Solid-State Structures and Conformations

A combined x-ray and neutron diffraction studies showed that 5-0-acetyl-l,2 3,4-di-O-isopropylidene-a-D-g/MCO-septanose (99) adopted a twist-chair conformation 0 [43]. Torsional angles are evaluated to identify the ring conformation. Thus, torsion angles of -31.1° for 06—Cl—C2—C3 and -46.0° for Cl—C2—C3—C4 of 

FIGURE 13.3 Molecular and solid-state structures of septanoside 98. 

FIGURE 134 Twist-chair ( TCj 4) conformation of septanosides 100-102 in the solid state. 

The twist-chair conformation was also seen intact when thiol substitution was intfoduced at anomeric center with 2,3,4,5-di-O-isopropylidene protecting group, namely, ethyl 2,3 4,5-di-0-isopropylidene-l-thio-p-D-glucoseptanoside 102 [49]. Positive signs and closer torsion angles of 53.5 for C6-06-C1-C2 and 35.7 for 06-C1-C2-C3 led to assign the conformational descriptor 

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Diffraction data give information about solid-state structures and conformations, which may be significantly different from solution conformations. On occasion, diffraction studies are used to support one possible structure from two or more possibilities. 

Both the measured dipole moments and H NMR spectra of 1,2-dithiane, its 4,4,5,5-tetradeutero analog, 3,3,6,6-tetramethyl-l,2-dithiane, and the m//ra r-isomers of 3,6-dimethyl-l,2-dithiane provided unequivocal evidence for the chair conformation adopted by the saturated six-membered ring, supported further by X-ray solid-state structures. Other conformers, for example the twist conformer, were not detected . 

Achiral objects can be assembled into chiral solid-state structures, and this is frequently the case for urea 1 when it encloses guests. Other compounds adopt a chiral conformation in solution and therefore may ultimately produce either chiral or achiral host structures. On the other hand, thiourea 2 forms an inclusion lattice that is achiral. This arrangement is nonetheless very effective in enclosing guest molecules. 

The similar observations of the sodium hydroxide concentration dependencies of the peak positions, relative peak intensities, and line-widths of schizophyllan revealed that, like lentinan, its molecule adopted single-helical conformation at lower NaOH concentrations (<0.13 N), while the transition to the random-coil state took place at 0.19 N. Grifolan produced soft gel at neutral pH whose NMR spectrum was similar to those of lentinan and schizophyllan [42]. However, different preparations of grifolan possessed two kinds of solid-state structure and formed different gel types that were described as a helix type (similar to that of lentinan) and a native type that had different structure. Both structures were investigated by NMR spectroscopy, the accent being made on the differences in gel-to-sol transitions induced by addition of alkali, DMSO or urea [86]. 

The relative stereostructure of 9-acetyl-7-hydroxy-l,2-dimethyl-7-meth-oxycarbonyl-4-phenyl-6-oxo-l, 4,7,8-tetrahydro-6/7-pyrido[l, 2-u]pyri-midine-3-carboxylate 122 was justified by an X-ray diffraction analysis (97JOC3109). The stereochemistry and solid state structure of racemic trans-6,9-//-l, 6-dimethyl-9 z-ethoxy-9-hydroxy-4-oxo-l,6,7,8,9,9 z-hexahydro-4//-pyrido[l,2- z]pyrimidine-3-carboxylate (123), adopting a cw-fused conformation, were determined by X-ray investigations (97H(45)2175). 

The ESR spectra of a large variety of sulfonyl radicals have been obtained photolytically in liquid phase over a wide range of temperature. Some selected data are summarized in Table 2. The magnitudes of hyperfine splittings and the observations of line broadening resulting from restricted rotation about the C—S bond have been used successfully in conjunction with INDO SCF MO calculations to elucidate both structure and conformational properties. Thus the spin distribution in these species is typical of (T-radicals with a pyramidal center at sulfur and in accord with the solid-state ESR data. 

The solid state structure of 33 reveals a dimeric molecule with an eight-membered B2C2N2O2 heterocyclic ring in a chair conformation. The N—B and B—O bond lengths of 1.585(2) and 1.541(2) A, respectively, as well as the C—N and C—O bond lengths of 1.294(3) and 1.285(3) A, respectively, indicate delocalization of the Ti-electron density in the acylamino moiety [65]. 

Structures 5 and 6 display the solid state structures of two representative distibine and dibismuthine adducts. The ligands bound to the central Sb and Bi atoms adopt a staggered conformation in relation to one another, with the bulky M(t-Bu)3 groups arranged in a trans-position. This is likely due to repulsive steric interactions. The central Sb—Sb [281.1(1) 32 283.9(1) pm 35] and Bi—Bi bond distances [298.3(1) 36 and 298.4(1) pm 37] are almost unchanged compared to the uncomplexed distibines and dibismuthines, as can be seen... 

In the last 20 years a great deal of effort has been focused towards the immobilization of chiral catalysts [2] and disparate results have been obtained. In order to ensure the retention of the valuable chiral hgand, the most commonly used immobihzation method has been the creation of a covalent bond between the ligand and the support, which is usually a solid, hi many cases this strategy requires additional functionalization of the chiral hgand, and this change - together with the presence of the very bulky support - may produce unpredictable effects on the conformational preferences of the catalytic complex. This in turn affects the transition-state structures and thus the enantioselectivity of the process. 

We also performed a single-crystal X-ray structure analysis of this lead compound. The solid state structure of this compound depicted in Fig. 3-15 shows a half-boat-like ( sofa ) conformation with the 9-phenanthryl group in a quasi-axial or r/Mf/.v/-flagpole position, and the a, 3-unsaturated exocyclic ester in a s-cis conformation. This cleft-like conformation is advantageous for the creation of centers with a high recognition ability, since one enantiomer fits in better than the other thus leading to selectivity. 

The solid-state structure of 68c, Figure 36, shows that the zinc atom is surrounded in a trigonal-planar fashion by one methyl group and two 2,4,6-trisilacyclohexyl ligands, of which one has a chair conformation, while the other one has a boat conformation. 

By contrast, the solid-state structures of several isolated Te62+ salts49,50 have been determined and all were shown to possess a Te6 homocycle in the boat conformation (Figure 9). 

At the end of the process, one obtains a large amount of information regarding the solid-state structure of a compound. Its three-dimensional structure and molecular conformation become known, as do the patterns of molecular packings that enable the assembly of the crystal. In addition, one obtains complete summaries of bond angles and bond lengths for the molecules in the crystal as well as detailed atomic coordinates for all of the atoms present in the solid. One generally finds... 

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