Nitrosopurines En Route to Potently Cytotoxic Asmarines

A nitrosopurine ene reaction easily assembles the asmarine pharmacophore and transmits remote stereochemistry to the diazepine‐purine hetereocycle. This reaction generates potent cytotoxins which exceed the potency of asmarine A (1.2 μM IC₅₀) and supersede the metabolites as useful leads for biological discovery.     

Aza‐Bambusurils En Route to Anion Transporters

Previous calculations of anion binding with various bambusuril analogs predicted that the replacement of oxygen by nitrogen atoms to produce semiaza‐bambus[6]urils would award these new cavitands with multiple anion binding properties. This study validates the hypothesis by efficient synthesis, crystallography, thermogravimetric analysis and calorimetry. These unique host molecules are easily accessible from the corresponding semithio‐bambusurils in a one‐pot reaction, which converts a single anion receptor into a potential anion channel. Solid‐state structures exhibit simultaneous accommodation of three anions, linearly positioned within the cavity along the main symmetry axis. The ability to hold anions at a short distance of about 4 Å is reminiscent of natural chloride channels in E. coli, which exhibit similar distances between their adjacent anion binding sites. The calculated transition‐state energy for double‐anion movement through the channel suggests that although these host–guest complexes are thermodynamically stable they enjoy high kinetic flexibility to render them efficient anion channels.     

Selective Modification of Chitin and Chitosan: En Route to Tailored Oligosaccharides

Chitin and chitosan are attractive biopolymers with enormous structural possibilities for chemical modification, creating platforms for new chemical entities with a broad scope of applications, ranging from material science to medicine. During the last few years, incredible efforts have been dedicated to the regioselective modification of these biopolymers paving the way for improved properties and tailored activities. Herein, the most recent advances in chitin/chitosan regioselective modification, reaction conditions, selectivity, and the impact on its applications are highlighted. Moreover, the recent focus on chitooligosaccharides, their regioselective and chemoselective functionalization, as well as their role in biological studies, including molecular recognition with several biological targets are also covered.     

A Stereospecific, Intermolecular Biaryl-Coupling Approach to Korupensamine A En Route to the Michellamines

By what means and how well can axial chirality be controlled in an intermolecular Suzuki biaryl cross-coupling reaction? The directionality of reductive elimination [Eq. (1)] is completely controlled by using a strategically positioned internal ligand L to afford a single biaryl atropisomer corresponding to the korupensamine A skeleton. TIPS=iPr(3)Si, Ts=H(3)CC(6)H(4)SO(2).     

Galactosidase-Assisted Synthesis En Route to Type I And Type II Structures of Chitooligomers

ABSTRACT

Transgalactosylation of chitobiose and chitotriose led to formation of terminally (β1-3)- and (β1-4)-galactosylated chitooligosaccharides ready for fucosylation to give Lewis^a and Lewis^x motifs. Their structures could be assigned employing HPAECPAD, methylation, ESI-MS/MS and NMR analyses.     

Stereodivergent Transformation of Azepino[3,4,5-cd]indoles En Route to Nature-Inspired Scaffolds

Azepino[3,4,5-cd]indole derivatives represent the core scaffold of important natural products and biologically relevant compounds. Therefore, the establishment of step- and atom-economic strategies to access this class of compounds is of paramount importance. To this end, complexity-to-diversity (CtD) strategy has become one of the most important tools that transforms complex molecules into diverse skeleta. However, many of the reactions that could be employed in CtD are restricted by the functional handles exist in these molecules. This limits the achievement of the desired skeletal diversity. Herein, an efficient and step-economic strategy to access a diverse collection of azepino-[3,4,5-cd]indole architectures through a cascade that combines Pictet-Spengler with Michael addition, is described. This was achieved by reacting cyclohexadienone acetaldehydes 2 a – 2 d with indolyl-4-ethyl amine 1 . Employing a CtD strategy on the developed azepino-[3,4,5-cd]indoles, a rapid rearrangement reaction that provided a modular, chemo- and diastereoselective access to diverse collection of spiro azepinocarbazole nature-inspired frameworks, was encountered.     

On the Chemistry of the Resveratrol Diastereomers

Summary.  The (E)- and (Z)-diastereomers of resveratrol were investigated with respect to their photochemical and thermal diastereomerization reactions. The free enthalpy difference between the two diastereomers was estimated to be in the order of common stilbenes, with the (E)-diastereomer more stable by about 11–14 kJ mol⁻¹. The Arrhenius activation barrier of about 280 kJ mol⁻¹ was found to be quite high and implies that thermal equilibration cannot account for the (Z)-diastereomer found in nature. A preparative access to the (Z)-diastereomer by photodiastereomerization is described. The ¹H and ¹³C NMR spectra of the two diastereomers were assigned and their absorption spectra and fluorescence quantum yields of the neutral and monodeprotonated species were determined. Corresponding author. E-mail: heinz.falk@jku.at Received October 15, 2002; accepted November 7, 2002 Published online February 20, 2003     

Homoleptic Tris(Pyridyl Pyrazolate) IrIII Complexes: En Route to Highly Efficient Phosphorescent OLEDs

Treatment of the metal reagent IrCl₃ ? nH₂O with two equivalents of 2‐pyridyl pyrazole (N^N)H (3‐tert‐butyl‐5‐(2‐pyridyl) pyrazole, (bppz)H and 3‐trifluoromethyl‐5‐(2‐pyridyl) pyrazole, (fppz)H), afforded the isomeric Ir^III metal complexes with a general formula cis‐[Ir(bppz)₂Cl₂]H ( 2 a ), trans‐[Ir(bppz)₂Cl₂]H ( 3 a ), cis‐[Ir(fppz)₂Cl₂]H ( 2 b ), and trans‐[Ir(fppz)₂Cl₂]H ( 3 b ). Single‐crystal X‐ray diffraction studies on 2 b and 3 a revealed the coexistence of two pyrazolate chelates and two terminal chloride ligands on the coordination sphere. Subsequent reactivity studies confirmed their intermediacy to the preparation of homoleptic mer‐[Ir(bppz)₃] ( 1 a ) and mer‐[Ir(fppz)₃] ( 1 b ) that showed dual intraligand and ligand‐to‐ligand charge‐transfer phosphorescence at room temperature. To attain bright, room‐temperature phosphorescence further, we then synthesized two isoquinolinyl pyrazolate complexes, mer‐[Ir(bipz)₃] ( 4 a ) and mer‐[Ir(fipz)₃] ( 4 b ) ((bipz)H=3‐tert‐butyl‐5‐(1‐isoquinolyl) pyrazole and (fipz)H=3‐trifluoromethyl‐5‐(1‐isoquinolyl) pyrazole). Their orange luminescence is mainly attributed to the mixed MLCT/ππ* transition, and the quantum yields were as high as 86 ( 4 a ) and 50 % ( 4 b ) in degassed CH₂Cl₂ solution at RT. The organic light‐emitting diodes (OLEDs) were then fabricated by using 4 a as a dopant, giving orange luminescence with CIE_x,y=0.55, 0.45 (CIE_x,y=the 1931 Commission Internationale de L’Eclairage (x,y) coordinates) and peak efficiencies of 14.6 % photon/electron, 34.8 cd A^?1, 26.1 lm W^?1. The device data were then compared with the previously reported heteroleptic complex [Ir(dfpz)₂(bipz)] ( 5 ) ((dfpz)H=1‐(2,4‐difluorophenyl) pyrazole), revealing the possible effect of the bipz chelate and phosphor design on the overall electrophosphorescent performance, which can be understood by the differences in the carrier‐transport properties.     

New Route to Polymeric Nanoparticles by Click Chemistry Using Bifunctional Cross-Linkers

A new route to functional polymeric nanoparticles (PNPs) of different chemical nature in the 3 to 20 nm size range is reported by combining both radical addition fragmentation chain transfer (RAFT) polymerization and “click” chemistry (CC) techniques. RAFT polymerization was employed for the synthesis of well-defined statistical copolymers with pending –Cl groups along the macromolecular chain. After transformation of the –Cl groups to –N₃ groups by treatment with sodium azide, an appropriate bifunctional cross-linker is employed to obtain PNPs under CC conditions promoting intramolecular cycloaddition (cross-linking). Following this new route, polystyrene, poly(alkyl (meth)acrylate), polymethacrylic acid, poly(sodium styrenesulfonate) and poly(N-isopropyl) NPs have been synthesized and in-deep characterized.     

值得引入物理化学教材的重要内容:势能面交叉

势能面交叉既是内转换和系间窜越的实际通道,也是联系光化学和热化学的重要桥梁。势能面交叉引起的非绝热过程广泛存在于各种生物、化学和材料的体系当中。笔者建议将势能面交叉的相关内容引入物理化学课程的教材中,因为这不但会使教材内容更加丰富和成体系,而且有利于学生了解科技发展前沿,激发他们探索未知的兴趣。     

Towards More Efficient,Greener Syntheses through Flow Chemistry

Technological advances have an important role in the design of greener synthetic processes. In this Personal Account, we describe a wide range of thermal, photochemical, catalytic, and biphasic chemical transformations examined by our group. Each of these demonstrate how the merits of a continuous flow synthesis platform can align with some of the goals put forth by the Twelve Principles of Green Chemistry. In particular, we illustrate the potential for improved reaction efficiency in terms of atom economy, product yield and reaction rates, the ability to design synthetic process with chemical and solvent waste reduction in mind as well as highlight the benefits of the real‐time monitoring capabilities in flow for highly controlled synthetic output.     

En route to hexaaza-kekulene

As synthetic precursors of hexaaza-kekulene 1 the dibenzo[$\text{b,j}$]= [1,10] phenanthroline systems 2, 3, 4, 5 and 6 were prepared. From 6 via 7 a product was obtained for which the dodecahydrohexaaza-kekulene structure 8 is suggested on the basis of ¹H-NMR comparison with the dipyrido[3,2-$\text{c}$;=2′,3′-$\text{h}$]acridine (14) for which two syntheses are reported.     

En route to tetra-t-butylethylene

Triene 22 was synthesized and converted by two different routes to the tetraaldehyde 27, the first “non-bridged” derivative of 1. An X-ray analysis of 27 reveals that the CC bond is twisted by 28.6°. Attempts to prepare 1 from 27 failed, thus far. Addition of t-butyllithium to 13, subsequent reaction with ethylene oxide and further reactions led to the highly strained tri-t-butylethylethylene 33; according to force field calculations 33 has the largest fragmentation strain of all synthesized hydrocarbons of this type.     

有机化学实验的绿色化路线设计

引入绿色化学的观念,把绿色化学融合到有机化学实验教学中。设计了2个有机化学合成实验,与传统的实验相比较,达到了在实验中减少环境污染的目的,让学生有机会了解化学发展的趋势,认识绿色化学研究的思想方法和技术路线,同时培养了学生的绿色环保意识。