Orientation anticlinic

In the pyranoses in the chair conformations, the vicinal hydroxyl groups can exist only in the anticlinal or gauche orientations. The gauche arrangement of a-glycols is normally encountered as diequatorial or axial-... 

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

While the existence of the anticlinic minimum in conventional SmC materials has been suggested by experiments with DOBAMBC,17 by far the majority of tilted smectics only exhibit the synclinic structure of the SmC or SmC under normal conditions. When the mesogenic compound is unichiral (i.e., enantiomerically pure) or enantiomerically enriched, a macroscopic polarization with the same orientation in each layer is produced, as discussed above. In the SmCA phase of MHPOBC, however, the anticlinic structure is the global minimum structure, as shown at the bottom of Figure 8.9. In this case, given that the chirality of the molecules is fixed, the polarization must alternate... 

Figure 8.9 Synclinic and anticlinic layer interface configurations, differing in relative azimuthal angle <(> between director orientations for pair of adjacent layers, are illustrated. It appears that many, if not all, SmC and SmC materials possess wells for both synclinic and anticlinic configurations. Materials for which antichnic well is global minimum for system in some temperature...

Anticlined isomorphous chains the two chains have parallel axes and opposite orientation of the pendant methyl groups. 

A counter-clockwise rotation would bring the CF bond synclinal to the sp2 lone pair. The synclinal overlap is weaker than the anticlinal,28 because of the negative overlap between sp2 and the fluorine AO in o CF. We have used this argument in our treatment of the Walden inversion (p. 179). Pushing the analogy a little further, we can equate the sp2-o CF interaction to an SN2 attack of the sp2 orbital on the CF bond. This attack must proceed with inversion (because the reaction occurs at a carbon center), so the sp2 orbital will be best oriented antiparallel to CF. 

There are emerging examples in which dppf functions as a unidentate ligand. This coordination mode has been postulated as the key intermediate in some catalytic processes (see Sect. 1.5.2.1). Diphosphines acting as a unidentate ligand were once thought to be unstable based on entropy arguments. In this mode of coordination, the expected antiperiplanar orientation of the pendant phosphine is rarely observed. The approximate anticlinal (eclipsed) conformation of most known unidentate complexes (t 122 — 123°) [42, 47, 51, 53] possibly reflects the higher influence of the... 

The Angel Field is located at the northern end of the NE-SW-oriented Madeleine Trend in the Dampier sub-basin, which forms part of the Silurian to Holocene Carnarvon basin, offshore Western Australia (Fig IB). The Madeleine Trend is a major fault-controlled anticlinal feature that follows the main depositional axis of the Dampier subbasin and along which several other oilfields are located, including the nearby Cossack and Wanaea Fields (Fig. IB). The stratigraphy of the Dampier sub-basin consists of marine and deltaic elastics and Tertiary carbonates (Fig. 3). 

Two chains of the same chirality and conformation are isomorphous with respect to each other. Thus, two isotactic poly(propylene) helices, each with the same conformational sequence. . . TG TG TG. . . are isomorphous. Two such chains are also isoclinal if the bond vectors in each chain always have the same positive or negative orientation. With anticlinal chains, however, the bond vectors of one chain have the reverse orientation to those of the other chain (Figure 5-11). Thus, although the chains of the polyamides 6 and 6,6 are parallel to each other, the former is anticlinal and the latter is isoclinal (Figure 5-10). 

The term is the result of the electrostatic free energy of the flexoelectrically induced part of the polarization. It can clearly be seen that this interaction originates in the flexoelectric effect and is always positive. The next nearest layers therefore favour an anticlinic orientation. The basic period of the structure, if this is the only interaction, would consist of four layers (Fig. 5.6, second row). In addition, if the flexoelectrically induced polarization is comparable to the piezoelectrically induced polarization, the indirect interaction with the next nearest layers can be surprisingly strong. 

This conformational profile can be modified by introduction of additional o-acceptors. For example, although the main conformer of 1,1-dimethoxyethene also has both methoxy groups syn-periplanar to the C=C bond, there is a significant population (38%) of a conformer where one methoxy group is oriented syn-periplanar and the other one anticlinal. In the latter conformer, the lone pair of the latter OMe group enjoys anomeric interaction with the a C-0 bond of the first substituent (Figure 6.119). 

The structure analysis of the piperidine derivatives and its drugs in the protein showed that the -substitutions are invariably positioned in the anticlinal orientation irrespective of the different substitutions in the rings and the non-covalent interactions. The bulky groups are in a equatorial orientation in the chair conformation (piperidine ring) and assumes opposite orientations in the boat and twist-boat conformations. Since the number of drug molecules analyzed here are very small, no significant conclusion could be reached concerning the confermers. 

The dipole pairing model is based on the pair formation of molecules in adjacent layers through the dipole-dipole interaction. Because of the experimental observation (2), pairing must be made between like enantiomers, as shown in Figure 9.14 [32]. Otherwise, anticlinic orientation cannot be formed in the racemic compoimds. Therefore, in this model, chiral molecular recognition is required. The pairing may be dynamic and may occur in optically resolved local enantiomeric domains. 

Another example of crystal lattice affected by disorder due to isomorphic substitution of different chains is provided by the a form of iPP (Fig. 2.15) [29,101,102]. As shown in Figure 2.25a, in the crystal the threefold helical chains of iPP of a given chirality may be up or down depending on the orientation of the bond vector b that connects the backbone methine carbon atoms to the lateral methyl carbon atoms, with respect to the c-axis. Isomorphous up and down (anticlined) [16] chains have a similar external steric hindrance because the methyl groups may assume the same position (Fig. 2.25b). Therefore, disorder in the positioning of up and down chains may be present in the crystals of the a form because if isomorphic and anticlined helices of iPP substitute each other in the same site of the lattice, the same steric interactions with neighboring chains are involved. 

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