Aaberg compound

In reality, the biending index of a compound varies according to its concentration and the nature of the product receiving it it is not, therefore, an intrinsic characteristic. In spite of this problem, refiners have long used the concept of blending index to predict and establish their refining flow sheets based on data drawn from their own experience. This approach is disappearing except in certain cases, for example, concerning the addition of oxygenates. In this manner, Table 5.11 gives estimated blending values for different alcohols and ethers when they are added in small quantities to an unleaded fuel close to the specifications for Eurosuper (RON 95, MON 85). Taking into account the diversity of situations encountered in regards to the composition of the receiving product stream, one does not retain a unique value for the blending value, but on the contrary, a margin for possible variation.  [c.203]

Two approaches to quantify/fQ, i.e., to establish a quantitative relationship between the structural features of a compoimd and its properties, are described in this section quantitative structure-property relationships (QSPR) and linear free energy relationships (LFER) cf. Section The LFER approach is important for historical reasons because it contributed the first attempt to predict the property of a compound from an analysis of its structure. LFERs can be established only for congeneric series of compounds, i.e., sets of compounds that share the same skeleton and only have variations in the substituents attached to this skeleton. As examples of a QSPR approach, currently available methods for the prediction of the octanol/water partition coefficient, log P, and of aqueous solubility, log S, of organic compoimds are described in Section 10.1.4 and Section 10.15, respectively.  [c.488]

However, the synthe.sis of each compound was considered as a specific task on its own. A suitable strategy for the synthesis of a target compound was mostly found on the basis of the intuition and experience of the acting chemists, i.e., the planning of a synthesis of a complex organic molecule was considered as an art form. No systematic approach was attempted to handle the strategic design of an organic synthesis. However, the growth of knowledge resulting from the rapid development of analytical methods and other techniques was demanding a more systematic approach for synthesis design.  [c.569]

The retrosynthetic analysis of a target compound is a systematic approach in developing a synthesis plan starting with the target structure and working backward to available starting materials.  [c.592]

The plane of the rotor blade cross-section representing the flow field configuration at the start of mixing in a partially filled single-blade mixer is shown in Figure 5.1. Initial distribution of the compound inside the mixer chamber corresponds to a fill factor of 71 per cent and is chosen arbitrarily. It is evident that the flow field within this domain should be modelled as a free surface regime with random moving boundaries. Available options for the modelling of such a flow regime are explained in Chapter 3, Section 5. In this example, utilization of the volume of fluid (VOF) approach based on an Eulerian framework is described. To maintain simplicity we neglect elastic effects and the variations of compound viscosity with mixing, and focus on the simulation of the flow corresponding to a generalized Newtonian fluid. In the VOF approach  [c.142]

Analysis Taking the heterocychc part first, we can remove the two heteroatoms as hydroxylamine (the approach of frames 258-261) to give us a 1,3-dicarbonyl compound.  [c.130]

Most programs take a retrosynthetic approach. This is a means for systematically working backward from the target compound to available precursors  [c.278]

Irrespective of the precision of these quantitative correlations, this approach is useful in emphasizing that relative rates depend on the nature of the reaction as well as of the aromatic compound.  [c.137]

In this chapter we have reviewed and extended our understanding of equilibrium chemistry. We also have developed several tools for evaluating the composition of a system at equilibrium. These tools differ in how accurately they allow us to answer questions involving equilibrium chemistry. They also differ in their ease of use. An important part of having several tools available to you is knowing when to use them. If you need to know whether a reaction is favorable, or the approximate pH of a solution, a ladder diagram may be sufficient to meet your needs. On the other hand, if you require an accurate estimate of a compound s solubility, a rigorous calculation using the systematic approach and activity coefficients is necessary.  [c.175]

For example, hopane (Fig. 12) is a conserved marker in cmde oils in at least the early stages of biodegradation (up to 80% degradation) (108) its concentration increases in the residual oil as biodegradation proceeds. Basing estimates of biodegradation on hopane allowed us to quantify the effect of the successhil bioremediation strategy following the Exxon Valde oil-spiU. (14,15). Even in refined products, the least degradable detectable analyte can serve this role. Thus, the trim ethyl -ph en an th ren es can be used to estimate quaUtatively the degradation of diesel oils in the environment, even though these molecules are themselves biodegradable (109). Of course, since these molecules are themselves biodegradable, estimates of the rates of biodegradation of more readily biodegradable compounds are systematically underestimated using this approach, but it can still provide very valuable information. The more halogenated polychlorinated biphenyls can serve a similar role in assessing the environmental fate of these products (55), and benzene is typically the last compound in BTEX plumes to be degraded, particularly under anaerobic conditions.  [c.39]

Combinatorial Chemistry. The development of stepwise methods of synthesis of polypeptides and polynucleotides has spawned a new methodology, combinatorial chemistry, for the production of new, functional molecules. Combinatorial chemistry is a way of rapidly generating a large number of closely-related compounds, followed by the screening of each of the compounds with respect to certain desired properties (eg, binding to a particular molecule) (151). A variety of methods of synthesis and screening utilize the combinatorial approach. These include parallel chemical synthesis and testing of multiple compounds (individually or in mixtures) in solution or on soHd supports and biotechnological or organism-based synthesis of biological oligomers coupled to methods of selection and amplification. In most appHcations, automated instmmentation has been developed that will produce a suite of compounds by covalent biochemical synthesis. These compounds may be small or large molecules (eg, oligopeptides and oligonucleotides of varying lengths) that are then screened by their abiUty to bind a biological molecule (eg, receptor or enzyme proteins). Once a compound that shows the desired binding properties is identified, a Hbrary of its variants can be produced, such that an optimized molecule can be obtained.  [c.206]

Unsaturated Polyesters. There are two approaches used to provide flame retardancy to unsaturated polyesters. These materials can be made flame resistant by incorporating halogen when made, or by adding some organic halogen compound when cured. In either case a synergist is needed. The second approach involves the addition of a hydrated filler. At least an equal amount of filler is used.  [c.461]

C-11 functionalized estrone derivatives are synthetically available by the following process (5) treatment of 7a-methylestrone acetate [36014-09-2] (15) with four equivalents of ceric ammonium nitrate in 90% acetic acid provides the 9a-hydroxy-liP-nitrate ester in good yield. Subsequent deoxygenation of the C-9 benzyUc position of the nitrate ester with retention of configuration is effected utilizing triethylsilane and boron trifluoride etherate to produce compound (16). Reduction with NaBH affords the key target (14). A nearly identical approach has been used to prepare (16) which was converted to (14) and also ethinylated at C-17 with acetylene and potassium /-butoxide to give CDB-3322 (6).  [c.233]

The aluminum containing compound having the largest worldwide market, estimated to be over 30 x 10 t in 1990, is metal grade alumina. Second, is aluminum hydroxide. In 1990 the market for Al(OH)2 should approach or exceed 3.5 million metric tons which is equivalent to 2.3 million tons on an alumina basis. The spHt between additive and feedstock appHcations for Al(OH)2 (16) is roughly 50 50. Additive appHcations include those as flame retardants (qv) in products such as carpets, and to enhance the properties of paper (qv), plastic, polymer, and mbber products. Significant quantities are also used in pharmaceuticals (qv), cosmetics (qv), adhesives (qv), poHshes (qv), dentifrices (qv), and glass (qv).  [c.136]

Markets. Industrial use of ammonia varies according to region. Eor example, industrial usage represents 20% of the ammonia production in the United States and Western Europe, 10% in the USSR, 1—10% in Asia, and 5% in Latin America and North Africa (79). Fertiliser ammonia consumed domestically in most countries is converted to straight or compound fertilisers such as urea, ammonium nitrate, diammonium phosphate, and various grades of mixed fertilisers. However, almost 29% of ammonia nitrogen in the United States is consumed as direct appHcation material. The use of nitrogen solution such as urea and ammonium nitrate (UAN) has also become popular in the United States and the USSR.  [c.355]

Walden inversion A phenomenon discovered in 1895 by Walden. When one of the atoms or groups attached to the asymmetric carbon atom in an optically active compound is replaced by a different atom, the product is sometimes a derivative of the optical isomer of the original compound. It is thus possible to pass from one isomer to the other without the formation and separation of a racemic compound. ( + )-Malic acid, when treated with PCI5 gives (— )-chlorosuccinic acid, which may be converted to ( —)-malic acid by AgjO or back to (-f)-malic acid by KOH. Similarly, ( —)-malic acid is converted to (-l-)-chloro-succinic acid which undergoes similar changes. A Walden inversion occurs at a tetrahedral carbon atom when the entry of the reagent and the departure of the leaving group are synchronous - the so-called bimolecular nucleophilic substitution mechanism. Since the reagent must approach from the side of the molecule opposite to that of the leaving group an inversion of optical configuration results.  [c.424]

O. Venard. Eddy current tomography a bayesian approach with a compound weak membrane-beta prior model. In Advances in Signal Processing for Non Destructive Evaluation of Materials, 1997.  [c.333]

Estimating free energy differences from an initial state ensemble has the advantage that a large number of potential modifications of a given compound can be investigated in a single calculation. Because extrapolation is based on an unperturbed ensemble the nature of the perturbation does not have to be predefined. The change in free energy associated with a particular mutation can be obtained quickly and efficiently by reanalysis of stored trajectories. Extrapolation based on the application of the perturbation formula to an appropriately biased ensemble holds considerable promise for use as a rapid, non-empirical means of estimating relative binding affinities for a wide range of possible chemical modifications of a lead compound. As such the approach could be used to guide experimental planning in drug design a task not practical using normal free energy calculations.  [c.160]

The prerequisite for using this approach is that the user has an intimate knowledge of the environmental databases, i.e., numeric, bibliographic, and full-text ones, which arc available on the Internet. This might not be the case all the time. A different way to proceed is to consult metadatabases and portals, databases which provide information on resources available on the Internet which contain environmental and chemical information, A working knowledge of the availability and functionality of thc.se mctadataba.scs and portals is the basis for taking this route. The third path uses search engines. First, the chemical compound has to be identified, then a search engine must be chosen and the documents comprising environmental information retrieved. This procedure often gives reasonable results [47]. Additionally, online journals can be consulted to find information on environmental chemicals. This aspea is not discussed in this section.  [c.274]

Screening systems normally use a predefined set of structural fragments called keys. For each key a preliminary (in a pre-process phase) substructure search is performed across the whole structural database. For each database compound a string of bits is constructed. Each bit of this string denotes the presence or absence of a key in the corresponding database compound. The kth bit in the bit-string is set to 1 if the kth key fragment is a substructure of the current database compoimd otherwise the kth bit is set to 0. In the same way during the substructure searching a bit-string of the query structure is constructed. This step is usually very fast since a few himdred isomorphism checks are performed for a set of quite simple structural fragments. Further, the query bit-string is compared with each of the bit-strings from the database. The target compounds are screened as follows each key which is present in the query structure must be present in the target structure (the corresponding bits are compared by using the fast bitwise operation AND, OR, and XOR). If at least one key that is present in the query graph is not present in the target graph, then this compound is pruned from a further processing. In this way a great many structures which are not likely to survive the isomorphism check are pruned early in the screening stage, thus escaping the much more time-consuming backtracking algorithm. This results in a much smaller set of structures being checked for structure isomorphism by means of the backtraddng algorithm. For example, in a typical screening session more than 90% of the database compounds which do not contain the query substructure are removed. Inasmuch as the time dependence of the screening procedure is linear, it decreases the time needed for substructure searching considerably (for example, this approach is 10 to 20 times faster than without the use of screening). Additionally, the screening procedure itself is very fast since it involves only a few isomorphism checks and is performed with the very fast bitwise operations.  [c.302]

A challenging task in material science as well as in pharmaceutical research is to custom tailor a compound s properties. George S. Hammond stated that the most fundamental and lasting objective of synthesis is not production of new compounds, but production of properties (Norris Award Lecture, 1968). The molecular structure of an organic or inorganic compound determines its properties. Nevertheless, methods for the direct prediction of a compound s properties based on its molecular structure are usually not available (Figure 8-1). Therefore, the establishment of Quantitative Structure-Property Relationships (QSPRs) and Quantitative Structure-Activity Relationships (QSARs) uses an indirect approach in order to tackle this problem. In the first step, numerical descriptors encoding information about the molecular structure are calculated for a set of compounds. Secondly, statistical and artificial neural network models are used to predict the property or activity of interest based on these descriptors or a suitable subset.  [c.401]

The BCUT descriptors have been proposed for the investigation of the similarity or diversity of large databases. They are an extension of the Burden approach and consider three classes of matrices whose diagonal elements correspond to values related to the atomic charge, atomic polarizability-related values, and atomic hydrogen bonding abihties. In addition, the off-diagonal terms include functions of interatomic distance, bond orders, etc. The Burden matrix b represents the hydrogen-depleted graph of a compound. The diagonal elements bj j are the atomic number Z of the atoms. The off-diagonal elements represent the bond order 0.1, 0.2, 0.3, or 0.15 for a single, double, triple, or aromatic bond respectively). The value for terminal bonds is augmented by 0.01. All off-diagonal elements for combinations i andj which cannot be related to a bond in the compound are set to the value 0.001. The ordered sequence of the n smallest eigenvalues of the b matrix can be used as a structure descriptor.  [c.428]

The abbreviation QSAR stands for quantitative structure-activity relationships. QSPR means quantitative structure-property relationships. As the properties of an organic compound usually cannot be predicted directly from its molecular structure, an indirect approach Is used to overcome this problem. In the first step numerical descriptors encoding information about the molecular structure are calculated for a set of compounds. Secondly, statistical methods and artificial neural network models are used to predict the property or activity of interest, based on these descriptors or a suitable subset. A typical QSAR/QSPR study comprises the following steps structure entry or start from an existing structure database), descriptor calculation, descriptor selection, model building, model validation.  [c.432]

Gasteiger and co-authors [71] implemented the following approach for full spectra simulation. The previously mentioned 3D MoRSE descriptor (Section 8.4.3), derived om an equation used in electron diffraction studies, allows the representation of the 3D structure of a molecule by a fixed (constant) number of variables. By using a fast 3D structure generator they were able to study the correlation between any three-dimensional structure and IR spectra using ANN. Steinhauer et al. [72] used RDF codes as structure descriptors. Together with the IR spectrum, a counterpropagation (CPG) neural network was trained to establish the complex relationship between an RDF descriptor and an IR spectrum. After training, the simulation of an IR spectrum is performed using the RDF descriptor of the query compound as the information vector of the Kohonen layer, which determines the central neuron. On input of this query RDF descriptor, the central neirron is selected and the corresponding IR spectrum in the output layer is presented as the simulated spectrum. Selzer et al. [73] described an application of this spectrum simulation method that provides rapid access to arbitrary reference spectra. Kostka et al. described a combined application of spectrum prediction and reaction prediction expert systems [74]. The combination of the reaction prediction system EROS and IR spectrum simulation proved to be a powerful tool for computer-assisted substance identification. More details and the description of a web tool implementing this method can be found in the "Tools Section  [c.530]

Another approach followed by Wang et al. led to an inhibitor of TTitrichomonas foetus hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRT) [88]. The X-ray structure of this enzyme is available and was used in a search for novel scaffolds. The Available Chemicals Directory (ACD) was screened with the program DOCK. Isatin and phthaHc anhydride were capable of mimicking the substrate purine base and acting as competitive inhibitors. A virtual library of substituted 4-phthaHmidocarboxanilides was constructed and synthesized on solid supports. The most active compound was used as lead for the further opti-  [c.615]

The integrals in the Hodgkin-Richards approach can be evaluated in a number of ways. Or approach is to position the molecules within a rectangular grid and to evaluate tl electrostatic potential due to each molecule at each grid point. The integrals in Equatic (12.19) are then determined numerically by summing over the grid points. This can 1 rather slow, particularly if the molecules are allowed to vary their relative orientatioi and conformations in order to find the location of maximum similarity. An alternative to represent the potential using an analytical function. For example, linear combinatioi of Gaussian functions can be fitted to the potential, enabling the similarity measure to 1 computed much more rapidly [Good et al. 1993].  [c.695]

We chose benzyli dene acetone (4.39, Scheme 4.11) as a model dienophile for our studies. The uncatalysed Diels-Alder reaction of this compound with cyclopentadiene is slow, justifying a catalytic approach. Reaction of 4.39 with paraformaldehyde and dimethyl amine under acidic conditions in an aqueous ethanol solution, following a literature procedure, produced the HCl salt of 4.42 (Scheme 4.11). The dienophile was liberated in situ by adding one equivalent of base.  [c.114]

The work that is described in Chapters 2 and 3 involves the Diels-Alder reactions of diene 1, that has been designed for bidentate coordination to Lewis-acid catalysts. The question arises whether the results obtained for this compound can be extended to other Diels-Alder reactions. This question is addressed in Chapter 4, in which attempts to catalyse Diels-Alder reactions of other potentially chelating dienophiles are described. Unfortunately, this approach was modestly successful. Apparently, coordination of organic molecules to Lewis acids in water is characterised by strict prerequisites. This conclusion called for an alternative approach of extendirg the scope of Lewis-acid catalysis of Diels-Alder reactions in water. We have made use of a strongly coordinating diamine auxiliary, which enables efficient coordination of the dienophile to the catalyst. A Mannich reaction is employed to covalently link the 2-(methylaminomethyljpyridine auxiliary to benzylidene acetone as a model dienophile. Coordination of Cu ion activated the adduct towards the Diels-Alder reaction with cyclopentadiene. Unfortunately, we did not succeed in the desired removal of the diamine auxiliary via a retro Mannich reaction. However, elimination of the diamine is possible, affording the Diels-Alder adduct which is functionalised by a vinyl group in 24 % overall yield.  [c.177]

The problems involved are exemplified here by Knorr s pyrrole synthesis (A. Gossauer, 1974). It has been known for almost a century that or-aminoketones (CjN components) react with 1,3-dioxo compounds (Cj components) to form pyrroles (C N-heterocycles). A side-reaction is the cydodimerization of the a-aminoketones to yield dihydropyrazines (C4N2), but this can be minimized by keeping the concentration of the a-aminoketone low relative to the 1.3-dioxo compound. The first step in Knorr s pyrrole synthesis is the formation of an imine. This depends critically on the pH of the solution. The nucleophilidty of the amine is lost on protonation, whereas the carbonyl groups are activated by protons. An optimum is found around pH S, where yields of about 60% can be reached. At pH 4 or 6 the yield of the pyrrole may approach zero. The ester groups of -keto esters do not react with the amine under these conditions. If a more reactive 1,3-diketone is used, it has to be symmetrical, otherwise mixtures of two different imines are obtained. The imine formed rearranges to an enamine, which cyclizes and dehydrates to yield a 3-acylpytrole as the "normal Knorr product (A. Gossauer, 1974 G.W. Kenner, 1973 B).  [c.150]

When planning the synthesis of a compound using an organometallic reagent or indeed any synthesis the best approach is to reason backward from the product This method is called retrosynthetic analysis Retro synthetic analysis of 1 methylcyclohexanol suggests it can be prepared by the reaction of methylmagnesmm bromide and cyclohexanone  [c.614]

Indirect Procedures. Numerous inorganic anions that do not form complexes with a complexing agent are accessible to a chelatometric titration by indirect procedures. Erequently the anion can be precipitated as a compound containing a stoichiometric amount of a titrable cation. Another indirect approach employing replacement mechanism is the reduction of a species with the liquid amalgam of a metal that can be determined by a chelometric titration after removal of excess amalgam. Eor example  [c.1168]

Historically, QSAR has been appHed primarily to dmg molecules however, more recently. Quantitative Stmcture—Property (Toxicology) Relationships (QSP (T) R) have been elaborated by a number of researchers (117). Regardless of the context of appHcation, QSAR has its roots in the use of Linear Free Energy Relationship (LEER) methods of deriving an equation which correlates the observed effect and the stmctural features responsible for the activity. The work of Hansch in relating hydrophobicity to biological activity is exemplary as a pioneering appHcation of LEER in QSAR (118). It is interesting to note, however, that many of the early investigations in QSAR involved analysis of the relationship of hydrophobicity to activity, the nature of which relationship is more often paraboHc rather than linear (119,120). QSARs are usually best derived from a series of compounds (typically differing only at one or two substitution points) for which the activities are weU-deterrnined by a stable biological assay. A QSAR table can be estabHshed wherein the columns are assigned to activities (thejys), and the metrics or properties (the xs which can be either observable properties such as log P, hplc retention times nmr chemical shifts computed values such as shape and size descriptors dipole moments atomic charges or conformational energies. Each row represents an individual compound or conformation. Statistical relationships can then be developed from this table by means of univariate or multivariate techniques such as linear or multiple linear regression (MLR), or partial least squares (PLS). If the statistical significance of the relationship is sufficiently high, then this relationship is robust enough to be used to predict or assess the activity of untested compounds. Usually the known data is divided into two groups, a training set and a test set to estabHsh the statistical model. As a rule of thumb, there should be between 3 and 10 times the number of observations (rows) as x-variables in order to derive a model which would have predictive power and be able to minimize chance correlations. Under these circumstances, there would be some constraints against pursuing QSARs if only a few observations (molecules for which activities or properties are known) are available. Often this is the case, yet investigators have proceeded with the development of the QSAR. A discussion of the use of Partial Least Squares (PLS) as a potential means of resolution for this difficult is treated herein in the context of the CoMFA paradigm. It should also be recognized that many more molecular conformations and property descriptors than ever before can now be computed. For example, in the QSAR and Diversity modules of the Cerius2 software from Molecular Simulations, Inc. (121), there are approximately 160 shape, size, and electrostatics descriptors which can be computed. This relative abundance is in sharp contrast to the small numbers of descriptors available to early investigators, who worked diligently with classical Hammett CJs and log P values to derive LFERs (122—125). A particularly lucid account of the Hansch Approach to appHcation of LFERs in QSAR which is illustrated with numerous examples is given by the Hansch coUaborator and speciaHst in QSAR, T. Fujita (126). An example of the classical linear equation is represented by equation 7  [c.168]

Smolder-Resistant Upholstery Fabric. Chemical finishing to improve the smolder resistance of cotton fabric received some attention during the late 1970s. It was thought that regulatory activity would impact the market position of cotton upholstery fabric research on semidurable and durable finishes capable of withstanding occasional scmbbing was initiated for cotton upholstery fabric. Two chemical treatments are of particular interest. In one system, a formulation comprised of borax, an acidic compound, and TMM was appHed by conventional pa dding as well as by low wet add-on techniques (112,113). Another successful approach consisted of the appHcation of various polymers as backcoatings to upholstery fabric (114). The most effective polymers for this purpose were copolymers in which one of the monomers contained halogen. One particularly effective copolymer contained butadiene, styrene, and vinyHdene chloride. Although the use of synthetic barrier fabrics have largely supplanted the need for this type of finishing, these finishes did provide an effective means for making cotton smolder resistant.  [c.490]

Development Based on Natural Products. Development, with or without the use of QSAR, of herbicides from natural products has been rare. Although a number of natural products have shown herbicidal activity, specificity has been lacking, the effective concentration has proven too high for practicaHty, or the cost of manufacture on a mass scale has been considered prohibitive. These characteristics have led to a variant of the lead-compound approach in which the microorganisms, usually fungi, that produce the biologically active natural products are appHed to the weed in the form of mycoherbicides(20—22), ie, herbicides based on fungal weed pathogens. The principal advantage of mycoherbicides over conventional herbicides is the relative ease of registration. The principal disadvantage is very limited target specificity a mycoherbicide is usually effective against only one weed species under rather specific environmental conditions. Future commercial development of mycoherbicides depends heavily on improvements in the formulations that are necessary for increased shelf-life and vitaHty of the living fungal spores that constitute the active ingredient of this type of pesticide (23).  [c.39]

The 11-position of estradiol analogues has been a fmitfiil site of exploration in the development of hormone antagonists, eg, the antiprogestin RU 486 (see Contraceptives). The Roussel group has also uncovered novel anti-estrogens by investigating various substituents at the liP-position. The bis dime thylarninophenyl compound [123955-65-7] (27) and the phenoxyoctanamide [134411-59-9] (28) are examples of such an approach. The dimethylaminophenyl analogue (27) is also, like (26), effective against MCF-7 mammary tumor cells, but is much less potent (IC g = 300 nAf) (17). The importance of the long alkyl side chain for potency is evident as compound (28) (IC g = 0.006 nAf) is extremely potent and has potential use in a variety of indications (18).  [c.235]

Cyclopropanes can also be obtained by the reaction of vinyltrialkylborates with aldehydes followed by treatment with phosphoms pentachloride and base (300), and by the rearrangement of 5-substituted alkynyltrialkylborates (308). It is also possible to utilize this approach for the synthesis of five- and six-membered rings (3). Trans-1,4-elimination ia cycHc systems leads to the formation of stereodefined acycHc 1,5-dienes or medium-ring dienes, depending on the starting compound (309).  [c.317]

Of the many other methods leading to isocyanates, only a few are practical enough in regard to availabiUty of starting materials to be of general apphcabihty. One of the more promising approaches utilizes olefinic substrates which add isocyanic acid in Markovnikov fashion to form alkyl isocyanates (2,9,87). This reaction is used to produce l,4-bis(2-isocyanatoisopropyl)benzene (38) from cumene [98-82-8] (37) in commercial quantities (22,57,88). One approach uses the slow addition of the olefin to an excess of solvent and isocyanic acid in the presence of a catalytic amount of inorganic acid (57,88). Reaction temperatures are preferably maintained between 25 and 80°C. In the case of a diolefin, such as diisopropenylbenzene, the reaction can be controlled to favor the production of either the mono- or the diisocyanate by controlling the stoichiometry of the isocyanic acid in the reaction mixture. Another approach involves the formation of the dichloro intermediate. The dichloro compound reacts at low temperatures with an excess of isocyanic acid in the presence of a Lewis acid.  [c.456]

The optimal stmcture for light extraction ia a LED is a transparent substrate chip, usually obtained by direct growth on a substrate which is optically transparent to the light emitted by the active region of the material. However, lattice matched growth on a transparent substrate is not possible for some compounds, eg, AlGaAs or AlGalnP. In the case of AlGaAs, TS stmctures are formed by growing a thick (>100 fiva) transparent epitaxial layer between the active layer and the lattice matched GaAs absorbiag growth substrate. After growth, the GaAs substrate is selectively removed by chemical etching, leaving a TS stmcture (17). For AIGalnP grown on GaAs, a newer approach has been developed whereia the GaAs substrate is selectively removed, and a GaP substrate is substituted ia its place by compound semiconductor wafer bonding under appHed uniaxial pressure and elevated temperature (18). The GaP substrate is appHed within a few micrometers of the active layer and facilitates current spreading and light extraction beneath the device.  [c.119]

The fluorine-19 [14762-94-8] F, nucleus has a sensitivity second only to that of H. Moreover, the frequency is similar enough to H s that often the signal can be detected by tuning the proton channel of a probe to the F frequency. In addition to the usual chemical shift and coupling experiments, F nmr of fluorinated ligands is widely appHcable in kinetic studies of en2yme—substrate interactions (23) (see Kinetic measurements). The success of this approach depends on the larger en2yme-induced changes in F chemical shifts, as compared to chemical shifts in the parent compound. Such protein-induced changes are typically 9—15 ppm in a downfield direction. The danger in interpreting such studies is that the replacement of hydrogen by fluorine may also have significant effects on the stmcture and stabiUty of a complex. A second problem for fluorine is the large chemical shift anisotropy (CSA). The effect of this property on the magnetic-field dependence of both and increases the natural hnewidth with increasing Bq.  [c.405]

The cyclization approach has been extremely successful ia the synthesis of highly selective 5-agonists. Substitution of D- or T.-penicillamine moieties ia the 2 and 5 position of enkephalin has led to compounds having a high degree of 5-selectivity (88). Two of the most selective analogues that have been used extensively as 5-receptor agonists are H-Tyr-cyclo[-D-Pen-Gly-Phe-D-Pen]-OH (DPDPE) and DPLPE (with L-Pen ia the 5 position). The 5-selectivity of these compounds was shown to result from steric interference at the p.-site, caused by the presence of the gem dimethyl groups in the 2-position side chain (89). Although DPDPE and DPLPE are extremely 5-selective, their absolute affinity for the 5-receptor is low compared to some the 5-selective linear enkephalin analogues. However, this problem has been solved by replacement of Phe with -chlorophenylalanine, which improves the 5-selectivity of the compound by fivefold over DPDPE because of an increase in affinity at the 5-site (90).  [c.447]

Alkylphenols can be synthesized by several approaches, including alkylation of a phenol, hydroxylation of an alkylbenzene, dehydrogenation of an alkylcyclohexanol, or ring closure of an appropriately substituted acycHc compound. The choice of approach depends on the target alkylphenol, availabihty of the starting materials, and cost of processing. The procedures discussed herein encompass commercial methods, general methods, and a few specific examples of commercial interest.  [c.58]

See pages that mention the term Aaberg compound : [c.195]    [c.151]    [c.153]    [c.700]    [c.726]    [c.728]    [c.22]    [c.14]    [c.519]    [c.484]   
Industrial ventilation design guidebook (2001) -- [ c.1423 ]