the catalyst

Abstract: Catalytic reactions are described for metal complexes containing bidentate phos- phine ligands that enforce wide bite angles in the complexes The calculated natural bite angles are in the range 100-1208 Three applications will be discussed: (i) nickel catalysed hydro- cyanation, for which the first active phosphine catalyst was found, (ii) palladium catalysed allylic alkylation, for which the selectivity also strongly depends on the bite angle, and (iii) rhodium catalysed hydroformylation, which leads to highly linear products

Stereoselective synthesis of three-membered ring compounds via ylide routes

Abstract: Three types of small ring compounds (epoxides, aziridines and cyclopropanes) can be synthesized stereoselectively via ylide route. Among them, the stereochemistry of vinyloxiranes, vinylaziridines, cyclopropylesters, amides and ketones can be tuned by the choice of reaction conditions and ylides with varying heteroatoms and ligands. Optically active epoxides and acetylenylaziridines can be prepared via camphor-derived chiral sulfonium ylide routes.