bluuebunny/crossref_metadata_2025 huggingface dataset:
Enantioselectivity Control with Metal Colloids as Catalysts
Authors: Bönnemann Helmut , Braun Gerhard A.
Published: July 2007

Metal colloids protected with chiral molecules can lead to a new type of enantioselective catalyst combining good selectivity control with an extraordinarily high activity in hydrogenation reactions. This concept has been applied for the first time in the form of platinum sols stabilized by the alkaloid dihydrocinchonidine.
Tripodal Synthetic Peptide Bundles
Authors: Trojandt Gunter , Herr Ulrike , Polborn Kurt , Steglich Wolfgang
Published: July 2007

The stereochemical course of the formation of the two diastereomers of trimethyl 2,2′,2″‐nitrilotris[2‐(benzoylamino)acetate], 2a and 2b, is described. The structures of both isomers were confirmed by X‐ray diffraction studies. Diastereomer 2b could be obtained in larger quantities by epimerisation of 2 a to 2 b with catalytical amounts of NaOMe. The (RRR/SSS)‐triester 2 b is a suitable template for the synthesis of tripodal peptide bundles. Saponification of 2 b yielded the C3‐symmetrical racemic triacid 4 b, which was coupled with amino acid methyl esters and dipeptide esters to give pseudo‐hexapeptides and pseudo‐nonapeptides, respectively. The resulting mixtures of diastereomers were easily separated by crystallisation. Their absolute configuration at the template unit (RRR or SSS) was established by means of the CD spectra. The pseudo‐hexapeptide (SSS)N(BzGly*ValOMe)3 (14) was saponified to yield the optically pure triacid (SSS)N(BzGly*ValOH)3 (23). Compound 23 is an ideally preorganised template for the production of longer tripodal peptides. This was illustrated by the synthesis of two pseudo‐pentadecapeptides. Peptide bundles with polar side chains (histidine and serine) or end groups (catechol or hydroxamate units) were synthesised by using the templates 4 b, 22 and 23 as anchors.
Methyl Aryl Ketones in the Synthesis of Hexaazabicycloicosane Cage Complexes
Authors: Angus Patricia M. , Bygott Alexia M. T. , Geue Rodney J. , Korybut‐Daszkiewicz Bohdan , Mau Albert W. H. , Sargeson Alan M. , Sheil Margaret M. , Willis Anthony C.
Published: July 2007

The template syntheses of various CoIII‐hexaazaicosane‐type cage complexes with aromatic substituents are described. The parent template, [Co(sen)]3+ ion {sen = 4,4′,4″‐ethylidynetris(3‐azabutan‐1‐amine)}, paraformaldehyde, a methyl aryl ketone and a base were reacted in acetonitrile to give aroyl substituents attached to the fully saturated sarcophagine cage (1‐aroyl‐8‐methyl‐3,6,10,13,16,19‐hexaazabicyclo[6.6.6]‐icosane cobalt(III) ion) and aryl substituents attached to an analogous imine‐type cage (2‐aryl‐8‐methyl‐3,6,10,13,16,19‐hexaazabicyclo[6.6.6]icosa‐2‐ene cobalt(III) ion). Phenyl, napthyl, phenanthryl, anthracenyl and anthraquinonyl substituents have been bound to the cage simply by this route to give molecules that can act as intercalators in DNA or as photosensitizers attached to reversible redox‐active metal centres and can couple an organic two‐electron reagent with an inorganic one‐electron reagent. An X‐ray crystallographic analysis of a phenanthroyl cage complex has also been carried out. These substances and their progeny should be useful as biological probes and fluorescent indicators and for energy capture and conversion.
Two‐Dimensional Electronic Conjugation: Statics and Dynamics at Structural Domains Beyond Molecular Wires
Authors: Opsitnick Elizabeth , Lee Dongwhan
Published: July 2007

Chemical architectures supporting a high degree of electronic conjugation serve as important functional components in devices and materials for advanced electronic and photonic applications. Increasing the spatial dimensionality of such constructs can fundamentally modify their optoelectronic properties and significantly alter intra‐ and intermolecular interactions that are crucial for understanding and controlling charge/energy‐transfer processes. In this article, emerging design principles in the construction of well‐defined conjugated platforms beyond molecular wires are highlighted. Both covalent and noncovalent approaches can be strategically employed to position one‐dimensional (1D) substructures in a spatially well‐defined manner in order to enhance both structural and functional complexity in a two‐dimensional (2D) setting. A predictable and controllable switching mechanism can be designed and implemented with mobile 2D electronic conjugation that operates by correlated motions of inherently rigid 1D subunits. This emerging “dynamic” approach complements and challenges the prevailing “static” paradigm of conjugated chemical architectures.
Cellular pathways of energy metabolism in the brain: Is glucose used by neurons or astrocytes?
Authors: Nehlig Astrid , Coles Jonathan A.
Published: July 2007

Most techniques presently available to measure cerebral activity in humans and animals, i.e. positron emission tomography (PET), autoradiography, and functional magnetic resonance imaging, do not record the activity of neurons directly. Furthermore, they do not allow the investigator to discriminate which cell type is using glucose, the predominant fuel provided to the brain by the blood. Here, we review the experimental approaches aimed at determining the percentage of glucose that is taken up by neurons and by astrocytes. This review is integrated in an overview of the current concepts on compartmentation and substrate trafficking between astrocytes and neurons. In the brain in vivo, about half of the glucose leaving the capillaries crosses the extracellular space and directly enters neurons. The other half is taken up by astrocytes. Calculations suggest that neurons consume more energy than do astrocytes, implying that astrocytes transfer an intermediate substrate to neurons. Experimental approaches in vitro on the honeybee drone retina and on the isolated vagus nerve also point to a continuous transfer of intermediate metabolites from glial cells to neurons in these tissues. Solid direct evidence of such transfer in the mammalian brain in vivo is still lacking. PET using [18F]fluorodeoxyglucose reflects in part glucose uptake by astrocytes but does not indicate to which step the glucose taken up is metabolized within this cell type. Finally, the sequence of metabolic changes occurring during a transient increase of electrical activity in specific regions of the brain remains to be clarified. © 2007 Wiley‐Liss, Inc.
Synthesis of 1,4‐oxathiocins and 1,3‐oxazines by the reactions of 2‐amino‐4,5‐dihydro‐3‐thiophenecarbonitriles with dibenzoyldiazomethane
Authors: Yamagata Kenji , Ohkubo Kimiko , Yamazaki Motoyoshi
Published: July 2007

The rhodium(II) acetate‐catalyzed reaction of dibenzoyldiazomethane with 4,5‐dihydro‐2‐(4‐morpholinyl)‐ or 2‐(1‐piperidinyl)‐ or 2‐(1‐pyrrolidinyl)‐3‐thiophenecarbonitriles 1 gave the corresponding 4H‐1,3‐oxazin‐4‐ones 3 through the cycloaddition of the cyano group of 1 to benzoylphenylketene which was formed by the Wolff rearrangement of dibenzoyldiazomethane. On the other hand, when bis(1,1,1‐trifluoroacetylacetonato)copper(II) was used instead of rhodium(II) acetate dimer, the corresponding 1,4‐oxathiocin‐7‐carbonitriles 2 and/or 3 were obtained. The formation of 2 could be explained by the mechanism involving the sulfonium ylide derived from the dibenzoylcarbenoid and 1.
Chiral polymethine dyes, V. syntheses, absolute configuration, spectroscopic, and chiroptical properties of chiral dinuclear tri‐ and pentamethinium as well as trinuclear [2.2.2]heptamethinediium cyanine dyes with 3‐sec‐butyl‐1,3‐dimethylindolyl end groups
Authors: Reichardt Christian , Budnik Ulrich , Harms Klaus , Schäfer Gerhard , Stein Jörg
Published: July 2007

Starting with natural monochiral (−)‐(S)‐2‐methyl‐1‐butanol (1a) generated by alcoholic fermentations, we have synthesized the new monochiral heterocyclic iminium salt (+)‐(3R,15S)‐3‐sec‐butyl‐1,2,3‐trimethylindoleninium tetrafluoroborate (11) in an eight‐step reaction sequence. Important steps of this sequence are the zeolite‐catalyzed Fischer synthesis of the monochiral indole 6a from the phenylhydrazone of ketone 5a and the separation of the diastereomeric salts 9 and 10 by fourfold fractional recrystallization from ethanol. With the new quaternary iminium salt 11, new monochiral dinuclear tri‐ (13) and pentamethinium cyanine dyes (14) as well as the trinuclear [2.2.2]heptamethinediium 17 and [1.1.1]tetramethinium cyanine dyes 18 have been synthesized. The absolute configuration of the four stereogenic centers in the cyanine dye 14 has been confirmed by means of an X‐ray structural analysis. Spectroscopical and chiroptical properties of all new cyanine dyes have been determined.
Catalytic gas‐phase oxidation of olefins mediated by Fe(C6H6)+ and a comparison of Fe(L)+ complexes (L = benzene, pyridine, naphthalene)
Authors: Stöckigt Detlef , Schwarz Helmut
Published: July 2007

The catalytic potential of Fe(L)+ complexes (L = benzene, pyridine, naphthalene) with respect to oxygen transfer to alkenes in the gas phase has been investigated by using Fourier‐transform ion cyclotron resonance mass spectrometry. Oxidation of Fe(L)+ by N2O leads to Fe(L)O+ with reaction efficiencies of 86% (L = benzene), 40% (L = pyridine), and 44% (L = naphthalene), respectively. While „naked”︁ FeO+ behaves as a powerful CC‐ and CH‐bond activation reagent, the ligated species Fe(L)O+ are entirely unreactive in that respect. However, oxygen transfer from Fe(L)O+ to olefins occurs at the collision rate with less than 10% formation of byproducts. Indirect evidence is presented suggesting that the O‐atom transfer from Fe(L)O+ to olefins does not generate ketones or aldehydes; rather, epoxide formation is brought about. The largest turnover number (nt) is obtained for the Fe(C6H6)O+/C2H 4 system with nt = 6.
Influence of substituents on the rotational energy barrier of atropisomeric biphenyls — studies by polarimetry and dynamic gas chromatography
Authors: Wolf Christian , König Wilfried A. , Roussel Christian
Published: July 2007

We have synthesized a series of 2,2′‐substituted biphenyls and particularly derivatives of 2,2′‐bis(trifluoromethyl)biphenyl (4) with either one or two fluoro, chloro, bromo, amino, nitro or methoxy substituents in the para positions for investigating the influence of these substituents on the rotational energy barrier of the biphenyls by polarimetry and dynamic gas chromatography (DGC). Axially chiral biphenyls with at least two bulky substituents in the ortho positions were separated by gas chromatography on selectively modified cyclodextrins. Due to competition between resolution and enantiomerization in the gas chromatographic column 2‐tert‐butyl‐2′‐isopropyl‐ (2), 2‐isopropyl‐2′‐(trifluoromethyl)biphenyl (5), and some para‐substituted derivatives of 4 show temperature‐dependent interconversion profiles and coalescence phenomena in analogy to dynamic NMR studies. The accelerating effect on the rate of enantiomerization of electron‐donating groups in the para positions and the decelerating effect of electron‐accepting groups has been determined. Dynamic gas chromatography is shown to be a useful supplement to studies of thermal enantiomerization kinetics of optically enriched or separated compounds by polarimetry for the investigation of energy barriers of atropisomeric biphenyls.
Molecular modeling of saccharides, 6. Small‐ring cyclodextrins: Their geometries and hydrophobic topographies
Authors: Immel Stefan , Brickmann Jürgen , Lichtenthaler Frieder W.
Published: July 2007

A detailed force‐field‐based evaluation of the molecular geometries of small‐ring cyclodextrins 1–3 with three, four, and five α(1→4)‐linked glucose residues and the starch‐derived α‐cyclodextrin (4) was performed by using molecular mechanics and high‐temperature annealing. The resulting minimum‐energy structures reveal that the progressive strain imposed by diminution of the cyclodextrin macrocycle from six glucose units (α‐CD, 4) to five (cycloglucopentaoside 3), four (tetraoside 2), and three (cyclotrioside 1) is reflected in a widening of the intersaccharidic bond angle, in a complex balance of the interrelated glucose tilt angles τ and the glycosidic torsions Φ and Ψ against each other, and most strikingly, in a specific unilateral distortion of the pyranoid rings, i.e. flattening at C‐4 towards the E1 envelope conformation. This successive levelling at C‐4, induced by a decrease of the two related ring torsion angles, is small in the pentamer 3, pronounced in 2, and fully realized in the cyclotrioside 1. – The respective contact surfaces of the minimum‐energy conformers and their molecular lipophilicity patterns (MLP), in color‐coded form, were also generated, allowing an assessment of their capabilities to form inclusion complexes. Accordingly, only the cyclopentaoside 3 exhibits a hydrophobic central cavity similar to that of α‐CD (4), the smaller cyclodextrins 2 and 1 are closed, yet contain a hydrophobic indentation for potential binding.
New sesquiterpene ethers and other constituents isolated from Brazilian vassoura oil
Authors: Weyerstahl Peter , Christiansen Christian , Marschall Helga
Published: July 2007

Three new sesquiterpene ethers, 1,6‐epoxyeudesm‐4(15)‐ene (7), 1,11‐epoxycadina‐4,9‐diene (9) and 5,11‐epoxycadin‐1(10)‐ene (10), some sesquiterpene alcohols, e.g. cis‐ and trans‐dracunculifoliol (3) and 4‐epi‐maaliol (12), as well as 2,2‐dimethyl‐6‐vinylchromene (14) were isolated from Brazilian Vassoura oil. The structures were elucidated by spectroscopic methods.
Radikal‐Stabilisierungsenergie – das MMEVBH‐Kraftfeld
Authors: Roth Wolfgang R. , Staemmler Volker , Neumann Martin , Schmuck Carsten
Published: July 2007

Radical‐Stabilization‐Energy – the MMEVBH Force Field

Making use of the VB method of Malrieu et al.[6–10] a force field has been developped, which allows to calculate heats of formation of hydrocarbons (conjugated and non‐conjugated olefins, radicals, and diradicals) with high accuracy. With this method radical stabilization energies (RSE) for a great number of delocalized radicals are calculated and compared with experimental values, derived from shock‐tube measurements of dissociation energies or from rotational barriers of substituted olefins. A detailed analysis of the RSE with respect to structure, substituents, strain, and aromaticity is presented.
Synthesis of vinca alkaloids and related compounds, 73. Synthesis and ring transformations of deethylaspidospermane derivatives. Synthesis of (±)‐20‐deethyl‐3‐oxotabersonine
Authors: Kalaus György , Juhász Imre , Greiner István , Kajtár‐Peredy Mária , Brlik János , Szabó Lajos , Szántay Csaba
Published: July 2007

The reaction of the key compound 14 with methyl 4‐formyl‐butanoate (9) or with 5‐benzoyloxypentanal (13) gave the D‐seco‐aspidospermane derivatives 17 and 18, respectively. Compound 17 was indirectly and 18 was directly converted to (±)‐20‐deethylvincadifformine (7) and (±)‐20‐deethyl‐20‐epivincadifformine (8). Epimerization occurred in both cases. The thioxo compound 23 was used for the synthesis of (±)‐20‐deethyltabersonine (28) and (±)‐20‐deethyl‐3‐oxotabersonine (27). Oxidative ring transformation of the (±)‐20‐deethylvincadifformine (7) gave (±)‐16‐deethylapovincamine (29).
Electrophilic additions to tetradehydrodianthracene
Authors: Herges Rainer , Neumann Helfried
Published: July 2007

5,12:6,11‐Di[1,2]benzenodibenzo[a,e]cyclooctene (tetrade‐hydrodianthracene, TDDA) (1) reacts with halogens to give transannular (anti) 4, 5 and ring‐opened (syn) products 6, 7. The product ratio shows a remarkable solvent dependence. Solvents of medium polarity favor anti addition, whereas syn addition is observed both with increasing and decreasing polarity. For instance, chlorine in toluene adds 100% syn, in dichloroethane 56 % anti, and in trifluorethanol again 100% syn. It is concluded that, depending on solvent polarity, three different mechanisms for electrophilic addition are operative.
A new synthesis of (±)‐phoracantholide, (±)‐dihydrorecifeiolide, and (±)‐muscone via α‐nitro ketones
Authors: Ballini Roberto , Marcantoni Enrico , Petrini Marino
Published: July 2007

Cα′ alkylation of α‐nitro cycloalkanones 1a–c with methyl iodide at −35 to −70°C in THF/heptane/DMPU/TMDA in the presence of LDA as a base furnished the methylated α‐nitro ketones 2a–c. Denitration of 2a, b with Bu3SnH/AIBN in dry benzene afforded the α′‐methylated ketones 3a, b, which were converted to (±)‐phoracantholide (4a) and (±)‐dihydrorecifeiolide (4b), respectively, by the Baeyer‐Villiger oxidation. Reduction of 2c with NaBH4 and dehydration of the obtained nitro alkanol 5 with Al2O3/DMAP furnished the corresponding nitro alkene 6. Nef conversion of 6 with sodium hypophosphite and Raney nickel afforded (±)‐muscone (7) in good yields.
Benzodiimidazoles by reaction of benzenetetramines and benzenehexamines with dimethylformamide and its dimethyl acetal
Authors: Mataka Shuntaro , Shimojyo Yoshiro , Hashimoto Iwao , Tashiro Masashi
Published: July 2007

Benzo[1,2‐d;3,4‐d′]‐ (9) and ‐[1,2‐d;‐4,5‐d′]diimidazole (10) were prepared by reactions of the corresponding benzenetetramines 1 and 2 with N,N‐dimethylformamide or N,N‐dimethylformamide dimethyl acetal 4. The corresponding reaction of benzenehexamine (3) gave the benzo[1,2‐d;4,5‐d′]diimidazole derivative 8.
Photochemistry of Phenyl Azides Bearing 2‐Hydroxy and 2‐Amino Groups Studied by Matrix‐Isolation Spectroscopy: Generation and Characterization of Reactive o‐Quinoid Compounds
Authors: Tomioka Hideo , Matsushita Takeshi , Murata Shigeru , Koseki Shiro
Published: July 2007

Broad‐band irradiation (λ > 370 nm) of 2‐hydroxyphenyl azide (1) in Ar at 10K monitored by IR resulted in the formation of at least three major products, all of which were shown to be photointerconvertible under these conditions. The two products showing carbonyl stretching absorption bands were assigned to an E/Z mixture of 6‐imino‐2,4‐cyclohexadien‐1‐one (3), while the product showing sharp absorption bands in the cumulenic double bond region was assigned to 6‐imino‐1,3,5‐hexatrien‐1‐one (4). These assignments were fully supported by comparison of the experimental IR spectra with the theoretical data calculated at the HF/6‐31G** level of theory. Similar irradiation (λ > 350 nm) of 2‐aminophenyl azide (6) in Ar at 10 K also afforded three photointerconvertible products, which were assigned to a mixture of (E,E)‐(E,Z)‐ and (Z,Z)‐1,2‐diimino‐3,5‐cyclohexadiene (8). The assignments were again supported by ab initio calculations. The reactions are discussed in terms of a 1,4 H shift in the photolytically generated phenylnitrenes (2 and 8) from the azides (1 and 7).
Comparison of the One‐Bond Nucleophilicities of Monomethyl‐ and Dimethyl‐Substituted 1,3‐Butadienes
Authors: Mayr Herbert , Hartnagel Manfred
Published: July 2007

The kinetics of the reactions of (E)‐2‐methyl‐1,3‐pentadiene (2a), 4‐methyl‐1,3‐pentadiene (2b), and (Z)‐1,3‐pentadiene (2c) with benzhydryl cations have been studied photometrically in dichloromethane solution. Substitution of these rate constants into the linear free enthalpy relationship lgk=s(N + E) yielded the nucleophilicity parameters N for these dienes. The influence of methyl groups on the nucleophilic reactivities of 1,3‐butadienes is discussed, and a system to estimate reactivities of further alkylated 1,3‐dienes is suggested.
Matrix Isolation of o‐Quinoid Compounds – 6‐Imino‐2,4‐cyclohexadien‐1‐one and 1,2‐Diimino‐3,5‐cyclohexadiene
Authors: Morawietz Jens , Sander Wolfram
Published: July 2007

6‐Imino‐2,4‐cyclohexadien‐1‐one (1b) and 1,2‐diimino‐3,5‐cyclohexadiene (1c) were generated in high yields from o‐azidophenol (5b) and o‐azidoaniline (5c), respectively, by irradiation in argon matrices at 10 K or by flash vacuum pyrolysis (FVP) with subsequent trapping of the products in solid argon. The hydrogen shift in the phenylnitrenes 6 formed as primary intermediates is very rapid, and thus the nitrenes are short‐lived transients even under the conditions of matrix isolation. Ring expansion to azacycloheptatetraenes was not observed. FVP produces the thermodynamically most stable E/Z isomers of the quinones as the main products. Photolysis of the matrix‐isolated quinones results in photostationary equilibria of the E/Z isomers. The stereoisomers were identified by comparison of the matrix IR spectra with ab initio calculations at the RMP2‐FC‐6/31G(d) level of theory.
The Configuration of (+)‐8‐Ethylnorlobelol‐I; a Correction of the Literature
Authors: Mill Sibel , Durant Alex , Hootelé Claude
Published: July 2007

The piperidine alkaloid (+)‐8‐ethylnorlobelo‐I is assigned the revised 2S,8,S absolute configuration as depicted in 7. LiAl(O‐tBu)3H reduction of the ketone rac‐5 proceeded with a high stereoselectivity to give, after hydrolysis of the carbamate function, the syn amino alcohol rac‐6. Inversion of the C‐8 configuration of ent‐6 yielded (‐)‐8‐ethylnorlobelol‐I ent‐7. A short nitrone‐alkene‐based synthesis of rac‐7 has been developed.
Relative and Absolute Configuration of the 3,12‐Dihydroxypalmitic Acids
Authors: Jakob Barbara , Gerlach Hans
Published: July 2007

The relative and absolute configuration of (+)‐3,12‐dihydroxypalmitic acid, a constituent of the Ipomea operculata M. resin, has been determined by synthesis. Dimethyl L‐malate was converted via (S)‐(+)‐1 into the oxirane (S)‐(‐)‐2. Reaction of (‐)‐2 with the Grignard reagent of 8‐benzyloxybromooctane provided (S)‐(+)‐3 in 84% yield, and this was converted into the aldehyde (S)‐(‐)‐6 via (S)‐(‐)‐4 and (S)‐(‐)‐5. Reaction with the lithium enolate of methyl acetate gave 7 and 8, which could be converted via 9 and 10, 11 and 12, 13 and 14 into the lactones (4S,13S)‐(+)‐15 and (4R,13S)‐(+)‐16 and finally into the methyl esters (3S,12S)‐(+)‐17 and (3R,12S)‐(+)‐18 and acids (‐)‐19 and (+)‐20. The erythro configuration of (+)‐17 was established by a stereoselective synthesis starting from (S)‐(‐)‐2 via (S)‐(+)‐21, (+)‐22, (+)‐23, the Grignard reagent of (+)‐24 and (R)‐(+)‐2‐(2‐benzyloxyethyl)oxirane to give (3S,12S)‐(‐)‐25, (‐)‐26, (+)‐27 and (+)‐28, which could be oxidized to the diacetoxy acid (3S,12S)‐(+)‐29. Saponification and esterification gave (3S,12S)‐(‐)‐19 and (3S,12S)‐(+)‐17, with properties identical to those of (+)‐17 obtained from the resin glycoside.
Synthesis of (−)‐Streptenol A, (±)‐Streptenol B, C and D
Authors: Blechert Siegfried , Dollt Heribert
Published: July 2007

2‐(2,2‐Dimethyl‐1,3‐dioxan‐4‐yl)acetaldehyde (3) was used for the preparation of streptenol A and B (Scheme 1) via a Grignard reaction with 1‐bromopent‐3‐ene. Hereby optically pure (4′R)‐3 gave the antipode of Streptenol A. Reaction with lithiated 1‐pentyne opened access to streptenol C and D. To obtain the dienone structure of streptenol C and D, a palladium‐catalyzed alkynone isomerization was induced. Kinetic differences in the acid‐mediated cleavage of the 1,3‐acetonide protected 1,3,5‐triol system caused the stereoselectivity in the natural products. So only the (3S*,5R*) acetonide of streptenol B reacted under mild hydrolytic conditions and gave after transacetalization first a 3,5‐protected streptenol B with pure relative stereochemistry and finally (3S*,5R*)‐streptenol B.
Chemistry of Opium Alkaloids, XLIII – A PM3 Computational Approach to the Selectivity in the Diels‐Alder Reactions of Thebaine and Analogues with Methyl Propenoate
Authors: Baas Jan M. A. , Woudenberg Richard H. , Maat Leendert
Published: July 2007

Geometrically fully‐optimized transition structures are calculated for the Diels‐Alder adducts of methyl propenoate and the 3‐demethoxy derivatives of the dienes thebaine, 6‐demethoxythebaine, 5β‐methylthebaine, and 5β‐methyl‐6‐demethoxythebaine using the semiempirical PM3 method. The number of calculated forms is limited according to a search of the conformational space of methyl propenoate and of the dienes. The lengths of the incipient bonds in the transition structures vary from 2.0 to 2.4 Å, but the average value of the lengths of these two bonds in the same structure is much more constant: 2.18 ± 0.02 Å. The effects of the methoxycarbonyl group and of the 6‐methoxy group on the lengths of the incipient bonds are in agreement with the relative magnitudes of the frontier orbital coefficients. Comparison of the energetic data with experimental results shows that the prediction of the stereo‐ and regioselectivities from the Gibbs energy of activation differences is good as far as it concerns the main products. The absence of minor products and the formation of the remarkably large amount of 8β‐ethoxycarbonyl‐6β,14β‐ethenomorphinan from 5β‐methyl‐6‐demethoxythebaine does not follow from the calculations.
Reactions of 3,5‐Diaryl‐1,2‐dithiolium Salts and 3,5‐Diaryl‐1,2,4‐dithiazolium Salts with Metal Cyclopentadienides
Authors: Hartke Klaus , Kraska Andreas , Massa Werner , Molinier Michel , Wocadlo Sigrid
Published: July 2007

Reaction of the 3,5‐diaryl‐1,2‐dithiolium salts 2 with the metal cyclopentadienides 4 and 9 leads to the formation of the tricyclic compounds 6 and 10 via a ring‐opened intermediate, which undergoes an intramolecular Diels‐Alder cyclization. Compounds 6 and 10 rearrange by treatment with boron trifluoride‐diethyl ether to the isomeric compounds 7 and 11. The structures 6f and 7f were confirmed by X‐ray analysis. Condensation of the 3,5‐diaryl‐1,2,4‐dithiazolium salts 3 with the metal cyclopentadienides 4 affords the tricyclic compounds 12, which do not rearrange by Lewis acid catalysis.
Stereoselectivity of 7‐Methyl‐7‐norborn(en)yl Cations
Authors: Herrmann Roland , Kirmse Wolfgang
Published: July 2007

7‐Methyl‐7‐norbornyl cations were generated, starting from 1‐methylbicyclo[3.2.0]hept‐2‐yl substrates. While the exo diazonium ion (26) reacted exclusively with migration of the C‐1–C‐7 bond, competitive shifts of C‐7 and C‐5 were observed with the endo diazonium ion (25) and the endo brosylate (24). The stereorandom capture of 7‐methyl‐7‐norbornyl cations was demonstrated by means of deuterium labels, thus supporting the classical structure 12 (C2v). The inductive and hyperconjugative effects of the 7‐Me group in 12 override the σ delocalization that is characteristic of the analogous secondary cation (7, C1). 1,2‐Dimethylbicyclo[3.2.0]hept‐2‐yl cations (34), labeled with deuterium, were shown to give 1,7‐dimethyl‐7‐norbornanol (35) without undergoing a degenerate shift of C‐5. A variety of 1‐methylbicyclo[3.2.0]hept‐6‐en‐2‐yl substrates were found to display a similar pattern of rearrangements as their saturated analogs. However, the 7‐methyl‐7‐norbornenyl cation (48) thus generated accepts nucleophiles stereoselectively, i.e., the delocalization of π electrons to C‐7 is not affected by methyl substitution.
Photoactive Phanes, II. – X‐ray Structure and Photoreactivity of pseudo‐gem Cinnamophane Dicarboxylic Acid {bis‐4,15‐(2′‐Hydroxycarbonylvinyl)[2.2]paracyclophane}
Authors: Meyer Ulrich , Lahrahar Nacer , Marsau Pierre , Hopf Henning , Greiving Helmut , Desvergne Jean‐Pierre , Bouas‐Laurent Henri
Published: July 2007

The pseudo‐gem cinnamophane dicarboxylic acid 1 was shown to undergo a stereospecific [2 + 2] photocycloaddition in the solid state, generating the truxinic acid derivative 2 in 100% chemical yield; the reaction can proceed to completion because it is not an “independent site” solid state reaction. The X‐ray structure of 1 reveals that its molecules are associated in dimers formed from four almost linear hydrogen bonds. A salient feature is the intramolecular centre‐to‐centre distance between the reacting double bonds of ≈3.37 Å, the smallest known to date. In contrast to trans‐cinnamic acid, the topochemical [2 + 2] photocycloaddition occurs also in solution, with a quantitative chemical yield and a quantum yield of ≈0.55 (in methanol).
Bimolecular Formation of Radicals by Hydrogen Transfer, 12. – Transfer hydrogenation of p‐Substituted α‐Methylstyrenes and of 9‐Methylenefluorene as a Criterion of Mechanism
Authors: Friebolin Heike , Roers Rolf , Ebenhoch Jochen , Gerst Matthias , Rüchardt Christoph
Published: July 2007

The uncatalyzed transfer hydrogenation of substituted α‐methylstyrenes with 9,10‐dihydroanthracene (DHA), xanthene (XAN), or 9,10‐dihydroacridine (DHAC) was studied mechanistically. The three hydrogen donors react at very similar rates and with similar activation parameters and with little discrimination between the various substituted styrenes. The kinetic isotope effects are also similar and the solvent effect is small. A hydrogen atom transfer mechanism (retrodisproportionation) is, therefore, preferred to a hydride transfer mechanism. This is supported by the very similar reactivity of the hydrogen transfer reaction of DHA and XAN with 9‐methylenefluorene. The product yields in all reactions investigated in this project were >90%.
New Synthetic Routes to Alkyl‐Substituted and Functionalized Perylenes
Authors: Schlichting Peter , Rohr Ulrike , Müllen Klaus
Published: July 2007

3,9(10)‐Dialkynyl‐ and 3‐monoalkynyl‐substituted perylenes (16 and 20) and their ω‐functionalized derivatives 27, 30, 33, 35 can be synthesized easily and in high yields as well as the 3‐alkyl‐9(10)‐alkynyl‐substituted perylenes 39 and 42. This is achieved by a palladium‐catalyzed coupling reaction of monobromoperylenes 19 and 38 and dibromoperylene 15 with 1‐alkynes and ω‐functionalized 1‐alkynes, respectively. The quantitative conversion of the alkynes into the corresponding alkyl‐substituted perylene derivatives 17, 21, 28, 31, 34, 36, 40 and 43 can be accomplished by catalytic hydrogenation. Further conversions of 28, 31, 34, 36, 40 and 43 provide the functionalized perylenes 29, 32, 37, 41 and 44.
Rotaxanes with Chiral Stoppers and Complexation of Cations by Podand Axles with Sugar Terminal Groups
Authors: Archut Andreas , Müller Walter Manfred , Baumann Sven , Habel Mathias , Vögtle Fritz
Published: July 2007

The new π‐donor/π‐acceptor type rotaxanes 15, 16 bearing chiral sugar units as blocking groups have been synthesized in 2% and 6% yield, starting from acetobromo glucose 10 and the podands 8 or 9, followed by the reaction of the resulting axles 11 and 12 with the dicationic bis(bipyridinio) wheel precursor 13 and the bis(bromomethyl) compound 14. The capability of the podands 11 and 12 and of the rotaxanes 15 and 16 to form podate‐like complexes with metal salts was investigated by NMR; the effect of metal thiocyanate salt addition was monitored. Distinct changes of proton shifts that indicate complex formation were only observed between the free axles 11, 12 and the metal salts, whereas no major effects were found in case of the rotaxanes 15, 16. Glucose entities were chosen as stopper units in order to support metal complexation as terminal groups and to introduce a source of chirality into the molecules.
Synthesis of Sterically Rigid Macrocycles by The Use of Pressure‐Induced Repetitive Diels–Alder Reactions
Authors: Benkhoff Johannes , Boese Roland , Klärner Frank‐Gerrit
Published: July 2007

Syntheses of syn‐dimethanotetrahydroanthracene, ‐tetracene, and ‐pentacene derivatives 8a–d, 9a–d, and syn‐25 are described. Highly stereoselective, pressure‐induced Diels‐Alder reactions of the bis‐dienophiles 8a and/or 9a with the bis‐diene 7 lead to the sterically rigid macrocycles 1a, 2a, and 3a/3b, which have well defined cavities of different sizes.
Differential Recognition of Chirality at C‐2 of 2‐Fluoro‐ and 2‐Methylmevalonate by Mevalonate Kinase – Absolute Configuration of Enzymatically Active 2‐Fluoromevalonate
Authors: Wilde Jürgen , Eggerer Hermann
Published: July 2007

Both 2‐methyl‐ and 2‐fluoromevalonate are alternative substrates of mevalonate kinase, but only the fluorinated compound is phorphorylated stereospecifically with respect to the chiral C‐2. The absolute configuration of the racemic pairs of 2‐fluoromevalonate is deduced from 1H‐ and 13C‐NMR data. The fluorine substituent shifts the equilibrium between the conformers of mevalolactone. Coupling constants found experimentally for 2‐fluoromevalonate are calculated satisfactorily with INDO‐MO data. Enzymatic analysis of components of the racemic pairs of 2‐fluoromevalonate with mevalonate kinase combined with NMR data yield the absolute configuration of the enzymatically active steroisomer, (2R,3R)‐2‐fluoromevalonate.
Synthesis and Stereochemistry of Diepoxynaphthacenes and Tetraepoxyheptacenes
Authors: Meier Herbert , Rose Bernd
Published: July 2007

Cycloaddition reactions of 1,4‐dihydro‐1,4‐epoxynaphthalenes (2) and benzo[c]furans (4), generated in situ from the tetracyclone adducts of 2, yield the diepoxynaphthacenes 5 (Schemes 1, 2). Out of 4 possible stereoisomers an exo‐exo and an exo‐endo geometry (A and C in Scheme 3) can be realized. The ratio A:C varies from pure A to pure C depending on the substituents of 2 and 4. Stereoisomeric heptacenes 7 can be obtained by a twofold cycloaddition of the same type (Scheme 4). Only one π bond of reactive alkynes such as cyclooctyne or acetylenedicarboxylate is capable of an addition to 4 (Schemes 5, 6).
Photochemical Formation of Heteromethylenecyclopropanes, 27. Annulated Tetrazolium Salts
Authors: Quast Helmut , Balthasar Jürgen , Fuss Andreas , Nahr Uwe , Nüdling Wolfgang
Published: July 2007

Lithiation of the annulated tetrazoles 6a, c with butyllithium yields the N‐lithiotetrazoles 7a, c which are allowed to react with alkyl halides. Alkylation at the α‐carbon atom is observed in the reaction with methyl iodide (→ 6b, d), 1‐bromo‐2‐chloroethane (7c→14a), and 1,3‐dibromopropane (→ 16, 17), while 1,2‐dichloro‐ and 1,2‐dibromoethane give other products, viz. 11–13. Quaternisation of 6 with dimethyl sulphate affords mixtures of 1‐methyl‐ (1) and 2‐methyltetrazolium salts (8) (3:1–4:1) from which the hexafluorophosphates 1 PF6 are obtained by crystallisation. Methyl triflate converts the ω‐azidonitriles 9 into the N‐methylnitrilium triflates 10 which immediately undergo an intramolecular 1,3‐dipolar cycloaddition to afford the 1‐methyltetrazolium triflates 1 F3CSO3. Cyclisation of 16 by intramolecular N‐alkylation furnishes the bisannulated tetrazolium bromide 3a · Br. Attempts to obtain the lower homologue 15, either from 14a or from 17, met with failure. Instead, 17 rearranges via 15b into 14b. The α‐branched tetrazole 24 is synthesised from ethyl cyanoacetate and the bromide 18. Double cyclisation of 24 affords the bisannulated tetrazolium chloride 3b · Cl. The analogous scheme, envisaged for the synthesis of the lower homologue 32, failed in the last step owing to the high strain of this system.
A Novel Synthesis and the Autoxidation of 2,4,6,8‐Tetraphenylbarbaralane
Authors: Quast Helmut , Becker Christian , Peters Eva‐Maria , Peters Karl , von Schnering Hans Georg
Published: July 2007

Addition to the diphenyldiketone 11 of the reagent prepared from rigorously dried cerium(III) chloride and phenyllithium yields the tetraphenyldiol endo,endo‐13 after extended periods of time. The configuration of this diol, which was previously assigned the (incorrect) configuration exo,exo‐13, is established by an X‐ray diffraction analysis. Dehydration of endo,endo‐13 with sulphuric acid in acetic acid affords a mixture of the oxatwistane 16 and the tetraphenyldiene 17 (3:1). Only the latter is obtained from endo,endo‐13 by the action of boron trifluoride–diethyl ether in dichloromethane solution. While attempts at allylic bromination of 17 with N‐bromosuccinimide in conventional solvents inevitably lead to complete decomposition, use of cyclohexane as solvent allows to obtain solutions of allylic dibromides that can be cyclised with the zinc‐copper couple to afford tetraphenylbarbaralane 9 in 58% yield based on 17. Thus, 9 is now available from the diphenyldiketone 11 in only three steps with an overall yield of 42%. – The autoxidation of 9, studied in various solvents, yields mixtures of products of which the epoxyendoperoxides 20 and 21, the unsaturated ketone 22, and a labile, probably oligomeric or polymeric product C are isolated in pure form. The structures 21 and 22 are elucidated by X‐ray diffraction analysis and independent synthesis, respectively. On warming, 20 rearranges into 21. Traces of acid convert the unknown autoxidation product A instantaneously into the unsaturated ketone 22. – Rate studies show that the autoxidation of 9 starts after a short initiation period. 2,6‐Di‐tert‐butyl‐4‐methylphenol exhibits powerful inhibitory effects. These results demonstrate the free radical nature of the autoxidation of 9. – The results are interpreted in terms of a mechanistic scheme involving initiation of the radical chain by addition of triplet oxygen to 9 to generate the diradical 28 followed by a radical chain 1:1 copolymerisation of 9 and oxygen to produce diradical 29. Eventually, endo addition of oxygen to 29 gives rise to the formation of the epoxyendoperoxides 20 and 21. Intramolecular hydrogen abstraction from the methylene group (C9) is accompanied by cleavage of a bridgehead bond (C1–C2) and the adjacent O–O bond to afford the extremely labile product A (perhaps 32 or 33) which undergoes acid‐catalysed elimination to furnish ketone 22.
Methylchalcogen Derivatives of 1,4,5,8‐Tetrathiatetralin
Authors: Nakatsuji Shin'Ichi , Amano Yoshiki , Kawamura Haruki , Anzai Hiroyuki
Published: July 2007

Tetrakis(methylthio)‐ and tetrakis(methylseleno)‐1,4,5,8‐tetrathiatetralin 3 and 4 were prepared by the reaction of tetrathiatetralin 2 with LDA (lithium diisopropylamide) and MeS‐SMe or MeSeSeMe in diethyl ether. A remarkable solvent effect was observed in that the similar reaction in tetrahydrofuran gave tetrakis(methylthio)tetrathiafulvalene 5 through a novel rearrangement. The iodine complex of 3 was prepared to be a rare example of complex formation in the tetrathiatetralin series.
Ozone Chemistry of Acetylenes: Trapping of α‐Oxo Carbonyl Oxides by Methanol and Stabilization of α‐Oxo Hydroperoxides by Derivatization
Authors: Griesbaum Karl , Dong Yuxiang
Published: July 2007

Ozonolyses of four symmetrically substituted (13a–13d) and of four unsymmetrically substituted acetylenes (17a–17d) gave all of the expected α‐oxo methoxyhydroperoxides (14a–14d, 18ae–18de, 19ae–19de) with good selectivities. The labile, non‐isolable hydroperoxides were converted into the corresponding O‐methyl oximes (15, 18f, 19f), which could be isolated as stable compounds.
Azo Bridges from Azines, XXIV. Tuning the Addition of Thiols to Parallel CC/NN Bonds from Transannular to Normal Addition
Authors: Hünig Siegfried , Brand Uwe , Peters Karl , Prokschy Frank , von Schnering Hans Georg
Published: July 2007

As an example of system A, compound 5 was reacted with several thiols. A radical route procedures transannular 1,4‐addition products (6a/b). In the presence of sulfur, the addition reaction is strongly enhanced, and a polar route yields 1,2‐adducts (7a/b). Structures 6a/b and 7a/b are supported by X‐ray analysis. The different reaction modes are discussed.
Pheromone Synthesis, CLXXXI. Synthesis of the Racemate and the Enantiomers of (Z)‐7,15‐Hexadecadien‐4‐olide, the Sex Pheromone of the Yellowish Elongate Chafer, Heptophylla picea
Authors: Nakayama Tôru , Mori Kenji
Published: July 2007

(4R,7Z)‐7,15‐Hexadecadien‐4‐olide (1), the female‐produced sex pheromone of the yellowish elongate chafer (Heptophylla picea), was synthesized by lipase‐catalyzed resolution of (±)‐10. The racemate and the opposite (S)‐enantiomer of the pheromone were also synthesized. Both (R)‐1 and (±)‐1 showed the pheromone activity.
A Hetero Retro Diels–Alder Reaction in Aqueous Solution: A Dramatic Water‐Induced Increase of the Equilibrium Constant and Inhibition of Cycloreversion
Authors: Wijnen Jan W. , Engberts Jan B. F. N.
Published: July 2007

The adduct of the Diels–Alder reaction of nitrosobenzene with cyclopentadiene is not stable in solution. The equilibrium constant for the reaction depends strongly on the medium and water induces a spectacular shift to the adduct. Comparison with the bimolecular addition of nitrosobenzene to 1,3‐cyclohexadiene enables separation of the effect of the aqueous medium on the rate constants for the forward and reverse reaction. In water, the former reaction is accelerated and the latter is retarded. The deceleration of the retro reaction in water is due to hydrogen‐bond stabilization of the adduct. The influence of cosolvents and micelles on the cycloreversion is discussed.
First Total Synthesis of (+)‐Membrine
Authors: Wirth Thomas
Published: July 2007

The easily accessible, optically active diselenide 1 is converted into the chiral selenium electrophile 2 and used for oxyselenenylation reactions of alkenes with functionalized nucleophiles. This reaction allows the synthesis of addition products 4, 6 and 10 with the selenium functionality as precursors for an intramolecular radical cyclization. This strategy is applied to the first total synthesis of optically pure (+)‐membrine (13) by a short synthetic sequence.
Band‐Shaped Structures by Repetitive Cycloaddition Reactions of Benzo[1,2‐b:4,5‐b′]bisthiete
Authors: Meier Herbert , Rose Bernd , Schollmeyer Dieter
Published: July 2007

Benzo[1,2‐b:4,5‐b′]bisthiete (1) undergoes stepwise thermal opening of the 4‐membered rings, thereby generating two highly reactive 8π systems (Scheme 1). In the presence of 1,4‐dihydro‐1,4‐epoxynaphthalenes 2, the dithiaheptacene derivatives 3 are formed, dehydration of which leads to the compounds 4 and 5 (Scheme 2). Repetitive cycloaddition reactions of 1 with the twofold 2π component 11 (Scheme 3) yield the oligomers 15 with statistically bent (and possibly cyclic) structures (Schemes 4 and 5). While 15 and the related oligomers 14 are freely soluble in many organic solvents, the dehydrated systems 16 (Scheme 6) are totally insoluble.
Diradical Wells, IX. Stereochemistry of Cyclopropane Ring Opening: The Case of 1,1‐Difluoro‐2,3‐diphenylcyclopropane
Authors: Roth Wolfgang R. , Wasser Thorsten , Boenke Martin
Published: July 2007

From the O2‐dependence of the trapping rate of 1,1‐difluoro‐2,3‐diphenylcyclopropane in supercritical CO2 in the temperature range 110–180°C and the rates of its geometrical isomerization and racemization of the trans‐isomer, the energy profile for the geometrical isomerization is derived. Assuming that 7 is a common intermediate, the temperature dependence of the ratio of the rates of isomerization/racemization leads to a lowering of 1.2 kcal · mol−1 for the activation energy for the disrotatory cyclization of the intermediate diradical compared to the conrotatory mode. The energy barriers associated with cyclization of the diradical amount to 2.1 and 0.9 kcal · mol−1, respectively.
Geminal Substituent Effects, 15. Enthalpies of Formation of a Series of Fluorinated Hydrocarbons and Strain‐Free Group Increments to Assess Polar and Anomeric Stabilization and Strain
Authors: Schaffer Frank , Verevkin Sergey P. , Rieger Hans‐Jürgen , Beckhaus Hans‐Dieter , Rüchardt Christoph
Published: July 2007

Enthalpies of combustion, ΔH, and enthalpies of vapourization, ΔH, for nineteen fluorohydrocarbons containing the groups CF, CF2, and CF3 were measured calorimetrically. From these data, enthalpies of formation, ΔHf(g), were calculated and were partitioned into new strain‐free group increments of the Benson and v. Schleyer type: CH3[F] = −42.1; CH2[F, C] = −37.1; CH[F, 2 C] = −35.7; C[F, 3 C] = −39.6; CH2[2 F] = −61.0; CH[2 F, C] = −71.7; C[2 F, 2 C] = −67.3; CH[3 F] = −110.8; C[3 F, C] = −131.4; C[4 F] = −150.4; F[C] = −195.7 kJ mol−1. These increments now allow the calculation of the enthalpies of formation of a large family of homologous fluorohydrocarbons. Fluorine substitution, in general, stabilizes saturated hydrocarbon structures. A new concept, which was proposed recently for ethers, acetals, ketals, and orthoesters, for the analysis of the values of the increments for CH3, CH2, CH, and C groups with their different neighbouring atoms (given in brackets), is successfully applied to fluorohydrocarbons. In this method, the number of pairwise geminal interactions XCY between all attached atoms X and Y is counted. The anomeric stabilization in geminal difluoromethylene groups taken from the FCF interaction is found to be 8 kJ mol−1 smaller than the OCO interaction for acetals and ketals, in agreement with calculations in the literature.
Synthesis of Novel C2‐Symmetrical Diamino Thioethers Derived from Amino Acids
Authors: Christoffers Jens
Published: July 2007

Chiral β‐amino alcohols 2, available from natural α‐amino acids by reduction, have been converted in a sequence of N‐protection, O‐activation, thioether formation, and deprotection to novel C2‐symmetrical β,β′‐diamino thioethers 1, which shall serve as chiral tridentate ligands in late transition metal complexes. Optical purity of compounds 1 was established by amide formation with (+)‐O‐acetylmandelic acid.
Diels‐Alder Reactions of Tetradehydrodianthracene with Electron‐Deficient Dienes
Authors: Sauer Jürgen , Breu Josef , Holland Uwe , Herges Rainer , Neumann Helfried , Kammermeier Stefan
Published: July 2007

Theoretical calculations and simple MO considerations predict that a coplanar and spatially close arrangement of double bonds in structures such as tetradehydrodianthracene 1, should both lower the LUMO and raise the HOMO energy of the π system. Thus the reactivity in Diels‐Alder reactions towards electron‐rich, as well as electron‐deficient, dienes should be enhanced. The first effect has already been shown in a previous paper, the latter statement is now also confirmed by the reactivity of 1 in Diels‐Alder reactions with inverse electron demand. In [4 + 2] cycloaddition reactions of 1 with 1,2,4,5‐tetrazines 2 the electronic effect of 3,6 substituents in tetrazines is far outweighed by steric factors resulting in an unexpected reactivity sequence: 2a ≫ 2b ≈ 2c > 2d.
Cyclophanes, XLI. Cyclophanes as Precursors for Saturated and Unsaturated Cage Hydrocarbons – the Catalytic Hydrogenation of [2.2]Paracyclophane
Authors: Hopf Henning , Savinsky Rainer , Jones Peter G. , Dix Ina , Ahrens Birte
Published: July 2007

The catalytic hydrogenation of [2.2]paracyclophane 1 has been reinvestigated with the following results: a) Best yields (95%) of the perhydrocyclophane 3 are obtained when 1 is hydrogenated over ruthenium on charcoal in ethanol at 200°C and 300 atm in the presence of lithium hydroxide; b) hydrogenation of 1 in ethyl acetate/acetic acid over platinum at room temperature and normal pressure leads to the monoene 7 and one diene, to which we assign structure 11 since dichlorocyclopropanation converts it to the bis‐adduct 12, whose structure was determined by X‐ray structural analysis; c) when the tetraene 13 (prepared from 1 by Birch reduction) is hydrogenated over palladium on charcoal in petroleum ether, the triene 14 is obtained in addition to the dienes 4 and 11. Again, dichlorocyclopropanation followed by X‐ray structural analysis of the resulting tris‐adduct 17 was employed for the structure determination of the hydrocarbon precursor. The strain energies of the anti‐Bredt hydrocarbons 11 and 17 have been calculated by molecular mechanics calculations.
Thermal Rearrangements, 28. Competing Reaction Pathways in the Thermal Cycloisomerization of 1,3‐Hexadien‐5‐ynes
Authors: Nüchter Uta , Zimmermann Gerhard , Francke Viola , Hopf Henning
Published: July 2007

The thermal cycloisomerization of the 1,3‐hexadiene‐5‐ynes 1a–e has been studied at temperatures above 500°C and reaction times of about 0.3 s (1a–d) and at a pressure of 1.5 · 10−2 Torr (FVP, 1e), respectively, at low partial pressures in quartz flow systems. While 1a and 1b cycloisomerize exclusively to the corresponding benzenes and pentafulvenes, substrates 1c–e cyclize primarily to 4‐substituted 1,2‐dihydronaphthalenes and biphenyls. The experimental results suggest a predominance of unimolecular reactions such as electrocyclic ring closures and 1,6‐C,H‐insertion processes of alkenylidene carbenes initially formed by an acetylene→vinylidene rearrangement.
An Improved Synthesis of 1,3,3‐Trimethyl‐2‐azabicyclo[2.2.2]octane and Its Nitroxyl Radical Product, a Rigid Analogue of TEMPO
Authors: Bakunov Stanislav , Tkachev Alexey
Published: July 2007

A new synthetic approach for the production of isoquinuclidine 6 from dipentene (rac‐limonene, 1) is described. The nitroxyl 5, a rigid analogue of TEMPO, has been prepared from 6 in 87% yield.