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Macrocycles
Macrocycles are often described as molecules and ions containing a ring of twelve or more atoms. Classical examples include the crown ethers, calixarenes, porphyrins, and cyclodextrins. Macrocycles describe a large, mature area of chemistry. Synthesis The formation of macrocycles by ring-closure is called macrocylization. Pioneering work was reported for studies on terpenoid macrocycles. The central challenge to macrocyclization is that ring-closing reactions do not favor the formation of large rings. Instead, small rings or polymers tend to form. This kinetic problem can be addressed by using high-dilution reactions, whereby intramolecular processes are favored relative to polymerizations. Some macrocyclizations are favored using template reactions. Templates are ions, molecules, surfaces etc. that bind and pre-organize compounds, guiding them toward formation of a particular ring size. The crown ethers are often generated in the presence of an alkali metal cation ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Macrocyclic Ligand
In coordination chemistry, a macrocyclic ligand is a macrocyclic ring having at least nine atoms (including all hetero atoms) and three or more donor sites that serve as ligands that can bind to a central metal ion. Crown ethers and porphyrins are prominent examples. Macrocyclic ligands exhibit high affinity for metal ions. History Porphyrins and phthalocyanines have long been recognized as potent ligands in coordination chemistry as illustrated by numerous transition metal porphyrin complexes and the commercialization of copper phthalocyanine pigments. In the 1960s the synthesis of macrocylic ligands received much attention. One early contribution involved the synthesis of the "Curtis macrocycles", in which a metal ion serves as a template for ring formation. Polyether macrocycles - or "crown" ligands - were also developed at that time. A few years later, three-dimensional analogs of crown ethers called " cryptands" were reported by Lehn and co-workers. Macrocyclic effect ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Macrocyclic Stereocontrol
Macrocyclic stereocontrol refers to the directed outcome of a given intermolecular or intramolecular chemical reaction, generally an organic reaction, that is governed by the conformational or geometrical preference of a carbocyclic or heterocyclic ring, where the ring containing 8 or more atoms. Introduction Stereocontrol for cyclohexane rings is well established in organic chemistry, in large part due to the axial/equatorial preferential positioning of substituents on the ring. Macrocyclic stereocontrol models the substitution and reactions of medium and large rings in organic chemistry, with remote stereogenic elements providing enough conformational influence to direct the outcome of a reaction. Early assumptions towards macrocycles in synthetic chemistry considered them far too floppy to provide any degree of stereochemical or regiochemical control in a reaction. The experiments of W. Clark Still in the late 1970s and 1980s challenged this assumption,Still, W. C.; Galynk ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Catenane
In macromolecular chemistry, a catenane () is a mechanically interlocked molecular architecture consisting of two or more interlocked macrocycles, i.e. a molecule containing two or more intertwined rings. The interlocked rings cannot be separated without breaking the covalent bonds of the macrocycles. They are conceptually related to other mechanically interlocked molecular architectures, such as rotaxanes, molecular knots or molecular Borromean rings. Recently the terminology " mechanical bond" has been coined that describes the connection between the macrocycles of a catenane. Catenanes have been synthesised in two different ways: statistical synthesis and template-directed synthesis. Synthesis There are two primary approaches to the organic synthesis of catenanes. The first is to simply perform a ring-closing reaction with the hope that some of the rings will form around other rings giving the desired catenane product. This so-called "statistical approach" led to ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Rotaxane
In chemistry, a rotaxane () is a mechanically interlocked molecular architecture consisting of a dumbbell-shaped molecule which is threaded through a macrocycle (see graphical representation). The two components of a rotaxane are kinetically trapped since the ends of the dumbbell (often called ''stoppers'') are larger than the internal diameter of the ring and prevent dissociation (unthreading) of the components since this would require significant distortion of the covalent bonds. Much of the research concerning rotaxanes and other mechanically interlocked molecular architectures, such as catenanes, has been focused on their efficient synthesis or their utilization as artificial molecular machines. However, examples of rotaxane substructure have been found in naturally occurring peptides, including: cystine knot peptides, cyclotides or lasso-peptides such as microcin J25. Synthesis The earliest reported synthesis of a rotaxane in 1967 relied on the statistical probability ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ring-closing Metathesis
Ring-closing metathesis (RCM) is a widely used variation of olefin metathesis in organic chemistry for the synthesis of various unsaturated rings via the intramolecular metathesis of two terminal alkenes, which forms the cycloalkene as the ''E-'' or ''Z-'' isomers and volatile ethylene.Carey, F. A.; Sunburg, R. J. Reactions Involving Transition Metals. ''Advanced Organic Chemistry: Reaction and Synthesis'', 5th Ed.; Part B; Springer: New York, 2010, pp. 761-767.Monfette, S.; Fogg, D. E. (2009). "Equilibrium Ring-Closing Metathesis". ''Chem. Rev.'' 109 (8): 3783-3816. . The most commonly synthesized ring sizes are between 5-7 atoms;Deiters, A.; Martin, S. F. (2004). “Synthesis of Oxygen- and Nitrogen-Containing Heterocycles by Ring-Closing Metathesis”. ''Chem. Rev.'' 104 (5): 2199-2238. . however, reported syntheses include 45- up to 90- membered macroheterocycles.Cain, M. F.; Forrest, W. P.; Peryshkov, R. V.; Schrock, R. R. Muller, P. (2013). “Synthesis of a TREN in Wh ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Porphyrin
Porphyrins ( ) are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). The parent of porphyrin is porphine, a rare chemical compound of exclusively theoretical interest. Substituted porphines are called porphyrins. With a total of 26 π-electrons, of which 18 π-electrons form a planar, continuous cycle, the porphyrin ring structure is often described as aromatic. One result of the large conjugated system is that porphyrins typically absorb strongly in the visible region of the electromagnetic spectrum, i.e. they are deeply colored. The name "porphyrin" derives from the Greek word πορφύρα (''porphyra''), meaning ''purple''. Complexes of porphyrins Concomitant with the displacement of two N-''H'' protons, porphyrins bind metal ions in the N4 "pocket". The metal ion usually has a charge of 2+ or 3+. A schematic equation for these syntheses is shown: :H2porp ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Macrolide
The Macrolides are a class of natural products that consist of a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered. Macrolides belong to the polyketide class of natural products. Some macrolides have antibiotic or antifungal activity and are used as pharmaceutical drugs. Rapamycin is also a macrolide and was originally developed as an antifungal, but is now used as an immunosuppressant drug and is being investigated as a potential longevity therapeutic. Macrolides are bacteriostatic in that they suppress or inhibit bacterial growth rather than killing bacteria completely. Definition In general, any macrocyclic lactone having greater than 8-membered rings are candidates for this class. The macrocycle may contain amino nitrogen, amide nitrogen (but should be differentiated from cyclopeptides), an oxazole ring, or a thiazole ring. Benzene rings are e ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Template Reaction
In coordination chemistry, a template reaction is any of a class of ligand-based reactions that occur between two or more adjacent coordination sites on a metal center. In the absence of the metal ion, the same organic reactants produce different products. The template effects emphasizes the pre-organization provided by the coordination sphere, although the coordination modifies the electronic properties (acidity, electrophilicity, etc.) of ligands. An early example is the dialkylation of a nickel dithiolate: : The corresponding alkylation in the absence of a metal ion would yield polymers. Crown ethers arise from dialkylations that are templated by alkali metals. Other template reactions include the Mannich and Schiff base condensations. The condensation of formaldehyde, ammonia, and tris(ethylenediamine)cobalt(III) to give a clathrochelate complex is one example. The phosphorus analogue of an aza crown can be prepared by a template reaction. Where it is not possible to ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Crown Ether
In organic chemistry, crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups (). The most common crown ethers are cyclic oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., . Important members of this series are the tetramer (''n'' = 4), the pentamer (''n'' = 5), and the hexamer (''n'' = 6). The term "crown" refers to the resemblance between the structure of a crown ether bound to a cation, and a crown sitting on a person's head. The first number in a crown ether's name refers to the number of atoms in the cycle, and the second number refers to the number of those atoms that are oxygen. Crown ethers are much broader than the oligomers of ethylene oxide; an important group are derived from catechol. Crown ethers strongly bind certain cations, forming complexes. The oxygen atoms are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chlorin
In organic chemistry, chlorins are tetrapyrrole pigments that are partially hydrogenated porphyrins. The parent chlorin is an unstable compound which undergoes air oxidation to porphine. The name chlorin derives from chlorophyll. Chlorophylls are magnesium-containing chlorins and occur as photosynthetic pigments in chloroplasts. The reduced chlorin variants are present in bacteriochlorophylls and are named ‘bacteriochlorins’ and ‘isobacteriochlorins’. Chlorins are excellent photosensitizing agents. Various synthetic chlorins analogues such as m-tetrahydroxyphenylchlorin (mTHPC) and mono-L-aspartyl chlorin e6 are effectively employed in experimental photodynamic therapy as photosensitizer. Chlorophylls The most abundant chlorin is the photosynthetic pigment chlorophyll. Chlorophylls have a fifth, ketone-containing ring unlike the chlorins. Diverse chlorophylls exists, such as chlorophyll ''a'', chlorophyll ''b'', chlorophyll ''d'', chlorophyll ''e'', chlor ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Effective Molarity
In chemistry, the effective molarity (denoted ''EM'') is defined as the ratio between the first-order rate constant of an intramolecular reaction and the second-order rate constant of the corresponding intermolecular reaction (''Kinetic Effective Molarity'') or the ratio between the equilibrium constant of an intramolecular reaction and the equilibrium constant of the corresponding intermolecular reaction (''Thermodynamic Effective Molarity''). EM_ = EM_ = EM has the dimension of concentration. High EM values always indicate greater ease of intramolecular processes over the corresponding intermolecular ones. Effective molarities can be used to get a deeper understanding of the effects of intramolecularity on reaction courses. In last decades, the frequency of use of effective molarity in scientific literature has shown a marked decrease, because this formalism is being progressively replaced by more important physical quantities. See also *Cyclic compound *Intramolecular reacti ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Molecular Knot
In chemistry, a molecular knot is a mechanically interlocked molecular architecture that is analogous to a macroscopic knot. Naturally-forming molecular knots are found in organic molecules like DNA, RNA, and proteins. It is not certain that naturally occurring knots are evolutionarily advantageous to nucleic acids or proteins, though knotting is thought to play a role in the structure, stability, and function of knotted biological molecules. The mechanism by which knots naturally form in molecules, and the mechanism by which a molecule is stabilized or improved by knotting, is ambiguous. The study of molecular knots involves the formation and applications of both naturally occurring and chemically synthesized molecular knots. Applying chemical topology and knot theory to molecular knots allows biologists to better understand the structures and synthesis of knotted organic molecules. The term ''knotane'' was coined by Vögtle ''et al.'' in 2000 to describe molecular knots by a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
