Isomerism structural features

The formation of enamines from carbonyl compounds and secondary amines usually entails as only questionable structural feature, the possible isomeric position of double bonds in the product. Molecular rearrangements have not presented synthetic limitations. A notable exception is the generation of o-aminophenols on distillation of enamines derived from 2-acylfurans 620,621). 

A careful investigation of the extent of isomeric products formed by reaction of several alcohols with thionyl chloride has been reported. The product compositions for several of the alcohols are given below. Identify the structural features that promote isomerization and show how each of the rearranged products is formed. 

Quenching rate constants for dienes and quadricyclenes have similar sensitivities to the electronic and structural features of the excited aromatic hydrocarbon. However, during this process quadricyclene isomerizes to nor-boraadiene with a quantum yield of 0.52, whereas dienes usually remain unchanged/10 Hammond has suggested that vibrational energy which is partitioned to the acceptor upon internal conversion of the exciplex can lead to isomerization(10a,103) ... 

Industrial applications of zeolites cover a broad range of technological processes from oil upgrading, via petrochemical transformations up to synthesis of fine chemicals [1,2]. These processes clearly benefit from zeolite well-defined microporous structures providing a possibility of reaction control via shape selectivity [3,4] and acidity [5]. Catalytic reactions, namely transformations of aromatic hydrocarbons via alkylation, isomerization, disproportionation and transalkylation [2], are not only of industrial importance but can also be used to assess the structural features of zeolites [6] especially when combined with the investigation of their acidic properties [7]. A high diversity of zeolitic structures provides us with the opportunity to correlate the acidity, activity and selectivity of different structural types of zeolites. 

The reaction of allenes with peracids and other oxygen transfer reagents such as dimethyldioxirane (DM DO) or hydrogen peroxide proceeds via allene oxide intermediates (Scheme 17.17). The allene oxide moiety is a versatile functionality. It encompasses the structural features of an epoxide, an olefin and an enol ether. These reactive intermediates may then isomerize to cyclopropanones, react with nucleophiles to give functionalized ketones or participate in a second epoxidation reaction to give spirodioxides, which can react further with a nucleophile to give hydroxy ketones. 

The isomeric bibenzoic acids (BBs), would appear to share similar structural features with naphthalene dicarboxylic acid. Like the PET-naphthalate copolymers, PET-bibenzoates have been demonstrated to possess moduli and glass transitions temperatures which increase with increasing levels of rigid comonomer [37-39], Unlike the PET/PEN copolymers, when the symmetrical 4,4 - I f I f monomer is substituted into a PET backbone, virtually every composition of PET-BB is semicrystalline the 2,4- and 3,4- isomers of BB, when... 

In general, things are simpler than that, much to our advantage. Within the limits set by the precision of the present estimates, structural features like the chair, boat, or twist-boat conformations of cyclohexane rings, as well as the butane-gawc/ze effects or the cis-tmns isomerism of ethylenic compounds leave no recognizable distinctive trace in zero-point plus heat content energies. Indeed, whatever residual, presently... 

In 1999 Uemuraetal. isolatedattenol A (100) and B (101) (Fig. 1.2.5), both marine natural products exhibiting a moderate cytotoxicity against P388 cells [57], from the Chinese Pinna attenuata. Since they are isomeric triols they differ only in the hydroxyl groups involved in the formation of the ketal functionality, so that this results in a l,6-dioxaspiro[4.5]decane and a 6,8-dioxabicyclo[3.2.1]octane unit as the main structural feature of attenol A and B, respectively. 

All six spectra showed strong fragment ions at m/e 205 corresponding to loss of a methyl group. The spectral features are in general agreement with those of trimethyl or methylethylphen-anthrene anthracene. Exact isomeric structures could not be determined because many isomers of the methyl-, C2-alkyl-, and Cg-alkylphenanthrene/anthracene are possible but only a few methylphenanthrene/anthracene standards were available to us. 

Four isomeric structures are possible. All of them have a bridgehead nitrogen atom. A characteristic feature in these triazolopyrimidines is the ease of a Dimroth rearrangement (99AHC) in two systems this results in the... 

ABSTRACT The acid-catalysed epimerization reaction of bioactive indole alkaloids and their derivatives is reviewed. The three mechanisms, which have been proposed for the (J-carboline-type indole alkaloids, are discussed. Through recent developments, evidence for all three mechanisms has been obtained, which shows the complexity of the epimerization reaction. The epimerization seems to depend on structural features and reaction conditions making it difficult to define one universal mechanism. On the other hand, the isomerization mechanism of oxindole alkaloids has been widely accepted. The acid-catalysed epimerization reaction provides a powerful tool in selectively manipulating the stereochemistry at the epimeric centre and it can also have a marked effect on the pharmacology of any epimerizable compound. Therefore, examples of this reaction in die total synthesis of indole alkaloids are given and pharmacological activities of some C-3 epimeric diastereomers are compared. Finally, literature examples of acid-catalysed epimerization reactions are presented. 

Complexes of molybdenum form the largest set of any of the transition metals. Many of these compounds display spectroscopic properties, isomerism and structural features that illustrate the coordination chemistry of polypyrazolylborate complexes. In addition, this system illustrates that trispyrazolylborate ligands are more bulky than their formal analogue C5H5. 

NMR studies have been mainly applied to elucidating the structural features of Schilf bases in solution. These studies are mainly concerned with SchiiT bases derived from benzaldehyde and its substituents, / -diketones, o-hydroxyacetophenones and o-hydroxyacetonaphthones. They were devoted to obtaining an insight into the keto-enol equilibrium (Scheme 1), syn and anti isomerism and steric distortions in different kinds of solvents. Relevant data and results are described in the following sections. 

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