S- to C-shaped Isomerization

2 S- to C-shaped Isomerization of Methylene-bridged Glycoluril Dimers 

The selective heterodimerization of glycoluril NH-compounds and glycoluril cyclic ethers (Section 4.4.1.1) and the general preference for the C-shaped diastereomers suggested that the mechanism of S-shaped to C-shaped equilibrium might proceed by an intramolecular rather than an intermolecular process under anhydrous acidic 

4 Implications for the Synthesis of Cucurbit[n]uril Analogs and Derivatives 

Building-Block Approach to Cucurbit [njuril Analogs 

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Polycondensations of monomers 19a. 21a and 22a with the tetrafimctional "star-centers" 24, 25 or 26 yielded star-shaped and hyperbranched poly-(ester-amide)s (15). As indicated by viscosity measurements variation of the monomer/star-center ratio allowed a control of the DP. The aliphatic protons of the "star-centers" also allow a determination of the DP by means of 1H-NMR spectroscopy (Figures 14 and 15). In the case of polyesters derived from the piperazine derivative 20 the NMR spectra need to be conducted at temperatures > 80°C, because lower temperatures yield a broad CH2-signal due to cis-/trans-isomerism and slow rotation around the CO-N-bond (27). 

CHO, and the double bond between carbons 11 and 12 of the side chain is in the less stable c/s configuration. When rhodopsin absorbs light energy, the less stable 11-c/s double bond is converted to the more stable 11-frans double bond. This isomerization changes the shape of the rhodopsin molecule, which in turn causes the neurons of the optic nerve to fire and produce a visual image. 

Step/ Such geometry changes are, of course, central to isomerizations via T-shaped three-coordinate intermediates. A crystal structure determination on c/s-[Pt(CH2CMe3)2(PEt3)2] indicates that loss of PEt3 (which precedes y-H migration) may considerably relieve steric strain. This may be an important factor in related ligand-loss processes. 

Thisayukta J, Nakayama Y, Kawauchi S, Takezoe H, Watanabe J (2000) Distinct formation of a chiral smectic phase in achiral banana-shaped molecules with a central core based on a 2,7-dihydroxynaphthalene unit. J Am Chem Soc 122 7441-7448 Tschierske C, Dantlgraber G (2003) From antiferroelectricity to ferroelectricity in smectic mesophases formed by bent-core molecules. Pramana J Phys 61 455-481 Ungar G, Percec V, Zuber M (1992) Liquid crystalline polyethers based on conformational isomerism. 20. Nematic-nematic transition in polyethers and copolyethers based on l-(4-hydroxyphenyl)2-(2-R-4-hydroxyphenyl)ethane with R=lluoro, chloro and methyl and flexible spacers containing an odd number of methylene units. Macromolecules 25 75-80 Urayama K (2007) Issues in liquid crystal elastomers and gels. Macromolecules 40 2277-2288 Vorlander D (1908) About transparently clear, crystalline liquids. Rep Ger Chem Soc 41 2033-2052... 

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