Position equivalency

A multiply bonded nitrogen atom deactivates carbon atoms a or y to it toward electrophilic attack thus initial substitution in 1,2- and 1,3-dihetero compounds should be as shown in structures (110) and (111). Pyrazoles (110 Z = NH), isoxazoles (110 Z = 0), isothiazoles (110 Z = S), imidazoles (111 Z = NH, tautomerism can make the 4- and 5-positions equivalent) and thiazoles (111 Z = S) do indeed undergo electrophilic substitution as expected. Little is known of the electrophilic substitution reactions of oxazoles (111 Z = O) and compounds containing three or more heteroatoms in one ring. Deactivation of the 4-position in 1,3-dihetero compounds (111) is less effective because of considerable double bond fixation (cf. Sections 4.01.3.2.1 and 4.02.3.1.7), and if the 5-position of imidazoles or thiazoles is blocked, substitution can occur in the 4-position (112). 

Most references involve studies of pyrazoloimidazoles. Bromine entered l//-imidazo[l,2-6]pyrazole at the 7-position (equivalent to the 4-position in pyrazole) (78MI1). Similar regiochemistry was observed when... 

In contrast to AMPA receptors, NMDA receptor channels display a prominent Ca2+ permeability, which is largely independent ofthe subunit composition. It has been shown by mutational analysis that the Ca2+ permeability of recombinant NMDA receptors is dependent on a residue at a position equivalent to the Q/R site of AMPA subunits. Both NR1 and NR2 subunits contain an asparagine (N) residue at this position. Replacing this N with an R within the NR1 subunit led to the formation of NMDA receptors with a strongly reduced Ca2+ permeability, whereas exchanging N for Q in the NR2 subunit had only a small effect,... 

Time equivalent to position Position equivalent to time in a... 

These are not the positive equivalent of an electron, a position, because such a particle would be eliminated instantaneously by combination with an ordinary election, but are virtual particles equivalent to the absence of an electron. They can be considered to be analogs of a vacancy, which is tbe absence of an atom. 

Aldehydo nitriles are readily available and can be used to prepare 4-amino-6-unsubstituted pyrimidines carrying one or more substituents in any of the other positions. Equivalents of the formyl group are also often used, for example, a 3-ethoxy-, 3-amino-, or 3-haloacrylonitrile. Most syntheses with /3-keto nitriles are carried out with equivalents thereof Such equivalents are /3-substituted /3-alkoxy-, /3-amino-, or /3-haloacrylonitriles. /3 -Ester nitrile reactions are also well established. Malononitriles and substituted malononitriles react readily with thiourea and N-substituted thioureas in refluxing ethanolic sodium ethoxide to form pyrimidine-4,6-diamines. An example is the reaction of N2-malononitrile 680 with N2-thiourea 681 to give N4- 4,6-diamino-2-(l//)-pyrimidinethione 682 which was then used in the synthesis of Ns-labeled adenine derivatives <2001JOC5463>. 

To compare doses resulting from different types of radiation it is necessary to formulate a measure of radiation effects on the crystal structure doses can be recalculated to the number of displacements per atom (dpa). A dose of 0.1 dpa, for example, means that one of ten atoms was displaced from its initial position. Equivalent values in dpa units may be calculated for different types of radiation from the effects of its interaction with the crystal lattice. To recalculate a-dose to dpa the following formula is used ... 

A multiply bonded nitrogen atom deactivates carbon atoms a or t to it toward electrophilic attack thus initial substitution in 1,2- and 1,3-dihetero compounds should be as shown in structures (136) and (137). Pyrazoles (136 Z=NH), isoxazoles (136 Z = 0), isothiazoles (136 Z=S), imidazoles (137 Z=NH, tautomerism can make the 4- and 5-positions equivalent) and thiazoles (137 Z=S) do indeed... 

Ellipticine (42a) possesses a unique pyrido[4,3-6]carbazole ring [218], The two methyl groups of the ellipticine molecule occupy positions equivalent to those of DMBA (4). In fact, structure-activity relationship study reveals that the presence of these methyl groups is essential for the expression of the inhibitory action against cytochrome P-450. The location of the pyridine ring appears to be the most important factor for the cytochrome P-450 binding, since isoellipticines (43), (44), (45) and (46) (none of which contains the characteristic 2-phenylnaphthalene pattern) are devoid of such inhibitory activity [226]. 

Fig. 2 Ribbon diagram of the structure of the a/p-tubulin dimer in 2D zinc-induced sheets (PDB entry 1TUB). The p monomer is on top. Some elements involved in PTX binding and lateral contacts are colored M loop in green, S9-S10 loop in blue, helix H7 in yellow, and helix H3 in orange. Small-molecule ligands are represented as sticks PTX in red, GDP and GTP in magenta. The a-tubulin S9-S10 loop occupies a position equivalent to that of PTX in P-tubulin...

In the introduction it was stated that the disturbances of the endocuticular matrix of Pieris brassicae larvae by the parent benzoylphenyl urea compound Du 19111 were caused by an influence on chitin formation, thus assigning to the benzoylphenyl ureas an insecticidal position equivalent to that of the fungicidal polyoxins. In the last part of this paper we shall try to adduce arguments supporting that statement. 

Sketches of designs that allow catalyst treatments and can be used with an integrating sphere have been published (Klier, 1968 Klier and Ralek, 1968 Kortiim, 1969, p. 242 Stone, 1983 Schoonheydt, 1984 Che and Bozon-Verduraz, 1997 Weckhuysen and Schoonheydt, 1999). Placement of the reference material and the sample next to each other, by using the same port of the sphere, yields exact position equivalency (Zecchina et al., 1975). Temperature gradients in the bed can be minimized by using a double set of windows separated by vacuum (Kellermann, 1979). Such a... 

The IV is defined as an average total number of double bonds per mole in a mixture of fatty materials. It is not a good descriptor for correlating physical and chemical properties with fatty acid composition in biodiesel. For example, the IV does not provide any information on structural factors such as number of allylic or bis-allylic methylene groups or location of double bond(s) within the hydrocarbon chain. Knothe (2002) recommended alternative indices termed allylic position equivalents (APE) and Ws-allylic position equivalent (BAPE) based on the total number of allylic and bis-allylic positions present in the fatty acid chains. 

Such a situation occurs in LaCrS3, in which the successive H layers [CrS2] are shifted by subceii/6 against the stacking sequence of the T layers. After six layer pairs, both layer sets meet in positions equivalent to those at the origin. The unit cell situation is unknown. 

In reactions of the imidazole neutral molecule or cation, attack by electrophiles at C-5 is preferred. However, when there is a free NH in the ring, tautomerism makes the 4- and 5-positions equivalent. Conditions in which the imidazole anion exists are reported to be characterized by electrophilic substitution at C-2. There would appear to be exceptions to this generalization, e.g. halogenation (Section 4.07.1.4.5). When the preferred positions are blocked it is usually possible to induce electrophiles to enter alternative ring positions and multiple substitution is commonly possible. In fact it is difficult to prepare mono-bromoimidazole, and imidazole can be dinitrated. More than one benzeneazo group can couple when imidazole reacts with diazonium salts. 

Studies on archaeal ribosomes have shown that mutations that confer resistance to the peptidyl-transfer inhibitor anisomycin (an antibiotic that also affects eucaryal ribosomes) also map to this central loop [87]. Sequencing of several archaeal LSU rRNA has also revealed that they have either a G or a U at a position equivalent to A2058, which in bacteria produces sensitivity to erythromycin. As a consequence of this change, archaea are insensitive to this drug [29,30,87,88,95]. 

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