Cis–trans isomerism

Summary

Cis–trans isomerism, also known as geometric isomerism or configurational isomerism, is a term used in chemistry that concerns the spatial arrangement of atoms within molecules. The prefixes "cis" and "trans" are from Latin: "this side of" and "the other side of", respectively. In the context of chemistry, cis indicates that the functional groups (substituents) are on the same side of some plane, while trans conveys that they are on opposing (transverse) sides. Cis–trans isomers are stereoisomers, that is, pairs of molecules which have the same formula but whose functional groups are in different orientations in three-dimensional space. Cis-trans notation does not always correspond to E–Z isomerism, which is an absolute stereochemical description. In general, cis–trans stereoisomers contain double bonds that do not rotate, or they may contain ring structures, where the rotation of bonds is restricted or prevented. Cis and trans isomers occur both in organic molecules and in inorganic

Official source

https://en.wikipedia.org/wiki/Cis%E2%80%93trans_isomerism

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Supramolecular self-assembly in water based on non-covalent bonding is attracting major attention due to the potential of hydrogels and aqueous polymers in biomedical applications. Although supramolecular polymerization in organic solvents is well established, the key design features, the assembly mechanisms in water and achieving control over the aggregate structures remain challenging. Here, we present the assembly and disassembly of geometrical isomers of a stiff-stilbene bis-urea amphiphile (SA) in pure water. A remarkable feature of this system is that the (E)-isomer forms supramolecular polymers in both pure water and organic solvents. Taking advantage of this unique property, the hydrophobic effect was studied by comparing the supramolecular assembly in both systems. The assembly process in water follows an enthalpy-driven nucleation-elongation (cooperative) supramolecular polymerization mechanism with a standard Gibbs free energy (Delta G degrees = -53 kJ mol(-1)) double the value of the one found in toluene. We attributed this distinctive feature to the hydrophobic effect in water. Furthermore, we discovered an isomer-dependent assembly process, which can be used to control aggregation in aqueous media. Due to the substantial geometric difference between (E)-SA and (Z)-SA, we compared their assembly in water to study the influence of different driving forces involved in the process. The supramolecular polymerization of (E)-SA was cooperatively influenced by hydrogen bonding, pi-stacking, and hydrophobic effects, whereas the assembly of (Z)-SA was mainly driven by hydrophobic effects. As a result, the fiber length of (E)-SA in water is much longer than that of (Z)-SA, presenting opportunities for geometrical control of aggregation in aqueous media. [GRAPHICS]

CHINESE CHEMICAL SOC2022

Dearomatization of electron poor six-membered N-heterocycles through [3+2] annulation with aminocyclopropanes

Shyamal Chakrabarty, Johannes Preindl, Jérôme Waser

Many abundant and highly bioactive natural alkaloids contain an indolizidine skeleton. A simple, high yielding method to synthesize this scaffold from N-heterocycles was developed. A wide range of pyridines, quinolines and isoquinolines reacted with donor-acceptor (DA)-aminocyclopropanes via an ytterbium(III) catalyzed [3 + 2] annulation reaction to give tetrahydroindolizine derivatives. The products were obtained with high diastereoselectivities (dr > 20 : 1) as anti-isomers. Additionally, the formed aminals could be easily converted into secondary and tertiary amines through iminium formation followed by reduction or nucleophile addition. This transformation constitutes the first example of dearomatization of electron-poor six-membered heterocycles via [3 + 2] annulation with DA cyclopropanes.

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Generation of photoswitchable peptide ligands by phage display

Silvia Bellotto

Photoswitchable ligands are used to control and study complex biological systems such as folding of proteins and peptides, enzymatic reactions or neuronal signaling. They are typically developed by conjugating photochromic compounds to known ligands so that exposure to light results in the change of the binding affinity of the ligand. Many light-responsive ligands are based on azobenzene, a molecule that undergoes a pronounced change in geometry upon photoisomerization from trans to cis in picoseconds. Currently, photoswitchable ligands are available only to a limited number of targets and their development by rational design is complex. Furthermore, they show low affinity and a relatively small change in binding affinity between the cis and trans conformation. In my PhD work, I aimed at overcoming these shortcomings by establishing an in vitro evolution method to generate light-controlled peptide ligands to targets of choice. Combining the expertise on phage display technology of the Heinis' lab and the knowledge about azobenzene photoswitches of the Wegner's lab, I screened large combinatorial libraries of cyclic peptides containing an azobenzene-based linker for the generation of light controlled ligands. [...]

EPFL2015