Mono- and dimethine dyes from 2-dimethylamino-5-formylfurans,-thiophenes,and -selenophenes

The chromaticity of diverse mono- and dimethine dyes obtained from 2-dimethyl-amino-5-formylfurans, -thiophenes, and -selenophenes was examined by means of the Kiprianov-Brooker deviation method. It is shown that the heterocyclic rings have considerably higher basicity than the 4-dimethylaniline ring, that the 2-dimethylamino-5-furan ring is the most basic ring, and that the 2-dimethyl-amino-5-thiophene ring is the least basic ring, whereas the analogous ring of the selenophene occupies an intermediate position that is closer to the thiophene ring.     

Computational evaluation of radical ring‐opening polymerization

The tendencies of ring‐opening processes in radical ring‐opening polymerizations were evaluated by AM1 and PM3 semi‐empirical calculations and 6‐31G*‐level calculations based on the density functional theory (DFT) B3LYP models. Sixteen cyclic monomers bearing vinyl or exomethylene groups were categorized into ring‐opening and no‐ring‐opening monomers by the evaluation of the differences of the internal energies and the lengths of the cleaving bonds between the ground states of the initial radicals and the activated states in the ring‐opening processes. Although the semi‐empirical calculations not parameterized to radical reactions resulted in the moderate categorization of the ring‐opening monomers, the DFT calculation clearly distinguished the ring‐opening and no‐ring‐opening monomers. The ring‐opening tendencies were also evaluated with the changes in the internal energies throughout the ring‐opening processes, but this method could not group the ring‐opening and no‐ring‐opening monomers clearly. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2827–2834, 2007     

Selective tandem enyne metathesis for the synthesis of functionalized cycloheptadienes

The regio- and site-selective ring expansion of dienes and the regioselective ring expansion of substituted cyclopentenes provide 1,3-cycloheptadienes by enyne metathesis under methylene-free conditions. Site-selectivity results from differential ring strain among two different cycloalkenes in diene reactants. The high regioselectivity found in the ring expansion of tetrahydroindene (THI) is explained on the basis of a selective ring opening by the second generation Grubbs' ruthenium carbene complex. The ring opening of substituted cyclopentenes and cyclopentene contained in a bicyclic ring system was also achieved. The ring expansion of bicyclic dienes provided seven-membered dienes contained in the bicyclo[5.2.0]nonane ring system. Details of the structural analysis are also discussed. A mechanistic analysis is provided to account for the data presented herein.     

Synthesis of new 14-membered macrolide antibiotics via a novel ring contraction metathesis

[reaction: see text] A novel ring opening ring closing metathesis (ROM-RCM) was demonstrated for cyclic conjugated dienes, effecting the excision of a C(2)H(2) unit and a net ring contraction. Applying the ring contraction metathesis, new 14-membered ring macrolide antibiotics were synthesized in a single step from existing 16-membered ring macrolides. This new class of macrolide antibiotics will provide access to new therapeutics for the treatment of macrolide-resistant bacterial infections.     

Synthesis of Ethyl 8‐Aryl‐8‐hydroxy‐3‐oxo‐8H[1,2,4]oxadiazolo‐[3,4‐c][1,4]thiazine‐5‐carboxylates by Ring‐Expansion Rearrangement

The title compounds were prepared by the ring–ring interconversion of ethyl 5‐nitroso‐6‐arylimidazo[2,1‐b]thiazole‐3‐carboxylates with hydrochloric acid. The effect of electron‐withdrawing substituent in the thiazole ring on the general applicability of the ring–ring interconversion has been also evaluated.     

非对称氮杂环丙烷的亲核开环反应及其区域选择性

本文系统地总结了各类亲核试剂对非对称氮杂环丙烷(吖丙啶)的亲核开环反应及开环的区域选择性.氮杂环丙烷亲核开环的区域选择性是一种空间效应和电子效应平衡的结果,非芳基和非烯基取代的氮杂环丙烷的亲核开环通常发生在氮杂环丙烷取代少的碳原子上,空间效应起主导作用;而芳基和烯基取代的氮杂环丙烷的亲核开环通常发生在氮杂环丙烷芳甲位和烯丙位的碳原子上,电子效应起主导作用,烯基取代的氮杂环丙烷的亲核开环还可以发生在烯基的β-碳原子上;分子内的亲核开环反应主要受成环时环大小的控制,成环时的倾向是五元环>六元环>七元环.对于亲核试剂,一般的亲核试剂也同时受电子效应和空间效应的影响; 而亲核性强的亲核试剂通常只受空间效应的影响.容易生成稳定自由基的亲核试剂容易发生单电子转移机理的开环反应,生成相当于亲核试剂进攻氮杂环丙烷中取代多的碳原子得到的开环产物.     

氧杂环丁烷的扩环反应

氧杂环丁烷是一类重要的小杂环化合物,也是重要有机合成中间体,在有机化学、药物化学和高分子化学中都有广泛的应用.作为具有较大环张力的小杂环化合物,氧杂环丁烷类化合物除可以发生开环反应外,也很容易发生扩环反应,构建含氧普通环到大环化合物.主要总结了氧杂环丁烷的扩环反应,包括重氮化合物作为卡宾前体与氧杂环丁烷的扩环反应、金属催化的分子间环加成和邻基参与的分子内环加成反应及亲核扩环反应等.分析了一些扩环反应的机理,并对扩环反应未来的发展提出了新的展望.     

Local angular momentum as ring strain descriptor

A new method is demonstrated to quantify local ring strain, which is based on the expectation value of orbital angular momentum along the internuclear axis. In contrast to energy based methods which provide overall ring strain, this method is able to identify the local strain in every part of the ring. The formalism is benchmarked on several cycloalkanes in which the presence of ring strain is well understood. The ring strain plays a decisive role in carbon nanotubes (CNTs) properties; for instance, the hydrogen storage capability of CNTs is related to their diameter, which in turn has a close relation to the ring strain in their C? C bonds. On this basis, the ring strain in five CNTs with different diameters is analyzed and the results reflected meaningful correlation between the CNTs diameter and ring strain. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A dissected ring current model for assessing magnetic aromaticity: a general approach for both organic and inorganic rings

A model based on classical electrodynamics is used to measure the strength of ring currents of different molecular orbitals, i.e., σ- and π-orbitals, and characteristics of ring current loops, i.e., ring current radii and height of current loops above/below the ring planes, among a number of organic as well as inorganic molecules. For the π-current, the present model represents an improvement of previous approaches to determine ring current intensity. It is proven that the present model is more precise than previous models as they could not explain presence of the minimum in the plot of NICS(πzz) versus distance close to the ring plane. Variations in the charge of molecules and the types of constituent atoms of each species affect the ring current radii of both σ- and π-current loops as well as the height of π-current loops above/below the ring plane. It is suggested that variation in the distribution of the one-electron density in different systems is the main source of differences of the ring current characteristics.     

Satellite Hole Investigation of the Vibrational Modes of 9-Aminoacridine upon Binding to DNA

The spectral features of satellite holes are used to investigate 9-aminoacridine-DNA interactions. The hole depths of the outer ring vibronic modes are reduced more than that of the inner ring vibronic modes, implying that inner ring motion is less perturbed than outer ring motion. As a result, the mode coupling between the inner ring and outer ring is reduced upon binding to DNA. However, similar hole frequency and width of the satellite hole corresponding to the NH₂ mode upon binding to DNA imply that the amino group of 9-aminoacridine sits outside the DNA.     

Convergent synthesis of the FGHI ring system of yessotoxin: stereoselective construction of the G ring

A convergent synthetic route to the FGHI ring system of yessotoxin, a marine ladder polyether, has been developed. The synthesis features convergent coupling of the diol and the aldehyde to form α-cyano ethers via acetal formation followed by ring closing metathesis and reductive etherification to construct the oxocane ring G and tetrahydropyran ring H, respectively. The β-methyl group on the G ring was stereoselectively introduced by alkylation of the corresponding ketone.     

Translational effects on electronic and nuclear ring currents

In previous works, it was predicted that electronic and nuclear ring currents in degenerate excited states of atomic and molecular systems persist after the end of driven circularly polarized atto- or femtosecond laser pulses on relatively long time scales, often on pico- or nanosecond time scales, before spontaneous emission occurs. Although this conclusion is true in the center of mass frame, it is not true in the laboratory frame, where the translation has to be considered. In this theoretical work, the analytic formulas for the ring current densities, electric ring currents, mean ring current radii, and induced magnetic fields at the ring center, depending on the translational wavepacket widths, are derived. It shows that the ring currents and the corresponding induced magnetic fields in the laboratory frame persist on shorter timecales due to spreading of translational wavepackets. The electronic ring currents in 2p(±) orbitals of the hydrogen-like systems decay on the femtosecond time scale, but the corresponding nuclear ring currents with giant induced magnetic fields (for example up to 0.54 MT for (7)Li(2+)) and very small mean ring current radii on the femtometer scale decay on the very short, zeptosecond time scale, according to the Heisenberg uncertainty principle. The theory is also applied to ring currents in many-electron atoms and ions as well as to nuclear ring currents in pseudorotating molecules. For example, in the first triply degenerate pseudorotational states |v(1)l(±1)> of the tetrahedral molecule OsH(4), the ring currents of the heavy central nucleus Os decay on the attosecond time scale.     

A new ring expansion for a chiral hexahydroazulene skeleton possessing an angular methyl group

A new synthetic route for a pseudoguaiane ring system is described. The synthesis features an Ireland-Claisen rearrangement for constructing the trans-fused ring system, followed by a new ring expansion to yield a bicyclo[5.3.0]decane ring system possessing an angular methyl group.     

Stacking interactions between nitrogen-containing six-membered heterocyclic aromatic rings and substituted benzene: studies in solution and in the solid state

The stacking interactions between an aromatic ring and a pyridine or a pyrimidine ring are studied by using a series of triptycene-derived scaffolds. The indicative ratios of the syn and anti conformers were determined by variable-temperature NMR spectroscopy. The syn conformer aligns the attached aromatic ring and the heterocycle in a parallel-displaced orientation while the anti conformer sets the two rings apart from each other. Comparing to the corresponding control compounds where a benzene ring is in the position of the heterocycle, higher attractive interactions are observed as indicated by the higher syn/anti ratios. In general, the attractive interactions are much less sensitive to the substituent effects than the corresponding nonheterocycles. The greatest attractive interactions were observed between a pyrimidine ring and a N,N-dimethylaminobenzene, consistent with a predominant donor-acceptor interaction. The interactions between a pyridine ring and a substituted benzene ring show that the pyridine is comparable to that of a NO2- or a CN-substituted benzene ring except for the unpredictable substituent effects.     

Variable and stereoselective synthesis of azasugar analogues by a ruthenium-catalyzed ring rearrangement

A novel ruthenium-catalyzed ring opening/ring closing tandem metathesis reaction with a catalytic transfer of stereocenters from a ring to an olefinic chain is described. This ring rearrangement serves as the key step in the stereoselective synthesis of the new azasugar analogues 1 and 2.     

Total Synthesis and Biological Evaluation of (+)‐Gambieric Acid A and Its Analogues

In this study, we report the first total synthesis and complete stereostructure of gambieric acid A, a potent antifungal polycyclic ether metabolite, in detail. The A/B‐ring exocyclic enol ether 32 was prepared through a Suzuki–Miyaura coupling of the B‐ring vinyl iodide 18 and the alkylborate 33 and subsequent closure of the A‐ring by using diastereoselective bromoetherification as the key transformation. Suzuki–Miyaura coupling of 32 with acetate‐derived enol phosphate 49 , followed by ring‐closing metathesis of the derived diene, produced the D‐ring. Subsequent closure of the C‐ring through a mixed thioacetalization completed the synthesis of the A/BCD‐ring fragment 8 . The A/BCD‐ and F′GHIJ‐ring fragments (i.e., 8 and 9 ) were assembled through Suzuki–Miyaura coupling. The C25 stereogenic center was elaborated by exploiting the intrinsic conformational property of the seven‐membered F′‐ring. After the oxidative cleavage of the F′‐ring, the E‐ring was formed as a cyclic mixed thioacetal (i.e., 70 ) and then stereoselectively allylated by using glycosylation chemistry. Ring‐closing metathesis of the diene 3 thus obtained closed the F‐ring and completed the polycyclic ether skeleton. Finally, the J‐ring side chain was introduced by using a Julia–Kocienski olefination in the presence of CeCl₃ to complete the total synthesis of gambieric acid A ( 1 ), thereby unambiguously establishing its complete stereostructure. The present total synthesis enabled us to evaluate the antifungal and antiproliferative activities of 1 and several synthetic analogues.     

Synthesis of tetracycline analogs and their bone affinities

Tetracycline analogs were designed and synthesized and their bone affinities were tested on hydroxyapatite. The results showed that the carbonyl-amide-enol structure in A ring and phenol-ketone structure in BCD ring may be responsible for tetracycline＇s high bone affinity and either A ring or BCD ring has a planar conformation is essential. 2007 Ling Ling Weng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.     

A Topological Approach to the Problem of Ring Structures

In the mechanized documentation of chemical literature, the definition of structural concept is very important. The usual for definitions for ring structures are inadequate. Essential ring structures are sometimes not recognized on the basis of these definitions and are therefore missed in a literature search. This is particularly true of bridged ring systems. The ring concept and ring condensation types are now redefined on a topological basis in the closest possible analogy to the intuitive approach of the chemist. In complicated molecular structures, these “fundamental rings” can be easily determined, either manually or by means of a programmed computer. The concept of the “ring complex” is defined and suggested as a preliminary screen in literature searches for ring structures. This will save machine time, and so reduce the cost of searches.     

Stereoselective synthesis of piperidinone and quinolinone systems via ring opening reactions using TiCl4/silyl reagents

Titanium(IV) chloride and silyl reagents mediated regio- and chemoselective ring opening reactions of oxa-bridged piperidinone ring systems were demonstrated. This methodology interestingly undergoes the stereoselective ring opening at the C-O bond of oxa-bridged piperidinone ring systems. Study of TiCl₄ with hydride or non-hydride silyl reagents furnished the product with selectivity. This protocol is highly valuable to synthesize a range of stereoselective piperidinones, quinolinones ring systems.