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Rings seven-membered, conformations

The NMR spectra of heterocyclic compounds with seven or more ring members are as diverse as the shape, size and degree of unsaturation of the compounds. NMR is perhaps the most important physical method to ascertain the structure, especially the conformational statics and dynamics, of large heterocycles. Proton-proton coupling constants provide a wealth of data on the shape of the molecules, while chemical shift data, heteroatom-proton coupling constants and heteronuclear spectra give information of the electronic structure. Details are found in Chapters 5.16-5.22. Some data on seven-membered rings are included in Table 10. [Pg.16]

An X-ray crystallographic analysis of perfluoro-2,7 -diazaheptafulvene (22) revealed that both seven-membered rings are in the boat conformation.60... [Pg.112]

Molecular orbital based molecular mechanical (MOMM) calculations have been carried out on 3H-2-benzazepinyl phosphonates.43 Similar calculations on 5//-dibenz[Z>,/]azepine lead to the prediction that there are two stable boat conformations for the seven-membered ring both of which are more stable than the planar form by 22.6 and 10.9 kJ moP, respectively the more stable conformer being the one in which the proton on nitrogen is exo lo the azepine... [Pg.210]

Benzopentathiepins are stable solids. Varacin, a cytotoxic metabolite, was isolated from a marine ascidian.398 The parent benzopentathiepin exhibits infrared absorptions at 1620, 1570, 1235 and 1020 cm" 1.399 X-ray structure analyses of benzopentathiepin and 6-methylbenzopen-talhicpin show that the seven-membered ring in both compounds is nonplanar and that it adopts a chair conformation. The S-S bond lengths are in the range 204-206 pm.400... [Pg.491]

P2j Z = 2 Dx = 1.33 R = 0.09 for 792 intensities. The molecule is tricyclic, with a five-membered and a seven-membered ring fused to the ribofuranose ring. Both five-membered rings have an envelope conformation. The seven-membered ring has the same conformation as is found in 2,5-arabinosylcytidine monophosphate.79 ... [Pg.235]

Three new spirans, including 131 and 132, incorporating a 1,5-benzodithiepin system have been synthesised from methyl benzynes. The structure of 131 was confirmed by X-ray crystallography, and the seven-membered ring was shown to have a chair conformation and the five-membered ring an envelope conformation . [Pg.368]

Five-membered carbocycles are the most easily formed [45, 107, 196, 200, 202,203]. Five-membered carbocyclic rings can be formed (with 2% MoF6 as the catalyst) even when the double bond is tetrasubstituted (Eq. 24) [200]. The stability of the catalyst toward the free OH group in this case is noteworthy. Evidently this particular t-butoxide-like alcohol does not react with this particular catalyst for steric reasons. Six-membered carbocyclic rings are also formed readily (Eq. 25) [200], as are seven-membered rings, especially if one takes advantage of a Thorpe-Ingold effect (e.g., Eq. 26) [20] or a similar conformational predisposition for the double bonds to remain near one another. [Pg.33]

Another example is the temperature-dependent study of the 31P-NMR spectra of the hydrogenation catalyst shown in Figure 11.11 [28]. At 240 K the 31P-NMR signals are equivalent, whereas at 123 K two sets of signals are observed which are attributed to a seven-membered ring in a twist-chair and a distorted boat conformation. [Pg.308]

Electrochemistry. The cyclic voltammogram of Compound 87a was measured and is compared to octa. S -benzyl porphyrazine, Ni[pz(A4)] A = difS -benzyl), 60a (from Section IV.B.l) in Table XXIV. Compound 87a has two reversible ring reductions, which are more positive than those measured for H2 pc and more negative than those measured for Ni[pz(.V-benzyl)8], 60a, suggesting that the conformational influences of the peripheral seven-membered ring make this pz harder to reduce than the pz with unconstrained peripheral thioethers. Because these compounds are of limited solubility and cannot be oxidized or reduced readily, they appear to be unsuitable for use as building blocks for molecular conductors. [Pg.527]

P2t Z = 2 Dx = 1.281 R = 0.06 for 1,338 intensities. The molecule contains a seven-membered, 1,3-dioxepane ring which has a conformation close to the twist-chair, with a two-fold axis through the mid-... [Pg.462]

While all strain effects in monoamines are basicity weakening, it is possible to find cases in di- and polyamines where strain is relieved upon protonation, leading to increased basicity. This phenomenon is observed in 1,4-diaminobutane derivatives where an almost linear N... H—(N+) hydrogen bond in the mono-protonated derivatives leads to a stable, seven-membered ring structure. Thus, for example, the measured PA of l,6-diazabicyclo[4.4.4]tetradecane (73) is 228.3 kcalmol-1, about 11 kcalmol-1 higher than its monoamine analog 75, despite the similar, inwardly pyramidalized, nitrogen conformation of both neutral amines. [Pg.68]

Axially chiral phosphoric acid 3 was chosen as a potential catalyst due to its unique characteristics (Fig. 2). (1) The phosphorus atom and its optically active ligand form a seven-membered ring which prevents free rotation around the P-0 bond and therefore fixes the conformation of Brpnsted acid 3. This structural feature cannot be found in analogous carboxylic or sulfonic acids. (2) Phosphate 3 with the appropriate acid ity should activate potential substrates via protonation and hence increase their electrophilicity. Subsequent attack of a nucleophile and related processes could result in the formation of enantioenriched products via steren-chemical communication between the cationic protonated substrate and the chiral phosphate anion. (3) Since the phosphoryl oxygen atom of Brpnsted acid 3 provides an additional Lewis basic site, chiral BINOL phosphate 3 might act as bifunctional catalyst. [Pg.399]


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See also in sourсe #XX -- [ Pg.40 ]




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Rings 3- membered, conformation

Rings conformations

Seven-membered

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