bluuebunny/crossref_metadata_2025 huggingface dataset:
Calix[4]arene‐Based (Hemi)carcerands and Carceplexes: Synthesis, Functionalization, and Molecular Modeling Study
Authors: van Wageningen André M. A. , Timmerman Peter , van Duynhoven John P. M. , Verboom Willem , van Veggel Frank C. J. M. , Reinhoudt David N.
Published: July 2007

The synthesis of 11 calix[4]arene‐based carceplexes obtained by solvent or doped inclusion is reported. Carceplexes with amides, for example, DMF, NMP, and 1,5‐dimethyl‐2‐pyrrolidinone, and sulfoxides, for example, DMSO and thiolane‐1‐oxide, were obtained by solvent inclusion. In these cases the yield of the carceplex decreases with increasing guest size. Potential guests that do not form carceplexes by solvent inclusion, such as 2‐butanone and 3‐sulfolene, could be incarcerated by doped inclusion with 1,5‐dimethyl‐2‐pyrrolidinone as a solvent “doped” with 5–15 vol% of potential guest. The amide bridges of the carceplexes were converted into thioamide bridges in essentially quantitative yield by means of Lawesson's reagent in refluxing xylene. The dynamic properties of the incarcerated guests were examined by 2D NMR spectroscopy. Whereas for most guests a preference for one orientation inside the calix[4]arene‐based (thia)carcerands was observed, for DMA, NMP, and ethyl methyl sulfoxide inside calix[4]arene‐based (thia)carcerandstwo different orientationswere present. The energy barriers for interconversion between the various orientations of DMA, NMP, and ethyl methyl sulfoxide inside calix[4]arene‐based (thia)‐carcerands were determined with 2D EXSY NMR. The energy barriers are higher for the thiacarcerands than for the corresponding carcerands with amide bridges. This may be due to the stronger hydrogen‐bond‐donating character of the thioamide group. Furthermore, molecular modeling simulations indicate that in case of the thiacarcerand the cavity is smaller as a result of a smaller diametrical distance between the NHatoms. Our results demonstrate that molecular modeling can be used to estimate the energy barriers for interconversion; the calculated activation energies showed good quantitative agreement with the experimental values.
The Crystal Polymorphism of Tetrolic Acid (CH3CCCOOH): A Molecular Dynamics Study of Precursors in Solution, and a Crystal Structure Generation
Authors: Gavezzotti A. , Filippini G. , Kroon J. , van Eijck B. P. , Klewinghaus P.
Published: July 2007

The possible configurations of two molecules of tetrolic acid in a solvent box containing 226 carbon tetrachloride molecules are studied by molecular dynamics with the GROMOS package and force field over periods of up to 2000 picoseconds. The cyclic hydrogen‐bonded dimer was the most persistent configuration, but events leading to the cleavage of one hydrogen bond and thus the formation of a precursor to the crystal catemer motif were found to occupy up to 10% of the simulation times. The experimental bond‐breaking enthalpy was correctly reproduced. Two different crystal structure generation procedures were employed to reproduce the two observed polymorphic crystal structures and to predict other possible polymorphs; in all cases, some unobserved structures had more cohesive packing energies than the observed ones. The possible application of molecular dynamics in the study of the preliminary steps in crystal nucleation is discussed.
Towards 2‐Silaallenes: Synthesis of Spirocyclic Precursors
Authors: Goetze Brigitte , Herrschaft Bernhard , Auner Norbert
Published: July 2007

Reactions of dichlorodivinylsilane (2) and LitBu in a molar ratio 1/1 and 1/2 lead to highly reactive intermediates, which can be trapped by suitable reagents. In the presence of trimethylmethoxysilane, norbornadiene, anthracene and diphenylacetylene, products are formed that provide evidence for the intermediate formation of both the neopentylsilene H2C=CH(Cl)Si=CHCH2tBu (3) and the 2‐silaallene tBuCH2CH=Si=CHCH2tBu (4). The formation of double cycloadducts from 4 is particularly interesting as a preparatively facile route for the synthesis of silaspirocycles such as 13, 16 and 17, which have been characterized by single‐crystal X‐ray structure analysis.
Kinetics and Mechanism of the Reactions of Amine Boranes with Carbenium Ions
Authors: Funke Marcus‐ A. , Mayr Herbert
Published: July 2007

The kinetics of the reactions of trialkylamine boranes and pyridine boranes with benzhydryl cations have been determined photometrically. Second‐order rate laws are obeyed, first‐order with respect to amine borane concentration and first‐order with respect to carbocation concentration. As for other reactions of carbocations with neutral nucleophiles, the rates of these reactions are only slightly affected by solvent polarity. The structure—reactivity relationships and kinetic isotope effects are in accord with a polar mechanism proceeding through a transition state where the migrating hydride is partly bound to the entering carbon and to the leaving boron atom. The rate constants correlate linearly with the electrophilicity parameters E of the carbenium ions. It is therefore possible to use the linear free enthalpy relationship logk2 = s(E+N) for determining nucleophilicity parameters N for the amine boranes and to compare their hydride‐donating abilities with those of other non‐charged hydride donors (silanes, germanes, stannanes, and dihydropyridines).
1,2‐cis‐C‐glycoside synthesis by samarium diiodide‐promoted radical cyclizations
Authors: Skrydstrup Troels , Mazéas Daniel , Elmouchir Mohamed , Doisneau Gilles , Riche Claude , Chiaroni Angèle , Beau Jean‐Marie
Published: July 2007

The samarium diiodide reduction of glycosyl pyridyl sulfones bearing a silicon‐tethered unsaturated group at the C2–OH position leads to the stereo‐specific synthesis of 1,2‐cis‐C‐glycosides in good yield after desilylation. These reactions proceed via an anomeric radical with subsequent 5‐exo cyclization. Unlike the corresponding glycosyl phenyl sulfones, the pyridyl derivatives react instantaneously with samarium diiodide and do not require a cosolvent such as hexamethylphosphoramide (HMPA). Under these reaction conditions radical cyclization precedes the second reduction step. Examples of 5‐exo‐trig and ‐dig ring closures are given. The synthetic utility of this method was demonstrated by a short synthesis of methyl C‐isomaltoside.
A Highly Convergent Total Synthetic Route to Glycopeptides Carrying a High‐Mannose Core Pentasaccharide Domain N‐linked to a Natural Peptide Motif
Authors: Danishefsky Samuel J. , Hu Shuanghua , Cirillo Pier F. , Eckhardt Matthias , Seeberger Peter H.
Published: July 2007

N‐Linked glycopeptides were synthesized by condensation of a highmannose anomeric amine bearing a pentasaccharide with aspartic‐acid‐containing tri‐ and pentapeptides through the agency of IIDQ. The pentasaccharide portion, corresponding to the „core”︁ region of all asparagine‐linked glycoproteins, was assembled by means of glycal‐derived thioethyl donors and glycal acceptors. The central mannose residue was established by inversion of the C2 hydroxyl of a glucosyl precursor in the pentasaccharide. The protecting‐group scheme employed allows the extension of the pentasaccharide through the terminal mannose units. While a fully convergent coupling of the high‐mannose carbohydrate to the peptide domain has thus been accomplished for the first time with a fully synthetic sugar, the stereochemical integrity of the anomeric center of the carbohydrate domain was not maintained and a mixture of glycopeptides was obtained.
[(TptBu, Me)CrR]: A New Class of Mononuclear, Coordinatively Unsaturated Chromium(II) Alkyls with cis‐Divacant Octahedral Structure
Authors: Kersten Jörg L. , Kucharczyk Robin R. , Yap Glenn P. A. , Rheingold Arnold L. , Theopold Klaus H.
Published: July 2007

Reaction of CrCl2 with TptBu, MeK yielded [TptBu, MeCr(3‐tBu, 5‐MepzH)Cl] (1) and [TptBu, MeCrCl] (2), while the same reaction in the presence of pyridine gave 1 and [TptBu, MeCr(py)Cl] (3). Alkylation of 3 with Grignard reagents produced the chromium(II) alkyls [TptBu, MeCrR] (4, R = Et; 5, R = Ph; 6, R = CH2SiMe3), which reversibly added to pyridine to form the five‐coordinate adducts [TptBu, MeCr(py)R] (7, R = Et; 8, R = Ph; 9, R = CH2SiMe3). 1, 2, 4, and 5 were structurally characterized by X‐ray diffraction. The four‐coordinate molecules 2, 4, and 5 adopt a highly unusual cis‐divacant octahedral coordination geometry, while 1 is the first five‐coordinate TptBu, Me‐complex of a first‐row transition metal. Despite their coordinative unsaturation, chromium alkyls 4‐6 do not polymerize ethylene, or even react with it. This observation is inconsistent with the catalytic activity commonly ascribed to divalent chromium in heterogeneous polymerization catalysts. Attempts to oxidize 4‐6 (e.g., with [Cp2Fe]BPh4) to cationic chromium(II) alkyls failed, yielding [TptBu, MeCr(thf)][BPh4] (10) instead.
Tetracenomycin M, a Novel Genetically Engineered Tetracenomycin Resulting from a Combination of Mithramycin and Tetracenomycin Biosynthetic Genes
Authors: Künzel Eva , Wohlert Sven‐Eric , Beninga Claus , Haag Sabine , Decker Heinrich , Hutchinson C. Richard , Blanco Gloria , Mendez Carmen , Salas Jose A. , Rohr Jürgen
Published: July 2007

The hybrid strain Streptomyces glaucescens Tü49 (pGB7) contains recombined genes of the tetracenomycin C (4) and the mithramycin (6) biosynthesis cluster. It was designed by the trans formation of plasmid pGB7 into the tetracenomycin producer Streptomyces glaucescens Tü49. Plasmid pGB7 carries the minimal polyketide synthase (PKS) genes of mithramycin biosynthesis (mtmP, mtmK, mtmS) along with its upstream (mtmX, which encodes a gene product of unknown function) and downstream flanking genes (the ketoreductasecoding mtmT1 and fragments of mtmO 1, which encodes an oxygenase). It was assumed that early intermediates of the wellknown biosynthesis of 4, such as tetracenomycin F2 (3) or very similar molecules, are likely to also serve as intermediates of the biosynthesis of aureolic acid antibiotics, such as 6. Thus, the enzymes of both parent biosynthetic pathways should be able to act on such intermediates, and several hybrid molecules were expected. Although the experiment resulted in new products, only the novel hybrid natural product tetracenomycin M (1), whose constitution was determined unambigously by spectroscopic methods, was obtained in larger amounts. The formation of 1 can be explained, if a combination of enzymes of both parent biosynthetic pathways is taken into consideration. When plasmid pGB7 is transformed into Streptomyces lividans TK21, that is, a strain which does not produce any secondary metabolites under our laboratory conditions, the production of SEK15 (2) is observed. The latter is well known as the product of the minimal PKS of decaketides, and its exclusive production indicates that the aureolic acid antibiotics are constructed via a single decaketide chain and that the enzyme products of the flanking genes mtmX, mtmT1, and mtmO1 cannot contribute in this second experiment.
Two New Closely Related MoV Hydroxymonophosphates Built Up of Cd[Mo6P4O25(OH)6]2 And Cd[Mo6P4O26(OH)5]2 Clusters
Authors: Guesdon A. , Borel M. M. , Leclaire A. , Raveau B.
Published: July 2007

Two new molybdenum(v) hydroxyphosphates have been synthesized hydrothermally, Na2Cd3(Mo2O4OH)6‐(PO4)2(PO3OH)6[N(CH3)4]4·10H20 (1) and Cd9(Mo2O4OH)12(PO4)6(PO3OH)10‐[N(CH3)4]8·15H2O (2). Their structures have been determined from single‐crystal X‐ray diffraction. The water molecules and hydroxyl groups have been deduced from valence calculations. Both compounds crystallize in the triclinic space group P1, with the cell parameters for 1 a = 12.340(2), b = 12.596(1), c = 14.717(2) Å, α = 107.24(1)°, β = 89.83(1)°, γ = 114.31(1)°, V = 1972.3(4) Å3, and for 2 a = 11.942(1), b = 13.339(2), c = 26.765(3) Å, α = 85.33(1)°, β = 86.87(1)°, γ = 64.08(1)°, V = 3821.3(9) Å3. The two frameworks can be described on the basis of similar [Mo6P4X31]n‐ (X = O, OH) anionic clusters, but 1 is a tridimensional structure, whereas 2 exhibits a monodimensional structure.
Stoichiometric and Catalytic Reactions of the Polysiloxane‐Bound (Ether‐Phosphine)Rhodium(I) Complex [ClRh(PO)(P ∼ O)] in Interphases
Authors: Lindner Ekkehard , Schneller Theodor , Auer Friedrich , Wegner Peter , Mayer Hermann A.
Published: July 2007

The reaction of four equivalents of the monomeric trimethoxysilyl(T)‐functionalized ether‐phosphine ligand CyP(CH2CH2OCH3)(CH2)3SiR3 (R = OMe [1 a(T0)], Me [1 b]) with [{μ‐ClRh(COE)2}2] yielded the monomeric pseudo 14 electron rhodium(I) complexes [ClRh(PO)(P∼O)] (2a(T0)2, 2 b). For the sol‐gel process the complex 2 a(T0)2 was protected by introduction of the volatile, reversibly binding ligand pyridine. Thus, the monomeric compound 2 a(T0)2 was co‐condensed with two and eight equivalents of the co‐condensation agent MeSi(OMe)2(CH2)6(MeO)2SiMe (D0‐C6‐D0) to give the polysiloxane‐bound congeners 2(Tn)2(Di‐C6‐Di)y (y = 2 and 8, respectively; i = 0–2; n = 0–3). The polysiloxane‐bound complex 2(Tn)2‐(Di‐C6‐Di)2 was treated with a variety of small molecules in the gas/solid and liquid/solid interphases. It was shown that a facile cleavage of the Rh‐O bond in the ether‐phosphine chelate occurred even in the solid state. The reaction of 2(Tn)2‐(Di‐C6‐Di)2 with carbon monoxide, carbon disulfide, and diphenylacetylene resulted in the irreversible coordination of the molecule to the metal. In the presence of pyridine, the polysiloxane‐bound complex 2(Tn)2‐(Di‐C6‐Di)2 oxidatively added hydrogen to give the octahedrally configurated complex [ClRhH2(Py)(P ∼ O)2] [6(Tn)2(Di‐C6‐Di)2]. Treatment of dry 2(Tn)2‐(Di‐C6‐Di)2 with ethene led to the reversible formation of the corresponding complex. Although the materials display low surface areas, at least 75% of the rhodium centers within the matrix are accessible to the rather bulky tolan molecules. The complexes 2(Tn)2(Di‐C6‐Di)y (y (y = 2, 8) show high activities and selectivities in the hydrogenation of tolan. The conversion was found to depend markedly on the amount of co‐condensate (D0‐C6‐D0) and on the polarity of the solvent. The polysiloxane‐bound complexes 2(Tn)2(Di‐C6‐Di)y are more active than their monomeric congener 2 a(T0)2.
Oxidative Nucleophilic Substitution of Hydrogen in Nitroarenes
Authors: Makosza Mieczyslaw , Staliński Krzysztof
Published: July 2007

Carbanions of 2‐phenylpropionitrile were found to add to nitroarenes in liquid ammonia to form σH adducts, which were oxidized with KMnO4 to yield products of oxidative nucleophilic substitution of hydrogen (ONSH) in the position para to the nitro group. Treatment of the carbanion‐nitroarene system with methyl iodide at ‐70°C indicated that the addition proceeds almost to completion. Thus, for the first time, the persistence of σH adducts, formed between free anions and mononitroarenes, has been demonstrated. It was also shown that KMnO4 oxidizes σH adducts to nitrobenzene faster than it does carbanions. This selective ONSH in the para positions of nitroarenes is a general process. However, some substituents hinder or inhibit the oxidation step.
Some Remarks on the Occurrence and Properties of Knödel
Authors: Lattermann Günter
Published: July 2007

Dr. Lattermann casts further light on the topic of shell‐crosslinked polymer micelles broached by Karen L. Wooley in the September issue.
Electroanalysis Utilizing Amperometric Microdisk Electrode Arrays
Authors: Ordeig Olga , del Campo Javier , Muñoz Francesc Xavier , Banks Craig E. , Compton Richard G.
Published: July 2007

This article reviews the literature dealing with electroanalytical applications of ultramicroelectrode arrays over the past twenty years. A brief theoretical description of the mass transport mode governing their behavior is given, after which the main fabrication methods are described. The applications described in the later sections of this review range from conventional electroanalysis, namely trace metal stripping methods, using mercury modified arrays to more recent advances in the field of biosensors (enzymatic, immunosensors and nucleic acid based ones).
Probiotic bacteria prevent hepatic damage and maintain colonic barrier function in a mouse model of sepsis
Authors: Ewaschuk Julia , Endersby Ryan , Thiel David , Diaz Hugo , Backer Jody , Ma Mang , Churchill Thomas , Madsen Karen
Published: July 2007

A breakdown in intestinal barrier function and increased bacterial translocation are key events in the pathogenesis of sepsis and liver disease. Altering gut microflora with noninvasive and immunomodulatory probiotic organisms has been proposed as an adjunctive therapy to reduce the level of bacterial translocation and prevent the onset of sepsis. The purpose of this study was to determine the efficacy of a probiotic compound in attenuating hepatic and intestinal injury in a mouse model of sepsis. Wild-type and interleukin-10 (IL-10) gene–deficient 129 Sv/Ev mice were fed the probiotic compound VSL#3 for 7 days. To induce sepsis, the mice were injected with lipopolysaccharide (LPS) and d-galactosamine (GalN) in the presence and absence of the peroxisome proliferator-activated receptor gamma (PPARγ) inhibitor GW9662. The mice were killed after 6 hours, and their colons were removed for the measurement of the cytokine production and epithelial function. The functional permeability was assessed by the mannitol movement and cyclic adenosine monophosphate–dependent chloride secretion in tissue mounted in Ussing chambers. The livers were analyzed for bacterial translocation, cytokine production, histological injury, and PPARγ levels. The tissue levels of tumor necrosis factor alpha, interferon gamma, IL-6, and IL-12p35 ribonucleic acid were measured by semiquantitative reverse transcription polymerase chain reaction. Mice injected with LPS/GalN demonstrated a breakdown in colonic barrier function, which correlated with enhanced proinflammatory cytokine secretion, bacterial translocation, and significant hepatic injury. A pretreatment with oral probiotics prevented the breakdown in intestinal barrier function, reduced bacterial translocation, and significantly attenuated liver injury. The inhibition of PPARγ with GW9662 abrogated the protection induced by probiotics.

Conclusion:

Orally administered probiotics prevented liver and intestinal damage in a mouse model of sepsis through a PPARγ-dependent mechanism. (Hepatology 2007.)
Chiral organometallic reagents, XII. On the racemization of α‐hetero‐substituted benzyllithium compounds
Authors: Ahlbrecht Hubertus , Harbach Jürgen , Hoffmann Reinhard W. , Ruhland Thomas
Published: July 2007

Benzyllithium compounds carrying a benzylthio substituent (6), a benzylseleno substituent (8), and an isopropyl(methyl)amino substituent (14) have been generated. Racemization barriers have been determined in THF by monitoring the coalescence of signals of diastereotopic groups in the 1H‐NMR spectra. The barrier for 6 amounts to a ΔH≠ of 7.5 kcal mol−1 and ΔS≠ of −12 cal mol−1 K−1 corresponding to a ΔG of 9.9 kcal mol−1. The barrier for the other compounds investigated varies from 9 to 10 kcal mol−1. The rate‐determining step for the racemization is discussed to be the transformation of the contact ion pair to the solvent‐separated ion pair.
1,3‐dipolar cycloaddition of diphenylnitrilimine to substituted dihydro‐3‐methylene‐2(3H)‐furanones
Authors: Štverková Slávka , Žák Zdirad , Jonas Jaroslav
Published: July 2007

Substituted 4,5‐dihydro‐3‐methylene‐2(3H)‐furanones 1 react as dipolarophiles with diphenylnitrilimine to give the corresponding spiro[furan‐pyrazolin]ones 2. From the compound 1a, 1,3‐diphenyl‐7‐oxa‐1,2‐diazaspiro[4,4]non‐2‐en‐6‐one (2a) is formed. In the other cases, a primarily formed cycloadduct undergoes rearrangement during the reaction to the bicyclic compound 5. Thus, addition of diphenylnitrilimine to (E)‐1b, (Z)‐1b and (E)‐1c leads to compound 5b, whereas reactions of (E)‐ or (Z)‐1f and (E)‐1g give the corresponding compound 5f and 5g, respectively. The structure of 5b was determined by X‐ray analysis. Labeled methyl formate (H13CO2CH3) was used for labeling the methylene carbon atom in 4,5‐dihydro‐3‐(p‐tosyloxymethylene)‐2(3H)‐furanone [(E)‐1b] in order to ascertain the structure of the intermediate cycloadduct as well as the pathway of the consecutive rearrangement.
Biologically active glycosides from asteroidea, XXXII. Glycosphingolipids from the starfish Astropecten latespinosus, 2. Structure of two new ganglioside molecular species and biological activity of the ganglioside
Authors: Higuchi Ryuichi , Matsumoto Shuji , Fujita Masanori , Komori Tetsuya , Sasaki Takuma
Published: July 2007

Two ganglioside molecular species [LG‐1 (1) and LG‐2 (5)] have been obtained from the water‐soluble lipid fraction of the chloroform/methanol extract of the starfish Astropecten latespinosus. On the basis of chemical and spectroscopic findings, the structure of 1 and 5 have been elucidated. They are new ganglioside molecular species. Negative FAB mass spectrometry provided important information both on the structure of the sugar moiety and on the molecular mass of the gangliosides. Partial hydrolysis by hot water has proved useful for structure elucidation of the complex oligosaccharide moieties. In addition, the major ganglioside molecular species, LG‐2 (5), shows antitumor activity against murine lymphoma L1210 cells in vitro.
Stereochemistry of 2‐methyl‐ and 2‐phenyl‐2‐adamantyl cations
Authors: Herrmann Roland , Kirmse Wolfgang
Published: July 2007

When both diastereomers of [5‐D]‐2‐methyl‐2‐adamantyl 2,4‐dinitrophenolate (18a) and [5‐D]‐2‐phenyl‐2‐adamantyl 2,4‐dinitrophenolate (18b) were solvolyzed in 40% aqueous acetone, 1:1 mixtures of (E)‐ and (Z)‐[5‐D]‐2‐R‐adamantan‐2‐ols (19a, b) were obtained. The stereoselectivity of secondary 2‐adamantyl cations, attributed to σ participation, is effectively suppressed by charge‐stabilizing groups at C‐2.
Addition and cyclization reactions in the thermal conversion of hydrocarbons with enyne structure, I. Detailed analysis of the reaction products of ethynylbenzene
Authors: Hofmann Jörg , Zimmermann Gerhard , Guthier Klaus , Hebgen Peter , Homann Klaus‐Heinrich
Published: July 2007

The pyrolysis of ethynylbenzene (C8H6, 1) was studied in a flow system between 700 and 1100°C (reaction time about 0.3 s) by using a mixture of 5 mol‐% of 1 in nitrogen and also in hydrogen at 700°C. The products were analyzed gas chromatographically. At 700°C in nitrogen, the main products were 1‐ and 2‐phenylnaphthalene (2, 3), 1‐methylene‐2‐phenyl‐1H‐indene (4), 1‐methylene‐3‐phenyl‐1H‐indene (5), and 5,10‐dihydroindeno[2,1‐α]indene (6). At higher temperatures, ethynylaromatics and more stable aromatics such as benzene, naphthalene, acenaphthylene, biphenyl, pyrene, fluoranthene, and six further C16H10 isomers where detected. With hydrogen as diluent, the dimer formation was reduced, mainly in favor of styrene. – The complex mixture of reaction products and the dependence of its composition on the pyrolysis temperature cannot be explained in terms of one reaction scheme only. It is suggested that H atoms act as important chain carriers. At temperatures around 700°C they mainly add to 1 yielding the phenylvinyl radicals 1a and 1b. These add to 1 forming dimers C16H12 via radicalic intermediates C16H13. With increasing temperature the 2‐phenylvinyl radical 1a not only reacts back to H + 1 but also decomposes by β(CC) cleavage into phenyl and ethyne. The latter channel is more endothermic by 33 kJ/mol. Additionally, isomeric ethynylphenyl radicals are increasingly formed by bimolecular H abstraction. Thus, with increasing temperature product formation is controlled by reactions of phenyl‐type radicals.
1‐ and 2‐fluoroalkyl carboxylates
Authors: Limat Dominique , Guggisberg Yves , Schlosser Manfred
Published: July 2007

The 2‐fluoroalkyl esters (1–4) of carboxylic acids are readily accessible by the base‐catalyzed acylation of fluorohydrines. A two‐step procedure is required to prepare the 1‐fluoroalkyl esters (10–14, 16): conversion of alk‐1‐enyl carboxylates („ene esters”︁) into the 2‐bromo‐1‐fluoroalkyl esters 5–9, 15, 17–19 by simultaneous treatment with N‐bromosuccinimide and hydrogen fluoride followed by the reductive replacement of bromine by tributyltin hydride. Alka‐1,3‐dienyl carboxylates („diene esters”︁) undergo 1,4‐addition of bromine and fluorine. The resulting 4‐bromo‐1‐fluoroalk‐2‐enyl carboxylates 20–25 undergo a substitution reaction with sodium benzenesulfinate to afford the sulfones 26–28 or oxidation with dimethyl sulfoxide to yield the 4‐oxo derivatives 29–30.
First stereoselective synthesis of (1S,2S,4R,5R)‐(−)‐β‐multistriatin
Authors: Henrichfreise Peter , Scharf Hans‐Dieter
Published: July 2007

Starting from 2,3‐O‐(3‐pentylidene)‐D‐glyceraldehyde (3), we prepared the bicyclic pheromone (1S,2S,4R,5R)‐(−)‐β‐multistriatin (1) on a gram scale. Key steps of the synthesis are a cis‐selective Wittig olefination followed by diastereo‐selective Michael addition and hydrogenation.
A convenient enantioselective synthesis of (R)‐(−)‐phoracantholide I, a component of the defensive secretion of the eucalypt longicorn Phoracantha synonyma
Authors: Enders Dieter , Plant Andrew , Drechsel Karin , Prokopenko Oleg F.
Published: July 2007

A short enantioselective synthesis of phoracantholide I [(R)‐4] (ee = 91 %), a component of the defensive secretion of the eucalypt longicorn Phoracantha synonyma, is described. Key steps of the synthesis are the enantioselective α‐alkylation of cyclononanone SAMP hydrazone [(S)‐1] (de > 93%) and subsequent Baeyer‐Villiger reaction of the ketone (R)‐3 with retention of configuration.
Efficient, simple procedures for the large‐scale preparation of building blocks for angiotensin (II) receptor antagonists
Authors: Jendralla Heiner , Wagner Adalbert , Mollath Martina , Wunner Joachim
Published: July 2007

The kg‐scale syntheses of p‐formylbenzeneboronic acid 3c and the biphenyl derivatives 4a, 4b, and 4c, building blocks for angiotensin (II) receptor antagonists, are reported.
New chiral building blocks from 1,2;5,6‐di‐O‐isopropylidene‐D‐mannitol: Synthesis of C2‐symmetrical and unsymmetrical mono‐ and bisepoxides and of a polyhydroxylated butenolide
Authors: Mulzer Johann , Pietschmann Catarina , Schöllhorn Bernd , Buschmann Jürgen , Luger Peter
Published: July 2007

The enantiopure epoxides 2–6 were synthesized from 1,2;5,6‐di‐O‐isopropylidene‐D‐mannitol (1). The terminal epoxides 2 and 3 are available from the desoxy mannitol 9 in three steps. The C2‐symmetrical epoxides 4 and 5 were generated via the C‐4‐inverted alcohol 15. The C2‐symmetrical bisepoxide 6 was prepared in a short sequence maintaining the symmetry throughout the entire synthesis. The crystalline butenolide 44 was prepared in five steps from 1 by Wittig olefination and lactonization.
Cycloadditions, 53. New spirocyclic chiral dienes — synthesis and diastereoselective Diels‐Alder reactions with N‐phenylmaleimide
Authors: Hoffmann Ralf , Mattay Jochen
Published: July 2007

Synthesis and diastereoselective Diels‐Alder reactions of new spirocyclic chiral dienes are reported. Starting with diketene and (−)‐menthone (1), we prepared the diastereomeric diketene‐menthone adducts 2 and 3. Further derivatization and olefination afforded four chiral dienes 12‐15. Diastereoselective Diels‐Alder reactions of these dienes with N‐phenylmaleimide (NPM) under high‐pressure conditions yielded the corresponding cycloadducts 16‐19. Olefination of 6‐diethylphosphonomethyl‐2,2‐dimethyl‐1,3‐dioxin‐4‐one 8 with formaldehyde led to unexpected compounds 9‐11 resulting from the originally formed olefination product.
Chiral calixarenes derived from resorcinol
Authors: Iwanek Waldemar , Mattay Jochen
Published: July 2007

A new type of chiral resorc[4]arene is described. The formation of both enantiomers of 5 is controlled by the chirality of the amines used in the Mannich reaction leading to the 1,3‐oxazine ring anellated to the resorcarene moiety. At −50°C ee increases up to 90%. Both enantiomers can be easily purified to >98% ee by recrystallization. The structures were proved by standard methods including X‐ray analysis.
Cyclophanes, XL. On the preparation of conjugated polyenes with [2.2.2]paracyclophanyl end groups
Authors: Heirtzler Fenton R. , Hopf Henning , Lehne Verena
Published: July 2007

Model substances for oligomeric systems based on the incorporation of [2.2.2]paracyclophane into the repeating unit are linear polyenes bearing a paracyclophanyl residue in the terminal positions. The preparation of the polyenes 2a–c is described. In addition to the characterisation as diarylpolyenes, consideration of model compounds revealed that the [2.2.2]paracyclophanyl residues interact with pendent substituents according to a magnetic anisotropic mechanism.
Diastereoselective side‐chain alkylation of (prolinolylmethyl)oxazoles and ‐oxadiazoles
Authors: Pohl Martin , Thieme Marion , Jones Peter G. , Liebscher Jürgen
Published: July 2007

Diastereoselective alkylation of the α‐position of (S)‐(prolinolylmethyl)oxazoles and ‐oxadiazoles 1 leading to side‐chain modified products 2 and 3 was achieved by reaction with LDA and alkylating agents or aldehydes. The products 2 and 3 are precursors of non‐natural chiral α‐amino acids. The influence of substituents, reagents, and reaction conditions on the diastereoselectivity of the alkylation was investigated. A model was proposed explaining the stereochemical outcome of the reaction. Corresponding α‐acylation of prolinolylmethyl heterocycles 1 with chloroformates affording chiral α‐amino esters 4 were not stereoselective.
On the formation of N‐heterocycles by Thermolysis of conjugated α‐azidoacrylates
Authors: Vogel Claus , Delavier Paul , Schnippenkötter Bernd
Published: July 2007

The synthesis of various o‐butadienyl‐α‐azidocinnamates 2 and β‐cycloalkenyl‐α‐azidoacrylates 9 is described. The thermolysis of compounds 2 in solution yields the new 8,9‐benzo‐6‐azabicyclo[3.2.2]nona‐3,6,8‐trienes 3 as major products, while that of 9 yields the cycloalkano‐2H‐pyrroles 10, which can rearrange to the cycloalkano‐1H‐pyrroles 11 at higher temperatures. A β‐(o‐vinyl)phenanthryl‐α‐azidoacrylate 12 was found to react on thermolysis both as a β‐cycloalkenyl‐α‐azidoacrylate and a o‐vinyl‐α‐azidocinnamate.
An unexpected product in attempted [4 + 2] cycloadditions of 2,2‐diethyl‐4,5‐dimethyl‐2H‐imidazole
Authors: Felderhoff Michael , Sustmann Reiner , Steller Ingo , Boese Roland
Published: July 2007

2,2‐Diethyl‐4,5‐dimethyl‐2H‐imidazole 1, formally a 1,4‐di‐azadiene, does not produce [4 + 2] cycloadducts with various electronically‐different dienophiles. In the presence of tri‐fluoroacetic acid, it tautomerizes to 2,2‐diethyl‐4‐methyl‐5‐methylene‐3‐imidazoline 2 and forms a dimer, the structure of which was determined by X‐ray crystallography.
Ab Initio Molecular Orbital Calculations on Allylic 1,3‐Strain of Electron‐Donor‐ and Electron‐Acceptor‐Substituted Alkenes
Authors: Tietze Lutz F. , Schulz Gerhard
Published: July 2007

Ab initio calculations on the conformations of several electron‐rich and electron‐poor alkenes 2, 8–15 were performed up to the MP2/6–31G*/RHF/6–31G* level. It was proven that allylic 1,3‐strain can be traced back to steric interactions between the allylic center and the (Z) substituent of the double bond. The electron density of the double bond seems to have no effect on the difference in energy between the global and the local minimum. The local minimum (180° conformation) of (Z)‐methyl‐substituted alkenes 2, 14 and 15 carrying a methyl, carboxyl and methoxy group, respectively, at the double bond in (E) position, lies 3.37–3.52 kcal/mol above the 0° conformation (global minimum); thus, the (E) substituent has no significant influence on this energy difference and therefore on the equilibrium distribution.
Chiral Tri‐ and Pentamethinium Cyanine Dyes with 1,2,3,4‐Tetrahydro‐6‐methylquinolyl End Groups: UV/Vis and CD Spectroscopy and Structure Correlations
Authors: Buß Volker , Ulbrich Dagmar , Reichardt Christian
Published: July 2007

UV/Vis and CD spectra of the chiral trimethinium cyanine dyes T2 to T5 and the pentamethinium cyanine dyes P2 to P5 have been studied for possible correlations between chiroptical properties and molecular structure. With a non‐planar conjugated π‐system as a consequence of steric interaction between the two end groups and the polymethine chain and with the methyl substituents in the 2‐ or 3‐ring positions acting as chiral anchors, the inherent chirality of these dyes manifests itself in the CD spectra which follow the same pattern throughout: monosignate CD absorptions in the long‐wavelength cyanine band region and bisignate in the aromatic‐end‐group region. A correlation between the sense of twist of the polymethine chain and the observed and calculated sign of the cyanine band is established, which allows the assignment of the absolute configuration for the 2‐, but not for the 3‐methyl substituted dye derivatives. Neither in the trimethinium nor in the pentamethinium dyes do the calculated energies, geometries or chiroptical properties indicate any interaction between the two end groups.
Asymmetric Strecker Synthesis of the Four α‐Quaternary 1‐Amino‐2‐methylcyclohexanecarboxylic Acids
Authors: Volk Franz‐Josef , Frahm August Wilhelm
Published: July 2007

The synthesis of the four 1‐amino‐2‐methylcyclohexanecarboxylic acids 13, 14, 15, and 16 from diastereomeric mixtures of the α‐amino nitriles 1–4 by successive application of conc. H2SO4, Pd/C H2, and conc. HCl is described. The amino nitriles 1–4 were prepared by asymmetric Strecker synthesis under various reactions, The formation of 1–4 is thermodynamically controlled in protic solvents (e.g. MeOH), whereas the reaction is under kinetic control in non‐protic solvents (e.g. hexane), The separation of the α‐amino amides 5–8, which were obtained by partial hydrolysis of 1–4, was achieved by CC, LPLC, and HPLC. The absolute configuration of all synthesized compounds was determined by means of only two X‐ray analyses with consecutive correlations. The steric control of the asymmetric Strecker synthesis is discussed.
Rearrangement of Donor‐Acceptor‐Substituted Vinylcyclopropanes to Functionalized Cyclopentene Derivatives: Evidence for Zwitterionic Intermediates
Authors: Buchert Matthias , Reißig Hans‐Ulrich
Published: July 2007

Donor‐acceptor‐substituted vinylcyclopropanes such as 1a, 3a, 5a, 5b, 7a, 9a, 9b, and 12a smoothly undergo thermal ring enlargement to functionalized cyclopentene derivatives 2, 4, 6, 8, 10, and 13 at relatively low temperatures (150–190°C) in good yields. This vinylcyclopropane‐cyclopentene rearrangement proceeds with low (1a, 3a), moderate (9a, b) or high (7a, 12a) stereoselectivity. In the case of 7a and 12a almost complete retention of configuration with respect to the migrating (methoxy) (phenyl)‐substituted carbon atom was observed. The definite mechanistic interpretation of these results is complicated by the stereoisomerization of the starting donor‐acceptor‐substituted vinylcyclopropanes which proceeds as competing reaction during ring enlargement. Nevertheless, all observations together, in particular the recorded solvent effects, support a stepwise mechanism of the rearrangement with a highly stabilized 1,3‐zwitterionic species as common intermediate for cyclopropane stereoisomerization and ring enlargement.
Aromatic Sulfonation, 130. – Relative Intramolecular Reactivity of the Insulated π Double Bond and the 14π Annulene Perimeter toward Sulfur Trioxide in the Sulfonation of Isopyrenes
Authors: Cerfontain Hans , Bakker Bert H. , Koeberg‐Telder Ankie , Wilmes Oswald
Published: July 2007

The sulfonation of isopyrene (1) and its 5‐methyl‐ and 5,10‐dimethyl derivative 2 and 3 with SO3 in both dichloromethane and nitromethane as solvent has been studied. Reaction of 1 and 2 leads to sulfodehydrogenation in which SO3 reacts as a monodentate electrophile, the initial products being 1‐5‐sulfonic acid (1‐5‐S) and 2‐10‐S, respectively. In the presence of a sufficiently large amount of SO3 1‐5‐S is subsequently converted into 1‐5,10‐S2. With 3, using 2.0 molequiv, of the sulfonating reagent in dichloromethane, the SO3 adds as a bidentate electrophile to the “central C=C bond” with formation of 7,14‐dimethyl‐1,6:8,13‐ethanediylidene‐[14]annulene‐15,16‐sultone (5) and the corresponding pyrosultone 6. Upon subsequent hydrolysis and neutralization with aqueous KOH the eventually obtained product is potasium 16‐hydroxy‐7,14‐dimethyl‐1,6:8,13‐ethanediylidene‐[14]annulene‐15‐sulfonate (4).
Oligomerization and Polymerization in Sodium Salts of Chlorocarboxylic Acids
Authors: Epple Matthias , Kirschnick Holger
Published: July 2007

Heating of the sodium salts of chlorocarboxylic acids leads to exothermic elimination of sodium chloride and oligomerization of the organic backbone of the molecule. Thermochemical data of sodium 2‐chloropropionate (1), sodium 3‐chloropropionate (2) and sodium 2‐chlorobutyrate (3) were derived. The reaction products were characterized by IR and NMR spectroscopy, and the degree of polymerization was determined by gel permeation chromatography (GPC).
Towards the Synthesis of o‐Diphosphaquinones: Benzodiphosphadihydropentalene – Naphthodihydrodiphosphete
Authors: Märkl Gottfried , Hennig Rüdiger , Nöth Heinrich
Published: July 2007

o‐Quinodimethanes 1 are important intermediates in organic synthesis that may be generated by thermal valence isomerization of benzocyclobutenes[1] or octa‐1,2,4,6,7‐pentaenes[2]. The preparation of a stable o‐quinodimethane has not been reported so far. Whereas o‐quinodiimines 2 containing strongly electron‐withdrawing groups at the nitrogen atoms are isolable[3], o‐diphosphaquinones 3 have not been detected.
Substituent Effects on the CC Bond Strength, 20. – Geminal Substituent Effects, 14. – Stabilization of the Cyano(dimethylamino)methyl Radical – Synergistic effect Due to Interaction Between α‐Amino and α‐Cyano Groups on the Radical Stabilization Energy
Authors: Welle Frank M. , Verevkin Sergey P. , Beckhaus Hans‐Dieter , Rüchardt Christoph
Published: July 2007

The thermolysis reactions of 3a and 2a, b were studied over a temperature range of 40°C and the activation parameters were determined. They were compared with the activation parameters of structurally comparable hydrocarbons of similar strain in order to obtain the radical stabilization enthalpies RSEs of the cyano(dimethylamino)methyl radical 1a. For this comparison the geminal interaction enthalpies of the cyano and the dimethylamino groups in the ground state had to be determined for the series of amino nitriles 4a–4c and 4j–4n by thermochemical methods. The geminal interaction in the ground state varies between −0.6 kcal/mol stabilization in 4a and a destabilization of +10.7 and +7.0 kJ mol−1 for secondary and tertiary α‐dialkylaminonitriles, respectively. A synergistic (i.e. more than additive) stabilization enthalpy of 26 kJ mol−1 of 1a was found in contrast to predictions in the literature. This stabilization is interpreted by conjugation between the substituents, which are separated by the radical center.
Sterically Congested Molecules, 17. – Novel Syntheses of α,α,β‐Tri‐tert‐butyl Compounds
Authors: Böhrer Gerald , Knorr Rudolf , Böhrer Petra , Schubert Bernhard
Published: July 2007

Pivaloin (2), prepared from ethyl pivalate (1) or from dimethyl oxalate (3), reacts with tert‐butyllithium by reduction (45% of 10) and addition (45% of 9), whereas the corresponding Barbier variant takes a different course. Productive transformations of the α,α,β‐tri‐tert‐butyl glycol 9 lead to α,β,β‐tri‐tert‐butylethanone (17, overall 4 steps), or α,α,β‐tri‐tert‐butylethene (20, 3 or 4 steps), or α,β,β‐tri‐tert‐butyl‐β‐hydroxyethanone (19, 3 steps, also by Grignard addition to bipivaloyl 4). Steric congestion is assessed by searches for restricted internal rotation. The alkene 20 and its epoxide 27 are studied with respect to NMR assignments and chemical degradation.
Amphiphilic and Mesogenic Carbohydrates, 8. – Liquid Crystalline Alkyl 2‐Deoxy‐2‐fluoro‐1‐thioglycopyranosides
Authors: Miethchen Ralf , Kowalek Heike , Niemanna Angela , Diele Siegmar , Schick Christoph
Published: July 2007

The homologous series of the amphiphilic alkyl 2‐deoxy‐2‐fluoro‐1‐thio‐α‐D‐glucopyranosides 5a–e and alkyl 2‐deoxy‐2‐fluoro‐1‐thio‐α‐D‐mannopyranosides 9a–e were prepared from the 2‐deoxy‐2‐fluoro‐D‐glycosyl fluorides 1 and 6, respectively. Their liquid crystalline properties were investigated by polarising microscopy, DSC measurements, and X‐ray measurements. The fluorine‐containing thioglycosides 5a–e as well as 9a–e form enantiotropic mesophases of the type smectic A. The effects of F for OH and S for O substitution are discussed with reference to the homologous alkyl‐1‐thio‐α‐D‐glucopyranosides 10 and alkyl 2‐deoxy‐2‐fluoro‐α‐D‐glucopyranosides 11.
Glycosyl Imidates, 78. An Efficient Synthesis of Galactinol and Isogalacatinol
Authors: Mayer Thomas G. , Schmidt Richard R.
Published: July 2007

2:3,4:5‐Di‐O‐cyclohexylidene‐D‐/L‐myo‐inositol (2) could be readily converted into 6‐O‐ and 1‐O‐unprotected L‐ and D‐myo‐inositol derivatives L‐ and D‐4 and L‐ and D‐6, respectively. Their reaction with trichloroacetimidate 7 as galactosyl donor in the presence of tin(II) trifluoromethanesulfonate as catalyst afforded the desired α(1–6)‐ and α(1–1)‐connected galactopyranosides 8, 11 and 12. Compound 11 could be readily deprotected to afford target molecule (galactinol) 1 in high overall yield; from 12, the diastereoisomer 13 was obtained.
Chiral Pyridinophanes as Hydrogen‐Bonding Supramolecular Building Blocks
Authors: de la Moya Cerero Santiago , Böhme Marc , Nieger Martin , Vögtle Fritz
Published: July 2007

Hydrogen‐donor and hydrogen‐acceptor substituted chiral cyclophanes with a [2](1,3)benzeno[2](3,5)pyridinophane framework have been synthesized, enantiomerically separated, and analysed by circular dichroism (CD). X‐ray structure determination and CD analyses show that these cyclophanes are able to act as hydrogen‐bonding donor/acceptor building blocks for the design of chiral supramolecular aggregates with suitable complementary partners. Starting from the pyridinophane N‐oxide 4, selective reactions lead to the new chiral pyridinophanes 5–7 in high yields. The X‐ray structure analysis of the pyridinophane‐2‐carbonitrile 5 reveals a multilayered herringbone structure as it forms dimeric aggregates. The CD analysis of the host 7 with the complementary guest molecules melamine and cyanuric acid elucidates the formation of supramolecular complexes in solution.
N‐Methyl Derivatives of Highly Substituted Porphyrins – the Combined Influence of Both Core and Peripheral Substitution on the Porphyrin Conformation
Authors: Senge Mathias O. , Kalisch Werner W. , Runge Steffen
Published: July 2007

N‐Methylation of the peripherally highly substituted octaethyltetraphenylporphyrin (1) with either methyl iodide or methyl triflate gave the respective mono‐, di‐, tri‐ and tetra‐N‐methylated derivatives. Use of methyl triflate under carfully controlled conditions often gave the N‐methylporphyrins in quantitative yield. The N‐methylporphyrins possess both core and peripheral steric strain and are good candidates for the study of the combined conformational effect of such substituent patterns. Introduction of successively more N‐methyl groups leads to a stepwise increase in the bathochromic shift of the bands by about 10–15 nm for each substitution. Crystal structure determinations of 11 and 12 showed that these molecules are indeed severely distorted and they exhibit degrees of nonplanarity which significantly exceed those observed for nonmethylated, dodecasubstituted nonplanar porphyrins and for 22,24‐dihydroporphyrin dications. After methylation of meso‐tetraalkylporphyrins 4–7 with methyl triflate only the respective N‐methylated porphyrins 14–17 could be recovered. Use of the extremely ruffled porphyrin 8 led to formation of a 5,21,22,23,24‐pentamethylated porphyrin cation with phlorin structure. Additionally, structural analysis of a cis‐21,22‐dimethyloctaethylporphyrin showed the first example of a porphyrin cation 20 with almost complete localization of the conformational distortion in the porphyrin half bearing the two N‐methyl groups.
Fluoride‐ and Base‐Induced Chemiluminescence (CIEEL) of a Bifunctional Spiroacridane‐Substituted Dioxetane
Authors: Adam Waldemar , Reinhardt Dirk
Published: July 2007

The intramolecular chemically initiated electron‐exchange luminescence (CIEEL) of the novel bifunctional dioxetane 2b was studied. By selective chemical triggering, two different functionalities, namely either a phenoxy (ArO−) or an amide (Ar2N−) ion electron donor, were released, dependent on the type of triggering agent used (either base or fluoride ions). From the CIEEL quantum yields (ΦCIEEL) for the triggered chemiluminescence and the fluorescence efficiencies (ΦFl) of the corresponding phenolate (7) and amide (5) emitters the singlet‐excitation yields (ΦS) were estimated. Due to the weak fluorescence emission and the low excitation propensity, the triggerable bifunctional dioxetane 2b constitutes an inefficient CIEEL system. For comparison, the activation parameters (EA, lg A, ΔH≠, ΔS≠) of the direct chemiluminescence in the thermal decomposition of the dioxetane 2a were determined by standard isothermal kinetic methods.
Galacturonic Acid Derivatives, IX. Regioselective Enzymatic Acetyl Ester Hydrolysis of D‐Galacturonic Acid Derivatives and Related Reactions
Authors: Vogel Christian , Kramer Sven , Ott Andrej‐Jakob
Published: July 2007

A number of D‐galacturonic acid derivatives (1, 3, 7 and 9) have been selectively deacetylated in enzyme‐catalyzed reactions. The 2‐O‐acetyl groups were selectively hydrolyzed from 2,3,4‐tri‐O‐acetyl‐β‐D‐galactopyranurono‐6,1‐lactone (1), and methyl [methyl (3) and benzyl (7) 2,3,4‐tri‐O‐acetyl‐β‐D‐galactopyranosid]uronates, affording the specifically deacetylated products in good to excellent yields. The methyl 1,2,3,4‐tetra‐O‐acetyl‐α‐D‐galactopyranuronate (9) was hydrolyzed at the anomeric position in 48% yield using the acylase from Aspergillus melleus (E1), whereas the lipases from wheat germ (E5 and E7) catalyzed the selective cleavage of the acetyl groups in both the 1‐ and 2‐O‐positions, furnishing the methyl 3,4‐di‐O‐acetyl‐D‐galactopyranuronate (12) in 46% isolated yield. The latter compound was obtained in nearly quantitative yield by selective enzymatic deacetylation at the 2‐O‐position of 7, followed by hydrogenolytic cleavage of the anomeric O‐benzyl group. Isopropylidenation of 12 furnished the hitherto unknown methyl 3,4‐di‐O‐acetyl‐1,2‐O‐isopropylidene‐α‐D‐galactopyranuronate (13).
Synthesis and Self‐Association of First‐Generation 1,3,4‐Oxadiazole‐Containing Dendrimers
Authors: Kraft Arno
Published: July 2007

Two 1,3,4‐oxadiazole‐containing dendrimers, 14 and 15, have been prepared employing a palladium‐catalyzed carbonylation reaction as the key step of the synthesis. In order to shorten the reaction sequence, a new oxadiazole synthesis has been developed based on the coupling of aryl iodides with aryl tetrazoles in the presence of a Pd(0) catalyst and carbon monoxide, and has been applied to the preparation of dendrimer 14. Both new dendrimers are highly soluble in chlorinated solvents, but 14, with fewer solubilizing tert‐butyl groups at its periphery, self‐associates in solution by π‐π interactions and possibly due to solvophobic effects. Considerable upfield shifts of the 1H‐NMR signals of the protons in the centre of the molecule indicate the formation of π‐stacks in solution. Vapour‐pressure osmometry studies allow a degree of association of ⩾3 to be estimated.