Two-membered ring system

An alternative and more likely interpretation would be that the electron pairs of the double bond are equivalent and both participate in the transition. This would correspond to a charge migration around a two-membered ring system, the transition being forbidden , as in the case of the B-bands, only by the comparatively small transition moment. These qualitatively similar interpretations account for the small intensities and low transition energies characteristic of the... 

It is possible to view this reaction as involving a ring contraction, because the three-membered ring has formed an allylic cation, i.e. the double bond can be considered as a two-membered ring system, which is then attacked by a solvent molecule. This is an example of a cyclopropyl/allylic cation rearrangement, which we first encountered in the chapter on nucleophilic substitution reactions. 

For the three- and four-membered rings, A 0 values are more negative than for the corresponding two-membered ring systems ... 

Dipolarophiles utilized in these cycloadditions leading to five-membered heterocycles contain either double or triple bonds between two carbon atoms, a carbon atom and a heteroatom, or two heteroatoms. These are shown in Scheme 9 listed in approximate order of decreasing activity from left to right. Small rings containing a double bond (either C=C or C=N) are also effective dipolarophiles, but these result in six- and seven-membered ring systems. 

Several five-membered ring systems readily available by 1,3-dipolar cycloadditions are shown in Scheme 10. The dotted line indicates how the system was constructed, the line bisecting the two new bonds being formed in the cycloaddition. The majority of chapters in these volumes make some reference to 1,3-dipolar cycloadditions. 

Reaction type 3 (equation 10), where the complete hetero-l,3-diene skeleton is incorporated into the newly formed ring system, occurs with compounds having both a nucleophilic center and an electrophilic center If these two functionalities are in positions 1 and 2, various types of six-membered ring systems become accessible 4,4-Bis(trifluoromethyl)-I,3-diaza-1,3-butadienes require only room temperature to react with acetyl cyanide to yield l,4,5,6-tetrahydropynmidin-6-ones [96] Likewise, certain open-chain 1,3-diketones (acetylacetone and acetoacetates) and the heterodiene form six-membered nng systems [97] (equation 19)... 

Treatment of hexafluoroacetone with a P(lll) species results either in formation of five membered ring systems via reductive CC coupling of two molecules of hexafluoroacetone [275, 276 277, 278, 279 280, 281] (equation 60) or in reductive fluoride elimination [282] (equation 61)... 

Perhaps the most notable difference between S-N and N-O compounds is the existence of a wide range of cyclic compounds for the former. As indicated by the examples illustrated below, these range from four- to ten-membered ring systems and include cations and anions as well as neutral systems (1.14-1.18) (Sections 5.2-5.4). Interestingly, the most stable systems conform to the well known Htickel (4n -1- 2) r-electron rule. By using a simple electron-counting procedure (each S atom contributes two electrons and each N atom provides one electron to the r-system in these planar rings) it can be seen that stable entities include species with n = 1, 2 and 3. 

To account for the course of this reaction theoretical calculations of the coordination of ketomalonate 37 to copper(II) and zinc(II) have revealed that the six-membered ring system is slightly more stable than the five-membered ring system (Scheme 4.30). The coordination of 37 to catalyst (l )-39 shows that the six-membered intermediate is C2-symmetric with no obvious face-shielding of the carbonyl functionality (top), while for the five-membered intermediate (bottom) the carbonyl is shielded by the phenyl substituent. Calculations of the transition-state energy for the reaction of the two intermediates with 1,3-cyclohexadiene leads to the lowest energy for the five-membered intermediate this approach is in agreement with the experimental results [45]. 

Other common five-membered heterocyclic amines include imidazole and thiazole. Imidazole, a constituent of the amino acid histidine, has two nitrogens, only one of which is basic. Thiazole, the five-membered ring system on which the structure of thiamin (vitamin Bt) is based, also contains a basic nitrogen that is alkylated in thiamin to form a quaternary ammonium ion. 

This section treats seven-membered ring systems with maximum unsaturation containing within the ring two or more oxygen, sulfur or nitrogen atoms, or a combination of these.1... 

The only heterocyclic seven-membered ring system with maximum unsaturation containing six heteroatoms is 1,3A4,5,2,4,6-trithiatriazepine (1). The methoxycarbonyl derivative 2 is a minor product (14%) of the complex reaction of tetrasulfur tetranitride with dimethyl acetylenedicarboxylatc in refluxing toluene, which gives mainly dimethyl l,2,5-thiadiazole-3,4-dicarboxylate (3, 67%) (see Houben-Wcyl, Vol. E8d, pl54ff which includes an experimental procedure). Two other products are the trithiadiazepine 4 (5%, see Section 4.4.1.1.1.) and the 1,2,4-thiadiazole derivative 5 (3%).385... 

A more deviating stoichiometry is found in the case of the inclusion compound of / with /-propanol 77 >. Here the assistance of two independent host molecules is required and results in a 2 1 stoichiometry. Nevertheless, even this unusual host guest ratio gives rise to a similar H-bond pattern (Fig. 18a and type lib in Fig. 19) as found for the inclusions of 1 with simpler alcohols (cf. Fig. 17a), namely the 12-membered ring system. Now, another interesting fact arises, signalling the flexibility of host 1 in its inclusion behavior. This is the formation of host dimers through H-bonds to ensure clathration. 

An example, where two C-C-bonds are formed and one C-C-bond is broken is the synthesis of the tricycle 3-285, which has some similarity with the eudesmane framework 3-286, developed by Kilburn and coworkers (Scheme 3.72) [113]. Thus, exposure of the easily accessible methylenecyclopropyl-cyclohexanone 3-281 to samarium(II) iodide led to the generation of ketyl radical 3-282, which builds up a six-membered ring system with simultaneous opening of the cyclopropane moiety. Subsequent capture of the formed radical 3-283 by the adjacent alkyne group afforded the tricycle 3-285 via 3-284 as a single diastereoisomer in up to 60% yield. It should be noted that in this case the usual necessary addition of HMPA could be omitted. 

From the method of preparation of the analogs of L-ascorbic acid and from their close resemblance to L-ascorbic acid in respect to chemical properties and absorption data, all these derivatives were assumed by analogy to have the characteristic unsaturated five-membered ring system already proved to be present in L-ascorbic acid. In two cases, namely that of D-gluco- (XV) and D-araboascorbic acid (XXXIX) the assumption has proved correct. 

While SN4 and STD exhibit essentially D2d symmetry, SN3 has Cs symmetry with a planar diene ring and an envelope-shaped cyclopentene ring. The maximum torsion in the folded ring of SN3 is 20.2°. The spiro-connection of two five-membered ring systems leads to some strain at the spiroatom (101.4° to 101.8° at A1 compared to 109.5° for tetrahedral... 

All polycyclic pigments, with the exception of triphenylmethyl derivatives, comprise anellated aromatic and/or heteroaromatic moieties. In commercial pigments, these may range from systems such as diketopyrrolo-pyrrol derivatives, which feature two five-membered heteroaromatic fused rings (DPP pigments) to such eight-membered ring systems as flavanthrone or pyranthrone. The phthalo-cyanine skeleton with its polycylic metal complex is somewhat unique in this respect. 

In pteridines, we have a pyrimidine ring fused to a pyrazine ring. There are, of course, a number of possible ways of combining these two six-membered ring systems pteridines are pyrazino[2,3-fi(]pyrimidines (see Section 11.9). 

The chemical shifts obtained in solution under different experimental conditions were compared to the shifts of the TMEDA complexes in the solid state. For 6, all data indicate a triplet structure of the CIP. The most compelling evidence for the triplet structure is the polarization of charge towards the two bridging carbons under CIP conditions. However, for 7, the charge distribution differs under CIP conditions in solution and in the solid state, as reflected by the chemical shifts. In solution, the shift data indicate again a triplet structure. In the solid state, however, the cations are not located at the bridging carbons, but shifted towards the five-membered ring systems. 

A molecular structure, similar to that of hexasilylated benzene derivative 229, was obtained from tetracyclic hexasilylbenzene 230 by Kira, Sakurai and coworkers, where the six silicon centres are incorporated in three five-membered ring systems (Scheme 80) °. In dimetalated compound 231, two lithium centres, coordinated by a quinuclidine ligand each, are capping the phenyl ring plane from both sides in the solid state. Moreover, it could be found that compound 231 has a thermally accessible triplet state, investigated by temperature-dependent ESR spectroscopy °°. 

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