Metathesis and metallotropy: a versatile combination for the synthesis of oligoenynes

We have demonstrated that the combined use of enyne metathesis and metallotropic [1,3]-shift of the corresponding alkynyl ruthenium carbenes is a powerful synthetic tool to construct oligoenynes. In this reaction, alkynyl carbene intermediates formed from an initial ring-closing metathesis reaction (RCM) undergo repetitive [1,3]-shifts and RCMs to give the final products. Linear poly-1,3-diynes containing repeating functionality of the type -[XCH2CCCCCH2]n- generated long-chain conjugated oligoenynes up to n = 5.     

A novel, selective, and efficient route to carotenoids and related natural products via Zr-catalyzed carboalumination and Pd- and Zn-catalyzed cross coupling

[structure: see text]. A highly efficient and stereoselective protocol for the syntheses of symmetrical and unsymmetrical carotenoids involving Zr-catalyzed carboalumination of conjugated oligoenynes and Pd- and Zn-catalyzed alkenyl-alkenyl coupling has been developed and applied to the syntheses of beta- and gamma-carotene and vitamin A. gamma-Carotene of > or =99% isomeric purity was prepared in three linear steps (five steps overall) from beta-ionone, enyne 8, (E)-ICH=CHBr, and (E)-Me3SiC triple bond CCH=CHBr in 32% overall yield.     

Study of cross-conjugated iso-polytriacetylenes and related oligoenynes

Two series of monodisperse cross-conjugated oligomers based on enyne repeat units have been realized. The first class of molecules, iso-polytriacetylenes (iso-PTAs, 2), was divergently synthesized using an iterative sequence of palladium-catalyzed cross-coupling reactions of vinyl triflate 5 with terminal alkynes. The second series of oligoenynes (17-20) are based on an octatetrayne backbone, and result from homocoupling of the differentially protected iso-PTA oligomers 8-11. The longest member of this series, 20, spans ca. 5.6 nm from Si atom to Si atom and is composed of a contiguous sequence of 44 sp and sp(2) carbons. The lowest energy electronic absorption band for iso-PTA dimers in the progression 13 --> 9 --> 16 is consistently red-shifted as a result of extending the cross-conjugated structure. A similar comparison within each series (i.e., 16, 6-7, or 17-20), however, suggests little effect on the electronic characteristics of these molecules as oligomer length is increased. The solid-state properties of one derivative, 17, are also described.     

超声膜扩散法制备纳米银粒子

采用超声膜扩散(Ultrasound-assisted reaction method, UAMR)的方法, 以NaBH4还原溶液中的银离子, 在聚乙烯吡咯烷酮(PVP)为稳定剂的条件下, 制备出了平均粒度为4.7 nm的银纳米粒子. 与超声滴加法(Ultrasound-assisted dropping reaction method, UADR)相比, 超声膜扩散法制备出来的银纳米粒子的粒径小, 且尺寸分布较窄.     

Lithium diisopropylamide-mediated reactions of imines, unsaturated esters, epoxides, and aryl carbamates: influence of hexamethylphosphoramide and ethereal cosolvents on reaction mechanisms

Several reactions mediated by lithium diisopropylamide (LDA) with added hexamethylphosphoramide (HMPA) are described. The N-isopropylimine of cyclohexanone lithiates via an ensemble of monomer-based pathways. Conjugate addition of LDA/HMPA to an unsaturated ester proceeds via di- and tetra-HMPA-solvated dimers. Deprotonation of norbornene epoxide by LDA/HMPA proceeds via an intermediate metalated epoxide as a mixed dimer with LDA. Ortholithiation of an aryl carbamate proceeds via a mono-HMPA-solvated monomer-based pathway. Dependencies on THF and other ethereal cosolvents suggest that secondary-shell solvation effects are important in some instances. The origins of the inordinate mechanistic complexity are discussed.     

Synthesis of tert-butyl peresters from aldehydes by Bu4NI-catalyzed metal-free oxidation and its combination with the Kharasch-Sosnovsky reaction

A new tert-butyl peresters synthesis directly from aldehydes and TBHP was developed via Bu(4)NI-catalyzed aldehyde C-H oxidation. Mechanistic studies suggest that the protocol proceeds via a radical process. Combining the method with the Kharasch-Sosnovsky reaction offers a practical approach for the synthesis of allylic esters from simple aldehydes and alkenes via a two-step one-pot procedure.     

On the relative migratory aptitudes of carbon and heteroatoms in borate complexes. A surprising alpha-thia effect

Dimethylsulfoxonium methylide reacts with dicyclohexyl alpha-thiophenylmethyl)methylborane via 1,2-alkyl migration and alkyldithiaborolane via 1,2-sulfur migration.     

Bimolecular recombination of charge carriers in molecularly doped polycarbonate

The general problems of the bimolecular recombination of charge carriers in molecularly doped polycarbonate are discussed. Experimental studies are performed via the transient radiation-induced conductivity method. Transient-current curves are numerically calculated via the multiple trapping model. The calculated and experimental curves are in good agreement. It is shown that the studied molecularly doped polymer undergoes bimolecular recombination via the Langevin mechanism.     

Polymer-protein conjugates from omega-aldehyde endfunctional poly(N-vinylpyrrolidone) synthesised via xanthate-mediated living radical polymerisation

Aldehyde omega-endfunctional poly(N-vinylpyrrolidone) was synthesised via quantitative conversion of a xanthate endfunctional precursor obtained via RAFT-mediated polymerisation.     

Three different dimerizations of 2-bromo-3-methyl-1,4-naphthoquinones

Three types of dimeric naphthoquinones, which possess structurally diverse skeletons, can be prepared in one step from 2-bromo-3-methyl-1,4-naphthoquinones. 2,2'-Dimeric naphthoquinones were prepared by a one-pot Stille-type reaction via vinylstannanes. Oxepines are formed by unexpected domino reactions via 1,4-dihydroxynaphthalene species. Epoxides are formed by a Michael/Darzens reaction via the o-quinone methides.     

Quasiclassical trajectory study of fast H-atom collisions with acetylene

Translationally hot H collisions with the acetylene are investigated using quasiclassical trajectory calculations, on a recent full-dimensional ab initio-based potential energy surface. Three outcomes are focused on: non-reactive energy transfer via prompt collisions, non-reactive energy transfer via the formation of the vinyl complex, and reactive chemical H-atom exchange, also via complex formation. The details of these outcomes are presented and correlated with the collision lifetime. Large energy transfer is found via complex formation, which can subsequently decay back to reactants, a non-reactive event, or to new products, a reactive event. For the present system, these two events are experimentally indistinguishable.     

Anion-anion assembly: a new class of anionic supramolecular polymer containing 3,4-dichloro-2,5-diamido-substituted pyrrole anion dimers

Two molecules containing two 2,5-diamido,3,4-dichloropyrrole units linked via 1,3- or 1,4-substituted benzene rings have been synthesized. The pyrrole groups in these compounds deprotonate in the presence of tetrabutylammonium fluoride and form pyrrole anion dimers via NH...N- hydrogen bonds. This dimerization process results in the formation of polyanionic chains linked via an unusual "orthogonal" hydrogen-bonding array.     

An efficient enantioselective synthesis of an indane acetic acid derivative: methyl (2S)-2-[(1S)-5-hydroxy-2,3-dihydro-1H-inden-1-yl]butanoate

We have developed a practical enantioselective synthesis of 1, a novel indane acetic acid derivative with two contiguous stereogenic centers. The key indene acetic acid framework was constructed via a robust, unprecedented Reformatsky process. One stereogenic center was set via a resolution, and the other via a highly diastereoselective hydrogenation of the indene acetic acid.     

Synthesis of the stereoisomers of 17,21-dimethylheptatriacontane - sex recognition pheromone of the tsetse fly

The title compounds were obtained both by application of directed resolution via diastereomeric amides and a novel asymmetric synthesis via ester enolates.     

Catalyst and ring size effects on periselectivity of oxonium ylide rearrangements

Diazoketones were subjected to carbene-transfer with Rh(II) or Cu(II) catalysts to probe the selectivity for rearrangement via five- or six-membered oxonium ylides. 4,5-Bis(benzyloxy) and 4-allyloxy-5-benzyloxy substrates 3a,b showed a large preference for rearrangement via the five-membered ylide under all conditions. However, a sharp divergence was seen with 5-allyloxy-4-benzyloxy substrate 3c, which underwent predominantly a [2,3]-shift to pyran 5c via the six-membered ylide with Cu(II) catalysis and a [1,2]-shift to furan 4c via the five-membered ylide with Rh(II) catalysis.     

A new route to iodine-labeled N-isopropyl iodoamphetamine via organoboranes

N-Isopropyl amphetamineboronic acid was prepared via a new in-situ transmetallation reaction involving sonication. The boronic acid intermediate was radioiodinated via a novel iodination procedure.     

Newer trace elements in Iranian diets

The intake of some “newer” trace elements via the daily diet by Iranian population groups was determined. The total mixed diets were prepared on the basis of dietary recording. Aluminium and nickel have been measured via atomic absorption spectrometry, vanadium and tin via radiochemical neutron activation analysis, and scandium via instrumental neutron activation analysis. The results for the Iranian diets are discussed and compared with corresponding data from other countries. The project has been performed in the frame of a co-ordinated research project of the International Atomic Energy Agency with participants from various countries. This revised version was published online in July 2006 with corrections to the Cover Date.     

Total synthesis of branimycin: an evolutionary approach

The first total synthesis of the macrolactone antibiotic branimycin (4) has been described. The key disconnection leads to a cis-dehydrodecalone core and a polyketide side chain which are connected via organometallic addition. The dehydrodecalone core was targeted via altogether five different approaches featuring various kinds of chiral elements and ring-closing methodology. In the end the most successful method starting from diepoxynaphthalene 109 was chosen to carry on with the synthesis. Thus the oxygen functions and carbon appendages were introduced via organometallic desymmetrization reactions to generate epoxy ketone 107, to which vinyl iodide 11 was added after conversion into the organolithium species. The synthesis was completed by introducing the ester side chain via Michael addition and subsequent macrolactonization.     

Utilization of associated petroleum gas via small-scale power generation

The utilization of associated petroleum gas via power generation in modern gas piston engines or gas turbines involves some difficulties due to a significant content of heavy C₅₊ hydrocarbons which are prone to detonation and soot and tar formation. Two pretreatment technologies were described for associated petroleum gas utilization via power generation. Selective oxycracking of heavy hydrocarbons in associated petroleum gas allows their conversion to lighter and high octane molecules. The conversion of associated petroleum gas into syngas with the use of volumetric radiant matrix not only enables utilization of associated petroleum via power generation but also opens prospects for its complete utilization with production of synthetic liquid hydrocarbons via Fischer-Tropsch synthesis route.     

A direct route to isoflavan quinones. The synthesis of colutequinones A and B

The first syntheses of colutequinone A and colutequinone B were achieved. Radical generation via phenyliodoso diacetate was superior to radical generation via ammonium persulfate.