Isomerism, structural

COT is prepared by the polymerization of ethyne at moderate temperature and pressure in the presence of nickel salts. The molecule is non-planar and behaves as a typical cyclic olefin, having no aromatic properties. It may be catalytically hydrogenated to cyclo-octene, but with Zn and dil. sulphuric acid gives 1,3,6-cyclooclairiene. It reacts with maleic anhydride to give an adduct, m.p. 166 C, derived from the isomeric structure bicyclo-4,2,0-octa-2,4,7-triene(I) ... 

The earlier sections have only considered the way atoms are bonded to each other in a molecule (topology) and how this is translated into a computer-readable form. Chemists define this arrangement of the bonds as the constitution of a molecule. The example in Figure 2-39, Section 2.5.2.1, shows that molecules with a given empirical formula, e.g., C H O, can have several different structures, which are called isomers [lOOj. Isomeric structures can be divided into constitutional isomers and stereoisomers (see Figure 2-67). 

Figure 2-67. Classification of isomeric structures of organic compounds.

The two substituted carbons are connected by a double bond in one structure but by a single bond in the other Because no such cases of isomerism m benzene derivatives were known and none could be found Kekule suggested that two isomeric structures could exist but mterconverted too rapidly to be separated... 

From UV studies of 4-phenyl-, 4-nitro- and 4-nitroso-pyrazoles, Habraken et al. (67RTC1249,72JHC939) conclude that the 4-pyrazolyl group acts as an electron-donating group. UV spectra of pairs of 1-aryl- and 2-aryl-indazoles and their utility in the determination of isomeric structures are discussed in (67BSF2619) many other UV data on indazole derivatives can be found in (71PMH(3)67). 

The fact that the isomeric structure of azolides is thermodynamically controlled has been used by Olofson and Kendall to prepare 1-alkylazoles regioselectively (70JOC2246). An asymmetric pyrazole yields two alkylated derivatives (Scheme 21 see Section 4.04.2.1.3 (viii)), but the alkylation with a powerful alkylating agent of the acetylated derivative leads to the less abundant isomer via the salt (249), which is too unstable to be isolated. 

The mid-block monomers are primarily isoprene and butadiene. These diolefins can polymerize in several ways. The isomeric structure of the final polymer has a strong impact on its properties and thermal stability. Isomeric composition is easily varied by changing the polymerization solvent or adding complexing agents. The typical isomeric structures for isoprene and butadiene mid-blocks are shown in Fig. 2. 

Absolute configurations of the amino acids are referenced to D- and L-glyceraldehyde on the basis of chemical transformations that can convert the molecule of interest to either of these reference isomeric structures. In such reactions, the stereochemical consequences for the asymmetric centers must be understood for each reaction step. Propose a sequence of reactions that would demonstrate that l( —)-serine is stereochemically related to l( —)-glyceraldehyde. 

It has been postulated that harmaline itself may under certain conditions consist of an equilibrium mixture of the isomeric structures 122 and 441, with the former strongly predominating . Both forms are regarded as existing in equilibrium with the harmalinium cation (440), from which the former may be derived by deprotonation at the... 

The structure of 82 was established by alkaline ring cleavage to benzilic acid amide and by hydrogenolysis to (C6H5)2CH—CONH— COCfiHs. These reactions also served to eliminate 83 as the structure of the 169° compound. The other possible isomeric structure, (C6H5)2C(CN)0C0C6H5, which could have formed after 0-acylation, was ruled out by its independent synthesis from bromodiphenyl-acetonitrile and silver benzoate. 

Two isomeric structures can be obtained for these products ( -113 and/or Z-113). The stereochemistry was conveniently elucidated on the basis of NMR data which showed coupling constant values 7(H,CO) consistent with the -isomers only. The formation of 113 was explained to occur via the enolate salts (86JHC199). Catalytic... 

Such ring system may be classified into three isomeric structures pyrrolo [l,2-a]quinoline, pyrrolo[4,5,l-jy]quinoline and pyrrolo[2,l-y]quinoline (Fig. 4). 

Similar number of the isomeric structures for thiazoloquinolines to that of the oxazoloquinolines can be drawn, but the reported examples for these isomers belong to only three ring systems thiazolo[3,2-u]quinoline, thiazolo[4,3-u]quinoline and thiazolo[5,4,3-i/]quinoline (Fig. 8). 

Alkenes also form a homologous series as the carbon number increases, the number of possible isomeric structures for each member increases more rapidly than in the case of the alkane series. 

Another class of compounds is called condensed-ring or fused-ring systems. These structures contain two or more aromatic rings that share a pair of carbon atoms. Examples include naphthalene, anthracene, and phenanthrene, the latter two being isomeric structures. 

TriN83 Trinajstic, N., Jericevic, Z. Computer generation of isomeric structures. Pure Appl. Chem. 55 (1983) 379-390. 

The equilibrium between oxepin and benzene oxide created interest in performing Diels-Alder reactions trapping one or both isomeric structures.1 The reaction of maleic anhydride or maleic imide with oxepin and substituted derivatives gives products 1 derived from the addition of the dienophile to the benzene oxide structure.2-l4-126 14 9 156 158 228 231-259... 

To date little is known about the chemistry of triazocines. Only three of the five possible isomeric structures are relevant in the context of this review as there are no reported syntheses of the 1,2,3- (I) and 1,3,6-triazo (5) compounds. A special review on triazocines is available.1... 

There are nine possible isomeric structures representing different annulations onto the triazine ring to form [1,3]diazolo[x,y-z][l, 2,4]tria-zines. Only eight are reported during the period covered by this review. 

Diazotization of 5-amino[l, 2,3]triazole 692 afforded (88BSB179) triaz-olo[l, 5-i>][l, 2,4]triazine 694 as a result of a Dimroth rearrangement of the initially formed isomeric structure triazolo[5,l-c][l,2,4]triazine 693. Molecular structure of 694 was determined by single X-ray diffraction (Scheme 146). 

Although there are various isomeric structures for this ring system, only one has been reported in the recent literature. 

Isomeric structures similar to those for the oxadiazolo analogues can be drawn for this group of compounds, but more are reported here. 

The presence of a 5-oxo group in the 1,2,4-triazines, as in 3-hydrazino-2,5-dihydro-5-oxo[l,2,4]triazines, does not alter the direction of cycliza-tion by the action of nitrous acid. The initial products are azido compounds 1016, which can cyclize spontaneously to tetrazolo[l,5-b][l,2,4]triazines 1017 (77JHC1221, 77JOC1866) rather than the previously reported isomeric structure. On the other hand, when the cyclization onto N-2 is impossible, as in derivative 1018, the azidotetrazolo equilibrium 1018 1019 exists (76JOC2860 77JOC1866). It exists in the solid state at least as the tetrazole derivative (Scheme 190). 

The mechanism of B polymerization is summarized in Scheme 4,9. 1,2-, and cis- and trews-1,4-butadiene units may be discriminated by IR, Raman, or H or nC MMR speclroseopy.1 92 94 PB comprises predominantly 1,4-rra//.v-units. A typical composition formed by radical polymerization is 57.3 23.7 19.0 for trans-1,4- c7a -1,4- 1,2-. While the ratio of 1,2- to 1,4-units shows only a small temperature dependence, the effect on the cis-trans ratio appears substantial. Sato et al9J have determined dyad sequences by solution, 3C NMR and found that the distribution of isomeric structures and tacticity is adequately described by Bernoullian statistics. Kawahara et al.94 determined the microslructure (ratio // measurements directly on PB latexes and obtained similar data to that obtained by solution I3C NMR. They94 also characterized crosslinked PB. 

The effective carbon number neff is helpful in characterizing surfactants with an inner functional group. Surfactants with isomeric structures can be compared by means of the hydrophobicity index / [69], which indicates the influence of the effective length of the alkane chain on cM ... 

R =Me, Et, n-Bu), to monomeric, orange-red dithiocarbamates, [CpTi(S2CNRz)X2], which contain 5-coordinate titanium. It should be noted that several isomeric structures are possible for these compounds. 

The crystal structures of four chlorinated derivatives of di-benzo-p-dioxin have been determined by x-ray diffraction from diffractometer data (MoKa radiation). The compounds, their formulae, cell dimensions, space groups, the number of molecules per unit cell, the crystallographic B.-factors, and the number of observed reflections are given. The dioxin crystal structures were performed to provide absolute standards for assignment of isomeric structures and have been of considerable practical use in combination with x-ray powder diffraction analysis. 

Absolute identification of the isomeric structure of primary standard materials by single crystal techniques ... 

Scheme 7 Initiation mechanisms proposed in literature for the CO-reduced Cr/Si02 catalyst. Vertical direction shows evolution of the initial species upon addition of one ethylene molecule. Horizontal direction shows all the possible isomeric structures characterized by an average C2H4/Cr ratio equal to 1, 2, and 3...

Tandem MS (DFS equipped with EI/F1/FD source) in conjunction with off-line direct inlet HPLC-UV was used for separation and quantification of isomeric antioxidants, C22H30O2S (MW 358 Scheme 6.3), as antioxidants in THF extracts of surgeons gloves [232]. Collision activation MS enabled differentiation between the three isomeric structures (Fig. 6.20). Quantification was achieved by chromatographic analysis of the isomeric species, which are not distinguishable by MS. On-line LC-MS facilitates this kind of analysis. 

The aim of the reaction of 15a with Cl—P=C(SiMe3)Ph was to synthesize the 2,3,1-diphosphasilabuta-1,3-diene derivative 42 (Scheme 11). However, it turned out that the Cl—P bond of the phosphaalkene exclusively adds to the Si=P bond, forming the addition product 43, which probably has the ( >isomeric structure.45 The corresponding (Z)-isomer 43 was not formed. On heating of 43, even for 8 hours at 110°C, no elimination of /Pr3SiCl was observed. Instead, ( )/(Z)-isomerization simply occurred (43 and 43 in the molar ratio of 1 0.8). 

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