Strain side groups, effect

Side differentiating effects like allylic 1,3-strain [45] are also observed leading to exo/endo isomers. This effect is particularly expressed in the reaction of compound 42. In this case, a methoxy group is involved in the corresponding steric interaction (compare transition state Q). A lower side differentiation is observed in the reaction of compounds 52 and 58. In the latter case, the allylic 1,3-strain effect is located on the olefinic side chain and the only one of the hydrogen atoms at the methylene carbon is involved in the corresponding interaction (transition state R). The 1,3-allylic strain effect was particularly important in the application of the reaction to natural product synthesis (see below) since it diminish the number of isomers. 

Conventional low molar mass LCs as well as linear LC-polymers can be macroscopically ordered by external electric or magnetic fields, which is widely applied in optoelectronics in the case of low molar mass LCs. For LC-elastomers it is very important to know whether a macroscopic mechanical deformation of the polymer network influences the liquid crystalline side groups and whether a mechanical stress or strain produces similar effects as observed for conventional LCs by external fields. 

The linear optical properties (UV-visible and Raman) of PDA crystals have been thoroughly characterized and are reasonably well-understood. The lowest energy optical transition is typically located at about 2.0 eV and is excitonic in origin. Distortion of the backbone due to deliberately induced disorder or strain caused by side group interactions shifts this transition to higher energies. Crystal strain (e.g. polymer chains in the monomer lattice) can shift the transition either way. More work/ theory and experiment, is needed to sort out and understand these effects. 

Li et al. (2007) also described a type of supramolecular shape memory elastomer that contains reversibly associating side groups, namely ureidopyridinone (UPy). In addition to its excellent shape memory effects (SMEs) - about 90% strain fixity and 100% strain recovery - a unique feature of this new SMP is the dynamies of its shape-memory response. More recently, Zhu et al. (2009) synthesized a novel supramolecular polyurethane for utiUzation as a SMP, by grafting the UPy nnit to the elastic polyurethane. Supramolecular SMPs based on o-cyclodextrin (a-CD) network through the inclusion of either PEG or polycaprolactone (PCL) with a-CD. In this system, the a-CD-PEG inclusion crystalhtes and a-CD-PCL crystallites serve as a fixing phase, while the naked PEG crystallites and PCL crystallites act as a reversible phase (Luo et al., 2008 Zhang et al., 2008). Fan... 

A similar steric effect was seen in Section 3 11 where van der Waals strain be tween methyl groups on the same side of the ring made as 1 2 dimethylcyclopropane less stable than its trans stereoisomer... 

While screening for p-lactam antibiotics stable to p-lactamases, a strain of Streptomyces lactamdurans was found to contain several such agents which have a 6-a-methoxy group whose electronic and steric properties protect the antibiotic from enzymatic attack. Cephamycin C (29a), one of these substances, is not of commercial value, but side chain exchange has led to much more potent materials. Of the various ways of effecting this transformation, one of the more direct is to react cephamycin C with nitrous acid so that the aliphatic diazo product (29b) decomposes by secondary amide participation giving cyclic iminoether 30. The imino ether moiety solvolyzes more readily than the p-lactam to produce 7-aminocephamycinic... 

The method used by the group was first to determine the direction of substrate tilt by means of reciprocal space maps in each of the <110> directions contained in the plane of the wafer. These are shown in Figure 7.14. A narrow slit was used instead of an analyser ciystal, so some analyser streaks are seen near the substrate peaks (on the right of each map— the origin in this case is on the left-hand sides). It is seen that in (a) and (b) the beam is oriented perpendicular to the substrate tilt axis, as these maps show only strain. In (c) and (d) the effect of the grading can be seen, since both the tilt and the strain are changing, but these views are insufficient to make a complete analysis. For this we need an... 

The structure of the cyclic ketone is of utmost importance. Reduction of cyclic ketone by complex hydrides is started by a nucleophilic attack at the carbonyl function by a complex hydride anion. The approach of the nucleophile takes place from the less crowded side of the molecule (steric approach or steric strain control) leading usually to the less stable alcohol. In ketones with no steric hindrance (no substituents flanking the carbonyl group or bound in position 3 of the ring) usually the more stable (equatorial) hydroxyl is generated (product development or product stability control) [850, 851, 852, 555]. The contribution of the latter effect to the stereochemical outcome of... 

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