Structure cyclic

The application of the above principles to simple cyclic structures is straightforward. The fi,Z notation should not be used to define configurations of cyclic compounds such as 1,2-cyclobutanediol. 

An arbitrary choice is made between methylene groups (2) and (3), giving (IRS,4RS) chirality to the trans form and (IRS,4SR) to the cis form. The result is independent of the arbitrary choice made. 

An alternative and more rigorous treatment considers C-1 and C-3 as centres of pseudoasymmetry and assigns them the appropriate symbols r and y (actually and see definitions below). (Note that according to this treatment, the cw-isomer is ls, 3s and the trans-isomer lr ,3r i.e., changing the configuration at one centre changes both descriptors.) For a full explanation, see Eliel and Wilen, p. 667. 

In the case of cyclic structures with several substituents (e.g., cyclitols), the (a,p)-convention may be clearer and unambiguous. See also cyclitols in Chapter 5. 

Organic Chemist s Desk Reference, Second Edition 

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The four kelohexoses are fructose, sorbose, allulose and tagatose. See glucose for example of isomerization between open chain and cyclic structures in a typical hexose molecule. 

The napthanes (C H2n), or cycloalkanes, are ring or cyclic saturated structures, such as cyclo-hexane (CgH 2) though rings of other sizes are also possible. An important series of cyclic structures is the arenes (or aromatics, so called because of their commonly fragrant odours), which contain carbon-carbon double bonds and are based on the benzene molecule. 

Cyclic structures Rings are described by breaking the ring between two atoms and then labeling the two atoms with the same number. 

Cyclic structures Ring dosures are described by a bond to a previously defined atom which is specified by a unique ID number. The ID is a positive integer placed in square brackets behind the atom. An " " indicates a ring closure. 

The neighborhoods of the atoms directly bonded to tbe chiral center must be defined. The neighborhood of an atom A. dircetly bonded to the ehiral eenter, is dc-fned as the set of atoms whose distance (in number of bonds) to A is less than their distance to any of the other three atoms bonded to the chiral center (Figure 8-9. In cyclic structures different neighborhoods can overlap. 

Aldonic acid (Section 25 19) Carboxylic acid obtained by oxi dation of the aldehyde function of an aldose Aldose (Section 25 1) Carbohydrate that contains an aldehyde carbonyl group in its open chain form Alicyclic (Section 2 15) Term describing an a/iphatic cyclic structural unit... 

Thus consideration of both reactions (5.EE) and (5.FF) forces us to examine the stability of cyclic structures as a function of ring size. 

Conversion of Aromatic Rings to Nonaromatic Cyclic Structures. On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically ate converted to o- and -quinoid stmctures and oxinane derivatives of quinols. Because of thein relatively high reactivity, these stmctures often appear as transient intermediates rather than as end products. Further reactions of the intermediates lead to the formation of catechol, hydroquinone, and mono- and dicarboxyhc acids. 

Microwave studies also provide Important Information regarding molecular force fields, particularly with reference to low frequency vibrational modes in cyclic structures (74PMH(6)53). 

Much work has been carried out in order to elucidate the molecular structure of poly(vinyl chloride). In 1939, Marvel, Sample and Roy dechlorinated PVC with zinc dust to give linked cyclic structures (Figure 12.9). 

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. 

Incorporation of stereogenic centers into cyclic structures produces special stereochemical circumstances. Except in the case of cyclopropane, the lowest-eneigy conformation of the tings is not planar. Most cyclohexane derivatives adopt a chair conformation. For example, the two conformers of cis-l,2-dimethylcyclohexane are both chiral. However, the two conformers are enantiomeric so the conformational change leads to racemization. Because the barrier to this conformational change is low (lOkcal/mol), the two enantiomers arc rapidly interconverted. 

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... 

The pattern of experimental results on charged species with cyclic conjugated systems is summarized in Table 9.1. It is consistent with the applicability of HiickeTs rule to charged, as well as neutral, conjugated planar cyclic structures. 

Acyclic C5. The C5 petroleum feed stream consists mainly of isoprene which is used to produce rubber. In a separate stream the linear C5 diolefin, piperylene (trans and cis), is isolated. Piperylene is the primary monomer in what are commonly termed simply C5 resins. Small amounts of other monomers such as isoprene and methyl-2-butene are also present. The latter serves as a chain terminator added to control molecular weight. Polymerization is cationic using Friedel-Crafts chemistry. Because most of the monomers are diolefins, residual backbone unsaturation is present, which can lead to some crosslinking and cyclization. Primarily, however, these are linear acyclic materials. Acyclic C5 resins are sometimes referred to as synthetic polyterpenes , because of their similar polarity. However, the cyclic structures within polyterpenes provide them with better solvency power and thus a broader range of compatibility than acyclic C5s. 

Alicyclic (Section 2.15) Term describing an a/iphatic cyclic structural unit. 

The nonpolar amino acids (Figure 4.3a) include all those with alkyl chain R groups (alanine, valine, leucine, and isoleucine), as well as proline (with its unusual cyclic structure), methionine (one of the two sulfur-containing amino acids), and two aromatic amino acids, phenylalanine and tryptophan. Tryptophan is sometimes considered a borderline member of this group because it can interact favorably with water via the N-H moiety of the indole ring. Proline, strictly speaking, is not an amino acid but rather an a-imino acid. 

In addition to unsaturated fatty acids, several other modified fatty acids are found in nature. Microorganisms, for example, often contain branched-chain fatty acids, such as tuberculostearic acid (Figure 8.2). When these fatty acids are incorporated in membranes, the methyl group constitutes a local structural perturbation in a manner similar to the double bonds in unsaturated fatty acids (see Chapter 9). Some bacteria also synthesize fatty acids containing cyclic structures such as cyclopropane, cyclopropene, and even cyclopentane rings. 

Nucleosides are compounds formed when a base is linked to a sugar via a gly-cosidic bond (Figure 11.10). Glycosidic bonds by definition involve the carbonyl carbon atom of the sugar, which in cyclic structures is joined to the ring... 

It has a cyclic structure and may be regarded as a substituted diamide of disulfuric acid, H2S2O7 (Fig. 15.49b). 

The second point concerns cyclic structures in molecules of high symmetry. Think of furan (Figure 10.6) and you might define the Z-matrix by starting... 

However, the cyclic structure of "hydrocotarnine acetic acid (20e) and the two condensation products of cotarnine with acetone (20f, 21) were even unambiguously determined by chemical methods alone. These compounds were not reduced either catalytically nor by sodium amalgam, although acetylhydrocotarnine acetic acid (22b) is easily reduced by both these methods. If even a small pro-... 

Emphasis in recent applications of the method has been placed on the synthesis of tetra- and penta-cyclic structures containing a di-hydro-j8-carboline system or its equivalent. Thus the tetracyclic system 100 was obtained from the amide (99) of tryptamine and hip-puric acid. ... 

Ring-chain tautomerism was studied for 1,3,4-thiadiazolines 296 and 1,3,4-thiadiazolidines 297 (Scheme 109) [78MI2, p. 159 84CHEC-I(6)545, p. 557 88KGS3 95ZOB705 96AHC(66)1, pp. 44, 46, 52]. In all these cases the equilibria are shifted to the cyclic structures. Quantitative aspects of these equilibria have been discussed elsewhere [96AHC(66)1, pp. 44 6]. 

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