Chair confirmation

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

An X-ray analysis of salts of lupanine, (8) HC1 2H20 and (8) HC104 H20, showed that ring c was in a boat conformation (cf. Vol. 10, p. 68) infrared studies of the salts and of the anhydrous hydrochloride indicate that this conformation is retained in solution, but that ring c is changed to a chair confirmation in anhydrous lupanine perchlorate. Conformations of mono- and di-protonated salts of 17/5-methyl-lupanine, cr-isolupanine, and 17/ -methyl-cr-isolupanine were also discussed on the basis of i.r. data.11... 

Dioxolane also pseudorotates essentially freely in the vapor phase. 2,2 -Bi-l,3-dioxolane (128) has been shown by X-ray crystallography to have a conformation midway between the half-chair and envelope forms. The related compound 2-oxo-l 3-dioxolane (129) shows a half-chair conformation. This result is confirmed by microwave spectroscopy and by NMR data. Analysis of the AA BB NMR spectra of the ring hydrogen atoms in some 1,3-dioxolane lerivatives is in agreement with a puckered ring. Some 2-alkoxy-l,3-dioxolanes (130) display anti and gauche forms about the exocyclic C(2)—O bond. 

For cyclooctane, a total of 11 conformations have been suggested for consideration and their relative energies calculated. The boat-chair was calculated to be the most stable conformation. This prediction was confirmed by analysis of the temperature dependence of the F-NMR spectra of fluorocyclooctanes. The activation energy for interconversion of conformers is 5-8 kcal/mol. A few of the most stable conformations are shown below. 

P212121 Z = 4 D = 1.25 R = 0.069 number of intensities not reported. This analysis was to confirm a configuration derived from n.m.r. data. The pyranoid rings are slightly distorted 4Q, due to fusion to the trioxacyclooctane ring, which has an almost ideal, boat-chair conformation. The overall molecule has a convex, sickle conformation. There is an error in the atomic coordinates reported, which do not correspond to the structure given in the paper. 

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 . 

The related tertiary phosphines PMe3 and PMe2Ph may also be met-alated by BunLi(tmeda), giving phosphinomethanide complexes that are shown by cryoscopy to be monomeric in benzene solution, but that crystallography confirms are dimeric in the solid state (126). Both complexes crystallize as dimers, [Li(CH2PMeR)(tmeda)]2 [R = Me (40), Ph (41)], with a similar structure to that of 39, containing a (CPLi)2 core. However, whereas 40 adopts a chair conformation, 41 is closer to a boat conformation with the phenyl groups in equatorial... 

The conformational behaviour in solution of a dermatan-derived tetra-saccharide has been explored by means of NMR spectroscopy, especially by NOE-based conformational analysis. RDCs were also measured for the tetrasaccharide in a phage solution and interpreted in combination with restrained MD simulations. The RDC-derived data substantially confirmed the validity of the conformer distribution resulting from the NOE-derived simulations, but allowed an improved definition of the conformational behaviour of the oligosaccharides in solution, which show a moderate flexibility at the central glycosidic linkage. Differences in the shapes of the different species with the IdoA in skew and in chair conformations and in the distribution of the sulphate groups were also highlighted.28... 

Of all these (c) is much less likely to be formed because it involves twisting of the allyl system while (a) and (b) are strain free. Again of these (a) is more favoured because the six p lobes are in a quasi planar arrangement. Moreover the molecular orbital calculations made by Dewar have shown that the central p lobes of the two allyl systems in the boat form appear to have a slight destabilizing effect and this has also been confirmed by Woodward and Hoffmann with the aid of correlation diagrams. But both the chair and boat forms have been found in practice. 

As a result of these unfavourable interactions, i.e., opposition to bond strain i.e., between the pair of hydrogens shown at the bottom) and also due to bowsprit interaction, the potential energy of the boat form becomes high and this is why the boat conformation is not the preferred one. Hassel in 1947 established by means of electron diffraction studies that cyclohexane exists predominantly in the chair form. This has also been confirmed by electron diffraction studies and results obtained from Raman and I.R. spectra. Calculations made on the basis of entropy show that only about one molecule in a thousand will be in the boat form. 

X-ray analysis of the 1,6-diol derivative (15) of the bicydo[4.4.1 ]-undecane ring system revealed the often calculated (9a,81) but rarely observed twist-chair cycloheptane ring. MM calculations confirmed that the observed conformation of 15 is the global minimum conformation (119). 

The concept of rocking-chair lithium batteries was confirmed experimentally by using lithiated oxides (LieFezOs, LiW02) as interaction anodes and other oxides (WO3, TiS2, V2O5) as cathodes in nonaqueous... 

Several 8a-substituted cw-octahydroquinazolin-2(l//)-ones 346C clearly prefer the N l)-in chair-chair conformation [71JCS(C)1812], as confirmed by the small values of the two /H.4,H-4a coupling constants (2-4 Hz). Similar small values were found for both yH-4,H-3 couplings of 4a-methyl-ds-octahydroquinazolin-2(l//)-one. This can be explained by either an N 1)-in or an N(2)-out conformation [N(3)-H equatorial], but the latter (347C) appears to be more probable because the 1,3-diaxial Me,NH interactions are less severe than Me,CH2 interactions (Table IV). 

Perhaps one of the most elegant object lessons demonstrating the importance of careful compositional analysis comes from the work by Beinert et al. (1983) on the 3Fe cluster. Even with structural analysis by numerous sophisticated spectroscopic methods and by X-ray diffraction, there was substantial controversy about whether the cluster was a modified cubane structure with four inorganic sulfurs and three ligands or a more open chair structure with three sulfurs and six ligands. The analysis of aconitase indicated four sulfurs and resolved the question in favor of the modified cubane. Further X-ray diffraction studies confirmed the cubane 3Fe 4S cluster for aconitase and ferredoxins from A. vinelandii and Desulfovibrio pgas (Stout et al., 1988 Stout 1988 Kissinger et al., 1989). 

The deformed-chair conformation of ring A in a 4,4-dimethyl-3-oxo-5or-steroid has been confirmed by an X-ray study of a 17 -benzoyloxyandrostane derivative. The results agree with those of an earlier study of the 17-iodoacetate, and with the geometry indicated by force-field calculations. Dipole moments calculated by the application of molecular mechanics to 5a-androstane-3,17-dione and its distorted 4,4-dimethyl derivative are larger than those observed the reasons for these deviations are not yet clear. 

Functionalization of the methyl groups in the photolysis of the nitrites of 6-hydroxy-4,4-dimethyl steroids is used as a probe for conformational analysis. It is confirmed that in the 4,4-dimethyl-19-nor-series ring A has the normal chair... 

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