Relationships between Isomeric Compounds

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

DL-Dihydrostreptose and its ribo isomer were similarly obtained. Birch reduction of 2-methyl-3-furoic acid, followed by addition of methanol, bromination, and dehydrobromination, gave 402 as a mixture of the isomers. Hydroxylation of 402 with osmium tetraoxide-so-dium chlorate, and subsequent treatment with acetone-sulfuric acid afforded three isomeric acetals (403-405). The structures of these compounds were assigned on the basis of their H-n.m.r. spectra. In addition, the relationship between 403 and 404 was established by hydrolysis and reglycosidation. The methyl esters 403-405 were quantitatively reduced to the corresponding alcohols. The mixture of alcohols obtained from 403 and 404 was converted into crystalline 5-deoxy-3-C-(hydroxymethyl)-l,2-0-isopropylidene-a-DL-ribofuran-ose (406), which was compared directly with a sample prepared from D-xylose. Methyl 5-deox y-3-C-(hydroxy methyl)-2,3-O-isopropy lidene-/3-DL-lyxofuranoside (407), obtained by reduction of 405 with lithium aluminum hydride, was hydrolyzed with dilute hydrochloric acid, to give a,/3-DL-dihydrostreptose.2,ifi... 

In this section we have treated a simple model of cis-trans isomerization by examining the time development of a compound state of the model system. Our purpose has been to develop relationships between observables, such as the quantum yield of product, and the fundamental properties of the model spectrum of states. For the particular case considered our results are described in Section XII-D. Insofar as our model system is designed to incorporate the principal features of the experimentally deduced reaction mechanism, formal agreement between the theoretical analysis and observations is assured. What then can we learn from such a treatment ... 

The evidence for isomerism varies from the obvious, such as a difference in phase, to the subtle, such as the temperature dependence of an NMR spectrum. These instances illustrate an important point, namely, that the detection of isomerism is dependent upon the existence of an energy barrier between the different forms and upon a method of detection sensitive enough to observe the barrier. A compound that appears to be a single species on one time-scale may prove to be a mixture on another. The relationship between the lifetime of a particular molecular structure and the various physical methods for studying it has been discussed by Muetterties.3 The essentially instantaneous method of X-ray diffraction (10 18s) may be contrasted with NMR techniques (10 1-10 9s) and the time scale for the experimental separation of isomers (>102s). 

In these compounds, there is a marked relationship between molecular geometry and biological activity. From values reported in the literature and according to our own studies, the E isomers, in which the residue originating from the aldehyde is in the transposition to the triazole, are markedly superior to the Z isomers in their biological activity. By suitable control of the reaction conditions, it is possible to achieve an almost complete isomerization to the unsaturated E-triazolylketones. Subsequent reduction leads to the more active E-alcohols. This group of N-vinylazoles includes the triazole derivative S 3308 (Sumitomo), currently under development as... 

Before the relationship between chirality and optical activity was known, enantiomers were called optical isomers because they seemed identical except for their opposite optical activity. The term was loosely applied to more than one type of isomerism among optically active compounds, however, and this ambiguous term has been replaced by the well-defined term enantiomers. 

The structure of the sex pheromone for the Fucus species, fucoserratene (11), was elucidated in 1973.16 The positions and geometries of alkenes were revealed by comparison of the gas chromatographic behavior with those of the isomeric conjugated 1,3,5- and 2,4,6-octatrienes. To date, a series of hydrocarbons and epoxides 1-11 and their stereoisomers have been identified within the pheromone bouquets of more than 100 different species of brown algae.17-23 Identification of these compounds was based on a combination of gas chromatography-mass spectrometry (GC-MS) analysis and by comparison with authentic synthetic compounds. These sex pheromones were all lipophilic, volatile compounds that consisted of C8 or Cn linear or monocyclic hydrocarbons or their epoxides. The monocyclic compounds have a cyclopropane, cyclopentene, or cyclo-heptadiene structure. Interestingly, the relationships between the chemical structures of pheromones and the taxonomical classifications of algae are unclear (Table 1). 

The question now arises, how may we determine which one of the various formulas, in the case of the five hexanes for instance, is to be assigned to each individual compound of definite physical properties To which one of the butanes, pentanes and hexanes do we assign the straight chain formula or the name normal In the case of the butanes the answer and the reason for it are found in a new synthesis of one of the butanes. We have given one synthesis of the two butanes, viz., from propyl iodide and methyl iodide. As one propyl iodide yields one butane and the other yields the isomeric butane, we know that one of the two isomeric butanes must have the straight chain or normal formula. But we do not know whether the propyl iodide from which the butane boiling at + i is prepared, is really the one possessing the normal or the iso formula. Therefore, it will be seen that the relationship between the isomeric propyl iodides and the isomeric bu-... 

Chemical compounds that have the same molecular formula but different structural formulas are said to be isomers of each other. These constitutional isomers (or structural isomers) differ in their bonding sequence, i.e. their atoms are connected to each other in different ways. Stereoisomers have the same bonding sequence, but they differ in the orientation of their atoms in space. Stereoisomerism can be further divided into optical isomerism (enantiomerism) and geometrical isomerism (cis—trans isomerism). The relationships between the different types of isomerism are shown in Figure 4.1. 

What, then, are the various types of selec-tivities (or specificities) encountered in xeno-biotic metabolism What characterizes an enzyme from a catalytic viewpoint is first its chemospecificity, i.e., its specificity in terms of the type(s) of reaction it catalyzes. When two or more substrates are metabolized at different rates by a single enzyme under identical conditions, substrate selectivity is observed. In such a definition, the nature of the prod-uct( nd their isomeric relationship are not considered. Substrate selectivity is distinct from product selectivity, which is observed when two or more metabolites are formed at different rates by a single enzyme from a single substrate. Thus, substrate-selective reactions discriminate between different compounds, whereas product-selective reactions discriminate between different groups or positions in a given compound. 

The reversible redox relationship between 1,4-benzenediols and 2,5-cyclohexadiene-l,4-diones (/ -benzoquinones, or just quinones) is a special one because of its relative ease. Conjugate addition and Diels-Alder cycloaddition are common reactions of quinones conjugate additions have biological importance. The isomeric but less stable 1,2-compounds are encountered much less often and arc accorilingly less important. 

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