Acyclic structures

Conversion of Cyclic to Acyclic Structures. Upon oxidation, the aromatic rings of lignin may be converted direcdy to acycHc stmctures, eg, muconic acid derivatives, or indirectly by oxidative splitting of o-quinoid rings. Further oxidation creates carboxyUc acid fragments attached to the lignin network. 

In acyclic structures, such effects are averaged by rotation, but in cyclic structures differences in C—H bond strengths based on the different alignments can be recognized. The C—H bonds that are in an anti orientation to the lone pair are weaker than the C—H bonds in other orientations. 

It is a yellow solid with an acyclic structure (Fig. 15.49a), cf N2O5 (p. 458). Moist air converts S3N2O2 to SO2 and S4N4 whereas SO3... 

Occasionally it happens that the oxo compound, produced by oxidation, forms a hydrate which is further oxidized to a dihydroxy compound. Attention must be given to the possibility (so far unreported) that when the hydrated species is in equilibrium with a trace of the ring-opened structure a sufficiently fast oxidation rate of the amino-aldehyde (i.e. the acyclic structure) could lead to the incorrect conclusion that the original material was not cyclic. 

DPT calculations confirmed the experimental findings that 2-ZnCl-l,3-oxazole 21 prefers a closed-ring structure while the corresponding 2-Li-l,3-oxazole 22 is not stable and prefers an acyclic structure (Scheme 17) [98CEJ814]. NBO analyses showed that the orbital of the C—M (M = Li, Zn) bond is -hybridized in the Zn... 

Another common ring system is the norbornane, or bicyclo[2.2.1.)heptane, structure. Like decalin, norbornane is a bicycloalkane, so called because two rings would have to be broken open to generate an acyclic structure. Its systematic name, bicyclo[2.2.1 heptane, reflects the fact that the molecule has seven carbons, is bicyclic, and has three "bridges" of 2, 2, and 1 carbon atoms connecting the two bridgehead carbons. 

Each of these compounds, 53-56, was shown to be a very effective competitive inhibitor of the enzyme with respect to the fructose 1,6-diphosphate, whereas several other analogs, including acyclic structures, had no effect. These and other results suggest that the furanose form of the sugar diphosphate is the active form in the enzymatic reaction (105). More recent studies using rapid quenching techniques and C-nmr measurements have confirmed this hypothesis and indicate that the enzyme uses the a anomer 52 much more rapidly than the 3 anomer 50 and probably uses the a anomer exclusively (106). 

Like amine oxide elimination, selenoxide eliminations normally favor formation of the E-isomer in acyclic structures. In cyclic systems the stereochemical requirements of the cyclic TS govern the product composition. Section B of Scheme 6.21 gives some examples of selenoxide eliminations. 

Compounds belonging to this series are of special interest because they exhibit the properties of both cyclic and acyclic structures as a result of ion complex tautomerism. They were obtained via borylation of hydroxy-alky lphosphines in the presence of tertiary amines [Eq. (64)], whereas primary and secondary amines give aminomethylphosphines under such conditions (84UK625 86JZV1641, 86MI1 90MI1). 

The series E(NSO)2 (39a-c E=S, Se, Te) exhibit planar, acyclic structures with a cis arrangement about the two S=N bonds. The central E-N bonds are essentially single bonds. The S=N bond distances fall in the double bond range with a significant shortening of this bond along the series. 

Physicochemical properties rather than reactivities were also explored. Molecular electrostatic potential (MEP) was calculated for the [l,2,4]triazolo[4,3- ]pyridine fragment 23, according to the CHELPG algorithm. This afforded a prediction of its H-bond acceptor ability in view of the synthesis of p38 MAP kinase inhibitors <2005JME5728>. Tautomerism was also examined for compound 24, also postulated as two possible acyclic structures. The ab initio self-consistent field (SCF)-calculated energies support 24a as the most stable tautomer <1999MRC493>. 

D-Ribonolactone is a convenient source of chiral cyclopentenones, acyclic structures, and oxacyclic systems, useful intermediates for the synthesis of biologically important molecules. Cyclopentenones derived from ribono-lactone have been employed for the synthesis of prostanoids and carbocyclic nucleosides. The cyclopentenone 280 was synthesized (265) from 2,3-0-cyclohexylidene-D-ribono-1,4-lactone (16b) by a threestep synthesis that involves successive periodate oxidation, glycosylation of the lactol with 2-propanol to give 279, and treatment of 279 with lithium dimethyl methyl-phosphonate. The enantiomer of 280 was prepared from D-mannose by converting it to the corresponding lactone, which was selectively protected at HO-2, HO-3 by acetalization. Likewise, the isopropylidene derivative 282 was obtained (266) via the intermediate unsaturated lactone 281, prepared from 16a. Reduction of 281 with di-tert-butoxy lithium aluminum hydride, followed by mesylation, gave 282. 

The application of this procedure to the fused polycyclic compound E, which already has a linear dual and only the last two steps (iii-iv) apply to it, leads to a linear acyclic structure F which may be traced back to the biogenetic cyclisation of squalene to lanosterol via cationic intermediates, as well as to the stereospecific cationic cyclisation of polyolefins studied by Johnson [18]. 

Later, imidazolidin-2-ylidenes such as 14, a saturated, more electron-rich and nonaromatic version of the imidazolin-2-ylidenes, were isolated. Isolation of a six-membered tetrahydropyrimid-2-ylidene 15 and of acyclic structures such as 1629,30... 

First of all, the introduction of an electropositive substituent onto the carbene center increases its nucleophilicity. In particular, the effect of a strong 7t-donating and electropositive ylide substituent is efficient as can be seen in the case of amino (ylide) carbenes and of cyclic vinylidenephosphorane type carbenes. Acyclic structures also increase the HOMO level but they also significantly diminish the S/T energy gap. 

It is estimated that far more than 50% of the published chemical literature concerns heterocyclic structures. One striking structural feature inherent to heterocycles, which continues to be exploited to great advantage by the drug industry, lies in their ability to manifest substituents around a core scaffold in a defined three-dimensional representation, thereby allowing for far fewer degrees of conformational freedom than the corresponding conceivable acyclic structures. 

The CRU is named using replacement nomenclature ( a nomenclature) [3,8], N.B. In replacement a nomenclature as conventionally applied to acyclic structures with several heteroatoms, terminal heteroatoms are not designated with a prefixes but are named as characteristic groups of the structure, i.e., as hydroxy, amino, carboxylic acid, etc. However, heteroatoms in such positions within the CRUs of ladder or spiro polymer molecules are not terminal units and the stmctures are not acyclic. Consequently, such atoms are designated with a prefixes, and thereby the simplicity afforded by the application of replacement nomenclature to polymer molecules is enhanced. 

The energy of the homodesmotic reaction does not exclusively reflect the effect of cyclic (bond) delocalization. The reference structure is hypothetical and one cannot write the equation of a reaction, where a cyclic and an acyclic structure participate, for which the difference between the energies of products and reactants was determined by a single factor, namely, aromatic stabilization (antiaromatic destabilization) (75TCA121). 

Antiaromatic structure (214) has an energy higher by 18.7 kcal/mol relative to the bent acyclic structure (3 2 state) and by 42.7 kcal/mol compared to the linear structure N3 ( Sg4 state) (88JA7225). 

Connectivity or topoiogy Many molecules have the architecture of a single chain, where all the atoms are connected together in a linear manner, such as in the normal paraffins. Many more have branching from the backbone, such as the isoparaffins. The starch molecule is very highly branched, so that there is no identifiable backbone. These are also acyclic structures, so that if you cut any bond the molecule will part as two separate molecules. 

A newer and general procedure for synthesizing 2,5- and 2,2,5-substituted 2,3-dihydrothiadiazoles (121) involves the reaction of a ketone or aldehyde with thioaroylhydrazines (120) (Scheme 22). With benzaldehyde, the reaction proceeds simply on mixing the aldehyde with (120) in ethanol at room temperature after 15 minutes, a single product was isolated in 88% yield. Previous workers have assigned the acyclic structure (122) to these compounds, but based on NMR measurements, it... 

Studied by trapping the intermediates with silylating agents. The chlorination of 3,3-methylchloro-2-thietanone at — 20°C leads to the acyclic structure 210 from which the five-membered heterocycle 211 can be formed after treatment with hydrogen sulfide and cyclocondensation of the resulting thiole by elimination of HCl. Expulsion of CI2 leads to (212) (Eq. 52). 

When 176 is treated with hydrochloric acid it gives the intermediate acyclic structure 265, which decomposes to sulfonyl isothiocyanates, isothiocyanates, and chlorformamidines. Solvolysis of 155 with ethanol, followed by addition of hydrogen bromide, leads to 266. Base hydrolysis of 156 leads to the zwitterionic structure 267. ... 

Oxathietanes and oxathietes have been cited in the literature only as reactive transient intermediates and not as isolated relatively stable products. The cycloaddition of dimethyl sulfoxide to the acetylene derivative 277 yielded via the 1,2-oxathiete 278 the acyclic structure 279 ° (Eq. 76). 

Undoubtedly, the vicinal groups play a fundamental role in the outcome of this reaction, especially with the sugars, where the favored conformation of the molecule at equilibrium is controlled at the outset by groups that determine whether the molecule exists as a cyclic or acyclic structure. The deamination of cyclic and acyclic amino sugar derivatives by nitrous acid will be considered in turn. 

As with alkanes, the boiling points and melting points of alkenes decrease with increasing molecular weight, but show some variations that depend on the shape of the molecule. Alkenes with the same molecular formula are isomers of one another if the position and the stereochemistry of the double bond differ. For example, there are four different acyclic structures that can be drawn for butene (C4H8). They have different b.p. and m.p. as follows. 

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