Stereochemistry Research Papers - Academia.edu (original) (raw)

The transversely bianisotropic uniaxial medium considered in the present article can be obtained, e.g., by mixing metal helices with an isotropic base medium in such a way that the axes of the helices are randomly oriented but... more

The transversely bianisotropic uniaxial medium considered in the present article can be obtained, e.g., by mixing metal helices with an isotropic base medium in such a way that the axes of the helices are randomly oriented but perpendicular to a fixed direction in space. The medium is a generalization of the well-studied chiral medium, and is somewhat similar to the recently studied axially bianisotropic uniaxial medium, which has interesting polarization properties. Plane-wave propagation in the medium is studied and the solution for the dispersion equation is given. Numerical examples for the wave numbers corresponding to the two eigenwaves of the medium are displayed. It is seen that, unlike in the axially bianisotropic uniaxial medium, there are no optical axes in the present medium, in general. © 1993 John Wiley & Sons, Inc.

Platinum-based heterogeneous catalysts are critical to many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the surface active-site atoms are used. Catalysts with... more

Platinum-based heterogeneous catalysts are critical to many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the surface active-site atoms are used. Catalysts with single-atom dispersions are thus highly desirable to maximize atom efficiency, but making them is challenging. Here we report the synthesis of a single-atom catalyst that consists of only isolated single Pt atoms anchored to the surfaces of iron oxide nanocrystallites. This single-atom catalyst has extremely high atom efficiency and shows excellent stability and high activity for both CO oxidation and preferential oxidation of CO in H2. Density functional theory calculations show that the high catalytic activity correlates with the partially vacant 5d orbitals of the positively charged, high-valent Pt atoms, which help to reduce both the CO adsorption energy and the activation barriers for CO oxidation.

Un estudio dedicado a la estereoquímica

QUÍMICA ORGÁNICA APUNTES DE ESTEREOQUÍMICA

Orthogonally protected chiral β-hydroxy-γ-amino acids can be accessed in >100 g quantities from readily available starting materials and reagents in three to four steps. These chiral synthons contain two adjacent stereocenters along with... more

Orthogonally protected chiral β-hydroxy-γ-amino acids can be accessed in >100 g quantities from readily available starting materials and reagents in three to four steps. These chiral synthons contain two adjacent stereocenters along with suitably protected functional groups (O-TBS, N-Boc) for downstream reactivity. Implementation of two existing aldol technologies allows rapid access to all possible stereoisomers of 1. The guiding principles during reaction optimization were reaction scalability and operational efficiency. Conversion of the amino acids to a variety of chiral building blocks in one to two steps demonstrates their synthetic utility.

Stereochemical configuration is determined by the relationship of atoms in three-dimensional space, yet remains most commonly represented in two-dimensional media such as printed publications or computer screens. Recommendations are... more

Stereochemical configuration is determined by the relationship of atoms in three-dimensional space, yet remains most commonly represented in two-dimensional media such as printed publications or computer screens. Recommendations are provided for the display of three-dimensional stereochemical information in two-dimensional diagrams in ways that avoid ambiguity and are likely to be understood correctly by all viewers. Examples are provided for all types of stereochemical configuration, with explanation of which styles are preferred and which should be avoided. Principal recommendations include:
• Know your audience: Diagrams that have a wide audience should be drawn as simply as possible.
• Avoid ambiguous drawing styles.
• Avoid the use of perspective diagrams and class-specific drawing styles (Fischer projections, Haworth projections, etc.) when structures are to be interpreted by computers.
• Use solid wedges to indicate bonds that project above the plane of the paper and hashed wedges to indicate bonds that project below the plane of the paper; in both cases, the bonds must be oriented with the narrow end at the stereogenic center.
• Avoid connecting stereogenic centers with a stereobond.

Stereoisomers have the same molecular formula and the same atom connectivity and their existence can be related to the presence of different three-dimensional arrangements. Stereoisomerism is of great importance in many different fields... more

Stereoisomers have the same molecular formula and the same atom connectivity and their existence can be related to the presence of different three-dimensional arrangements. Stereoisomerism is of great importance in many different fields since the molecular properties and biological effects of the stereoisomers are often significantly different. Most drugs for example, are often composed of a single stereoisomer of a compound, and while one of them may have therapeutic effects on the body, another may be toxic. A challenging task is the automatic detection of stereoisomers using line input specifications such as SMILES or InChI since it requires information about group theory (to distinguish stereoisomers using mathematical information about its symmetry), topology and geometry of the molecule. There are several software packages that include modules to handle stereochemistry, especially the ones to name a chemical structure and/or view, edit and generate chemical structure diagrams....

In his inaugural presidential address at the 1898 annual meeting of the British Association for the Advancement of Science, William Crookes called upon scientists to solve the “Wheat Problem”—that is, the threat of global scarcity—by... more

In his inaugural presidential address at the 1898 annual meeting of the British Association for the Advancement of Science, William Crookes called upon scientists to solve the “Wheat Problem”—that is, the threat of global scarcity—by inventing processes to fix atmospheric nitrogen to produce fertilizer. This project became a driving force of modernity and seemed to realize its promise: that humanity would use inventive genius to triumph over scarce material resources. Nitrogen is the element of life and of death, of the timeless cycle of waste and rebirth and of modernity. Why, then, this essay asks, does Primo Levi devote the chapter on nitrogen in his 1975 memoir The Periodic Table to the manufacture of lipstick, a paradigmatically inessential luxury good? The answer, I argue, can be found in a second intervention delivered at the very same meeting by the incoming president of the Chemistry Section, Francis Robert Japp. His paper, which sparked lively debate among prominent scientists, proposes a scientific vitalism in the form of some “directive force” proper to life that is evinced in molecular asymmetry. In the chapter “Nitrogen,” Levi narrativizes the clash between the philosophical underpinnings of the two interventions to reclaim an aesthetic vitalism that neither conflicts with nor is reducible to the laws of chemistry and physics.

Chemical examination of the Chinese marine sponge Xestospongia testudinaria led to the isolation of 39 new brominated polyunsaturated compounds, which were designated with the trivial names xestospongienes A–Z and Z1–Z13. The structures... more

Chemical examination of the Chinese marine sponge Xestospongia testudinaria led to the isolation of 39 new brominated polyunsaturated compounds, which were designated with the trivial names xestospongienes A–Z and Z1–Z13. The structures of these compounds were elucidated by analyzing extensive spectroscopic (IR, MS, 1D, and 2D NMR) data and the results from CD spectral assignment, chemical conversion, and Mosher reaction.

a-Chiral amine Asymmetric organocatalysis Bifunctional organocatalyst a b s t r a c t Enantiomerically pure a-chiral amines, have been commonly utilized as resolving agents and chiral auxiliaries and are currently found in 40% of active... more

a-Chiral amine Asymmetric organocatalysis Bifunctional organocatalyst a b s t r a c t Enantiomerically pure a-chiral amines, have been commonly utilized as resolving agents and chiral auxiliaries and are currently found in 40% of active pharmaceutical ingredients. Hence, development of highly stereoselective metal-free protocols regarding atom-economy and large-scale applications becomes a major issue. In this respect, chiral bifunctional H-bonding squaramides have been successfully applied for both amine synthesis and functionalization of amines in the last decade. This survey summarizes asymmetric synthesis of chiral amines by various carbon-carbon and carbon-nitrogen bond formation with squaramide catalysis as a particular focus of interest.

A convergent total synthesis of amphidinolactone A, a cytotoxic macrolide from the cultured dinoflagellate Amphidinium sp., is described in 13 linear steps. The key step in the synthetic sequence involves an intramolecular... more

A convergent total synthesis of amphidinolactone A, a cytotoxic macrolide from the cultured dinoflagellate Amphidinium sp., is described in 13 linear steps. The key step in the synthetic sequence involves an intramolecular Nozaki–Hiyama–Kishi (NHK) reaction for the construction of the 13-membered lactone ring by union of two fragments derived from a single chiral epoxide. The stereochemical outcome of the NHK reaction has been supported by computational studies.

A novel experiment was prepared to help chemistry educators to improve the stereochemistry curriculum of secondary school students and undergraduate students. Based on polarimetry and Vitamin C, chemistry educators are now provided with... more

A novel experiment was prepared to help chemistry educators to improve the stereochemistry curriculum of secondary school students and undergraduate students. Based on polarimetry and Vitamin C, chemistry educators are now provided with an experiment that takes into account the delocalization of electrons, and the interactions with plane-polarized light in atomic bonds.

Solar fuel production often starts with the energy from light being absorbed by an assembly of molecules; this electronic excitation is subsequently transferred to a suitable acceptor. For example, in photosynthesis, antenna complexes... more

Solar fuel production often starts with the energy from light being absorbed by an assembly of molecules; this electronic excitation is subsequently transferred to a suitable acceptor. For example, in photosynthesis, antenna complexes capture sunlight and direct the energy to reaction centres that then carry out the associated chemistry. In this Review, we describe the principles learned from studies of various natural antenna complexes and suggest how to elucidate strategies for designing light-harvesting systems. We envisage that such systems will be used for solar fuel production, to direct and regulate excitation energy flow using molecular organizations that facilitate feedback and control, or to transfer excitons over long distances. Also described are the notable properties of light-harvesting chromophores, spatial-energetic landscapes, the roles of excitonic states and quantum coherence, as well as how antennas are regulated and photoprotected.