bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of the potentiometric titration curves of poly(acrylic acid), poly(methacrylic acid) and poly(ethacrylic acid) in terms of species population distributions
Authors: Jager Jan , Engberts Jan B. F. N.
Published: September 2010

For atactic poly(methacrylic acid) (PMAA) and poly(ethacrylic acid) (PEA) the conformational transition from a compact coil to a random coil upon ionization in aqueous solution is revealed by an analysis of the potentiometric titration curves. The treatment, proposed by Strauss, is couched in terms of successive ionization constants of a suitably sized subunit of the polymer chain. Consideration of the bimodal population distribution indicates that the conformational transition in PEA is more pronounced than in PMAA. This is most likely the result of the more hydrophobic side‐chain in PEA. In agreement with previous work, no “two‐state” conformational transition is observed upon ionization of poly(acrylic acid) (PAA).
X‐Ray structure of tetrameric 1‐lithio‐3‐methoxypropane
Authors: Klumpp G. W. , Geurink P. J. A. , van Eikema Hommes N. J. R. , de Kanter F. J. J. , Vos M. , Spek A. L.
Published: September 2010

The crystal structure of (LiCH2CH2CH2OMe)4 [(3)4] consists of a distorted (CH2)4Li4 cube, four parallel edges of which are spanned by CH2CH2OMe chains. The interatomic distances provide insights into the nature of alkyllithium‐Lewis base (ethers, tertiary amines) interactions and into the structural basis of the differential reactivity and stability of primary and secondary alkyllithium tetramers.

NMR of hydrocarbon solutions indicates that under these conditions the solid state tetramer is in equilibrium with a stereoisomer.
Synthesis with sulfones, Part XLIII. Preparation ofγ‐functionalized vinyl sulfones by nucleophilic substitution of allylidene disulfones; stereospecific reduction of 3‐hydroxy‐(E)‐1‐propenyl sulfones to (Z)‐allylic alcohols
Authors: Cuvigny T. , du Penhoat C. Herve , Julia M.
Published: September 2010

The substitution of a sulfonyl moiety of allylidene disulfones by various nucleophiles (PhSNa, PhSO2Na, H2O and R2NH) leads toγ‐functionalized (E)‐vinyl sulfones. The stereospecific reduction of 3‐hydroxy‐(E)‐1‐propenyl sulfones affords (Z)‐allylic alcohols in greater than 98% purity.
Structure‐odour relationships of some new synthetic sandalwood aroma chemicals: Synthesis and olfactive properties in a series of bicyclo[4.4.0]decan‐3‐ols
Authors: Witteveen Jan G. , van der Weerdt Anton J. A.
Published: September 2010

The relationship between chemical structure and sandalwood fragrance in a series of four isomers of 8‐tert‐butylbicyclo[4.4.0]decan‐3‐ol (4a‐d), the unsaturated analogue 5 and the methyl‐substituted derivative 17 has been examined. The sandalwood fragrance of decalol 4c suggests a potentially new class of compounds having a sandalwood odour.
The synthesis, properties and reactivity of model systems for protonated chromophores in biliproteins
Authors: Hoef K. v. D. , Hempenius M. A. , Koek J. H. , Lugtenburg J. , Fokkens R.
Published: September 2010

Three 2,3,22,23,24‐pentahydro‐21H‐biline‐1,19‐dion‐10‐enium salts have been prepared: 17,18,22,23‐tetramethyl‐2,3,22,23,24‐pentahydro‐21H‐biline‐1,19‐dion‐10‐enium methanesulphonate (1) with a central flexible structure, its 10‐syn‐locked 22,23‐methylene homologue (2) with a central Z‐syn structure and its 8,12‐isopropylidene homologue (3), the first bile pigment with a central E‐anti structure. For comparison, the corresponding E‐anti 22,23,24‐trihydrobilinedionium salt 3a was also prepared. The 2,3 single bond induces a more delocalized charged system, having higher fluorescence quantum yields than observed for the fully conjugated systems. The flexible systems 1, 1a, which adopt an E‐anti conformation, do not show fluorescence. The E‐anti systems have a larger Stokes shift than do the Z‐syn systems. In the pentahydrobilinedionium salts, the highest positive charge resides on the central C10 atom as apparent from 1H NMR spectroscopy and as was corroborated by the finding that the site of nucleophilic attack is invariably at C10. The pentahydro salts also undergo electrophilic H‐D exchange at positions 5, 7 and 12. From the spectral data presented it would appear that the chromophore in (allo)phycocyanine will occur as a protonated system in a rigid protein environment in the E‐anti conformation.
Bacteriorhodopsin: The influence of the cyclohexene‐ring methyls
Authors: Courtin J. M. L. , Verhagen L. , Biesheuvel P. L. , Lugtenburg J. , van der Bend R. L. , van Dam K.
Published: September 2010

Four ring‐demethylated retinals, viz. 1,1,5′‐tridemethylretinal, 1,1′‐didemethylretinal, 1,5‐didemethylretinal and 1‐didemethylretinal, have been synthesized via new and simple schemes. The properties of these modified retinals, their protonated Schiff bases and the corresponding bacteriorhodopsins have been studied and compared with the native system. These bacteriorhodopsin analogues have also been tested for their proton‐pump efficiencies. A large decrease in proton‐pump activity was found for the analogues lacking the 5‐methyl group. On the whole, the opsin shifts of the modified bacteriorhodopsins were much lower than those of the native system. UV‐Vis and 1H NMR data support a planar 6‐s‐trans conformation for the demethylated retinals in solution rather than a twisted 6‐s‐cis conformation (torsion angle 40‐60°) as found in retinal. This explains the lower opsin shift and the better fit of these demethylated retinals in bacteriorhodopsin's binding site, which, in its native form, contains a 6‐s‐trans chromophore.
The unexpected formation of trans‐β‐lactams in the reaction of α‐iminoesters (RNC(H)C(O)OR′) with diethylzinc. Crystal structure of trans‐1‐(N‐t‐butyl)‐3‐(N‐t‐butyl‐N‐ethylamino)‐4‐ methoxycarbonylazetidin‐2‐one
Authors: van Vliet M. Robert P. , Klaver Wim J. , Jastrzebski Johann T. B. H. , van Koten Gerard , Goubitz Kees
Published: September 2010

Reaction of iminoesters, RNC(H)COOR′, (R  tBu, i‐Pr; R′  Me, Et) with diethylzinc affords in a selective quantitative reaction trans‐β‐lactams. An X‐ray crystal structure determination of the obtained trans‐1‐(N‐t‐butyl‐3‐(N‐t‐butyl‐N‐ethylamino)‐4‐methoxy‐ carbonylazetidin‐2‐one has been carried out. The formation of the trans‐β‐lactams is believed to proceed via zinc enolates.
Conformational transmission in silatranes. Confirmation of the pentacoordinated structure in solution
Authors: van Genderen Marcel H. P. , Buck Henk M.
Published: September 2010

A 300‐MHz 1H‐NMR conformational analysis study of a set of pentacoordinated silicon compounds (silatranes) and their tetracoordinated counterparts is presented. The solution conformation of the silatranes shows a close resemblance to known X‐ray structures, although the silatrane conformation was found to depend upon the solvent polarity. Comparison of the conformation of a 2‐methoxyethoxy fragment in the tetra‐ and pentacoordinated systems showed the existence of conformational transmission, similar to that known for phosphorus compounds. Enhanced electrostatic repulsion between the vicinal oxygens in the 2‐methyoxyethoxy fragment produces a trans conformation around the CC bond in the pentacoordinated silatranes. The conformational transmission effect is smaller in silatranes than in phosphorus compounds, due to a relatively weak fifth bond.
Solvent dependence of the rotamer population around the interglycosidic C(5)C(6) bond of (1→6)‐β‐linked digalactopyranosides
Authors: de Vries Nanne K. , Buck Hendrik M.
Published: September 2010

A 300‐MHz 1H NMR study of the rotamer population around the interglycosidic C(5)C(6) bond of methyl 2,3,4‐tri‐O‐methyl‐6‐O‐(2,3,4,6‐tetra‐O‐acetyl‐β‐D‐galactopyranosyl)‐β‐ (1) and ‐α‐D‐galactopyranoside (2) is reported. The existence of the (1→6)‐β‐linkage in these compounds is established by homo‐ and heteronuclear 2‐D NMR techniques. It is shown that the dependence on solvent polarity of the population around the C(5)C(6) bond for the dissaccharides is almost identical to that for monosaccharides. The influences of stereoelectronic effects and solute–solvent interactions on the conformational properties of the interglycosidic linkage are discussed briefly.
A new approach to 1, 2‐dilithioethane; easy preparation of dilithiomethane
Authors: van Eikema Hommes N. J. R. , Bickelhaupt F. , Klumpp G. W.
Published: September 2010

1,2‐Dilithioethane (I) is postulated to be formed together with dilithiomethane (VIII, main product) from methylene iodide and lithium 4, 4′‐di‐tert‐butylbiphenyl in THF at T < ‐70°C. With Me3SnCl the tin derivatives of I, VIII and their C3 and C4 homologues are obtained.
Evidence for α‐lactone formation in the thioacetylation of some α‐hydroxy acids with the aid of Mitsunobu‐type reagent
Authors: Strijtveen Bert , Kellogg Richard M.
Published: September 2010

(S‐Mandelic acid, (S)‐lactic acid, and (S)‐2‐hydroxy‐3‐phenylpropionic acid react with thioacetic acid in the presence of the salt of diisopropyl azodicarboxylate and triphenylphosphine to give the α‐(acetylthio)‐substituted products. (S)‐Mandelic acid reacts with predominant retention of configuration whereas (S)‐lactic acid and (S)‐2‐hydroxy‐3‐phenylpropionic acid react predominantly with inversion. The involvement of α‐lactone intermediates is very likely.
1,2,3‐Triarylpropenones as starting materials for 2‐pyridinethiones
Authors: Paniagua Esther , Rubio María J. , Seoane Carlos , Soto José L.
Published: September 2010

2‐Pyridinethiones 3 are obtained in good yields from 1,2,3‐triarylpropenones 2 by treatment with cyanothioacetamide 1. Disulfides 4 result from oxydation of 3. Pyridinethiones 3 undergo methylation at the sulfur atom, leading to (methylthio)pyridines 5, which can be hydrolized to carboxamides 6. Thieno[2,3‐b]pyridines 7 are obtained from 3 upon reaction with methyl chloroacetate.
Synthesis of the antibiotic 1,5‐dideoxy‐1,5‐imino‐D‐glucitol; concomitant formation of the D‐mannitol analogue
Authors: Broxterman H. J. G. , van der Marel G. A. , van Boom J. H. , Neefjes J. J. , Ploegh H. L.
Published: September 2010

The easily accessible 1,2‐O‐isopropylidene‐3‐O‐benzyl‐α‐D‐glucofuranose was converted in six steps into 1,2‐O‐isopropylidene‐3,6‐di‐O‐benzyl‐5‐deoxy‐5‐azido‐α‐D‐glucofuranose. The latter afforded, after acidolysis followed by hydrogenolysis, 1‐deoxynojirimicine and a small quantity of 1‐deoxymannonojirimicine. The antibiotic thus obtained had an inhibitory effect on the trimming of N‐linked carbohydrates in IgM.
Synthesis of an analogue of the antithrombin binding region of heparin containing α‐L‐idopyranose
Authors: van Aelst S. F. , van Boeckel C. A. A.
Published: September 2010

The synthesis of a heparin‐like pentasaccharide, containing 2,6‐di‐O‐sulphated α‐L‐idopyranose instead of the naturally occurring 2‐O‐sulphated α‐L‐idopyranuronate, is described. Although conformational properties of these compounds are similar, the analogue is not biologically active.
Photochemistry of 2,5‐dialkyl‐1,3,5‐hexatrienes. The influence of the ground‐state conformation, controlled through steric substituent effects
Authors: Brouwer A. M. , Bezemer L. , Cornelisse J. , Jacobs H. J. C.
Published: September 2010

The photochemical reactions of a number of 2,5‐dialkyl‐1,3,5‐hexatrienes are described. Variations in the selectivity of the reactions are observed, which can be rationalized by considering differences in the composition of the equilibrium mixture of conformational isomers, brought about by steric substituent effects. This result is in agreement with the NEER principle. In addition, examples are given of changes in efficiency of light‐induced isomerizations, which are not restricted by the NEER principle. A reaction mechanism, involving a biradical intermediate, is proposed, which accounts for the formation of all of the reaction products containing a three‐membered ring. MO correlation diagrams show that the formation of the three‐membered ring as a primary step must occur in a conrotatory fashion.
1H NMR and 13C NMR spectroscopy of pyrene dianions
Authors: Rodenburg L. , Floor M. , Lefeber A. , Cornelisse J. , Lugtenburg J.
Published: September 2010

A simple technique for the preparation of anions of polycyclic aromatic hydrocarbons is described. 1H and 13C NMR spectra of dianions of pyrene and derivatives of pyrene are reported and discussed. The electron density is greatest at the carbon atoms in the perimeter of the dianions. The presence of an electron‐donating substituent at position 2 causes an increase in the charge at positions 1 and 3.
An analysis of complex alkyl‐substituent effects in the water‐catalyzed hydrolysis of 1‐acyl‐1,2,4‐triazoles
Authors: Mooij Herman J. , Engberts Jan B. F. N. , Charton Marvin
Published: September 2010

Pseudo‐first‐order rate constants and thermodynamic activation parameters have been determined for the water‐catalyzed hydrolysis of the 1‐acyl‐1,2,4‐triazoles 1–9 in water and in aqueous acetonitrile [x(H2O) = 0.80]. The reaction occurs via water‐catalyzed nucleophilic attack of water at the amide carbonyl group. Variation of the alkyl group in the acyl part of the substrate led to the sequence of reactivities (in water): R = t‐Bu > i‐Pr > Et > Me > n‐Pr > s‐Bu > i‐Pent > neo‐Pent. This complex behavior strongly suggests the operation of a composite steric effect most likely involving contributions from conformational preference (SEI), change of coordination number at carbonyl carbon (SE2), steric repulsion between O2 and the alkyl group (SE3), and steric inhibition of solvation (SE4). Modelling of the relative rates in water in terms of the expanded branching equation gave satisfactory results. Two possible transition states are proposed for the neutral hydrolysis, one involving intramolecular hydrogen bonding between the second water molecule and N2 of the 1,2,4‐triazole ring. The different sequence of reactivities for neutral hydrolysis in aqueous acetonitrile as well as the rates for hydroxide‐ion‐catalyzed hydrolysis in water (B AC2 mechanism) support the analysis of substituent effects.
The electron spin resonance spectra of the radical cations of p‐terphenyl, triphenylene and triptycene
Authors: Courtneidge John L. , Davies Alwyn G. , Mcguchan Deborah C.
Published: September 2010

The ESR spectra of the radical cation of p‐terphenyl (1), triphenylene (2), and triptycene (3) have been investigated in fluid solution.
Reactions of (C5Me5)M(2,3‐dimethyl‐1,3‐butadiene)Cl (M = Ti, Hf) with symmetrical acetylenes
Authors: Hessen Bart , Teuben Jan H.
Published: September 2010

The reactivity of the electronically unsaturated diene complexes (C5Me5)M(2,3‐dimethyl‐1,3‐butadiene)Cl (M = Ti, Hf) with symmetrical acetylenes RCCR (R = H, Me, Ph) is reported. Whereas the reaction of the Hf complex with ethyne resulted in a product derived from initial insertion into the metal‐diene bond, elimination of the diene was observed in the other reactions. Two types of metallacyclo‐2,4‐pentadiene products, formed by oxidative coupling of acetylenes, were isolated: monomeric (C5Me5)Hf(C4Ph4)Cl and dimeric [(C5Me5)TiCl]2(C4Me4).
An experimental and theoretical study of the dimetallation of vinylacetylene
Authors: Brandsma Lambert , Hommes Hendrik , Verkruijsse Hermann D. , Kos Alexander J. , Neugebauer Wolfgang , Baumgärtner Wolfgang , von Ragué Schleyer Paul
Published: September 2010

The direct dimetallation of vinylacetylene (1‐buten‐3‐yne) at the 2‐ and 4‐positions can be achieved by the butyllithium‐potassiumtert‐butoxide reagent, but not when butyllithium is used alone or in the presence of TMEDA. Supporting experimental and calculational (ab initioand MNDO) results suggest that the potassium acetylide, KC≡C‐CH = CH2, is formed first. Further metallation occurs at the activated C‐2 vinyl position. Preferential substitution at the C‐2 position int‐BuC≡CCH = CHt‐Bu also was demonstrated. Neither dianion nor dilithium models account for the dimetallation results; in both cases, 1,4‐arrangements are indicated to be better than 2,4. The 2,4‐dimetallated vinylacetylene is a useful intermediate allowing the synthesis of derivatives inaccessible otherwise.
Alumina‐ and Y‐zeolite‐catalyzed reaction of cyclohexanol with tert‐butylhydroperoxide
Authors: Floor M. , Kieboom A. P. G. , van Bekkum H.
Published: September 2010

The use of γ‐alumina and exchanged Y‐zeolites as catalysts in the liquid‐phase oxidation of cyclohexanol by tert‐butyl hydroperoxide (tBHP) has been studied. With γ‐alumina, cyclohexanone is formed, whereas the zeolites NaCeY and NaLaY give cyclohexyl formate as the main product. This compound is thought to originate from a zeolite‐mediated rearrangement of tBHP to acetone and methanol, oxidation of methanol to formic acid and electrophilic addition of formic acid to cyclohexene (formed in situ from cyclohexanol).
Synthesis and structure of a bis‐ortho‐chelated aryllithium compound: 2,3,5,6‐tetrakis[(dimethylamino)methyl]phenyllithium dimer
Authors: van der Zeijden Adolphus A. H. , van Koten Gerard
Published: September 2010

The reaction of 1,2,4,5‐(Me2NCH2)4C6H2 with one equivalent of n‐butyllithium affords Li2[C6H(CH2NMe2)4‐2,3,5,6]2 (1). The molecule consists of a dimeric unit containing three‐centre, two‐electron lithium‐bonded aryl groups. The distorted tetrahedral coordination sphere of the lithium atoms is completed by two N‐coordinating ortho‐CH2NMe2 substituents. Complex 1 is the first dimeric aryllithium compound characterized in which the coordination around lithium is completed solely by intramolecular donating hetero atoms.
Studies in gas‐phase thermal hydrogenolysis. Part IV: Chlorobenzene and o‐dichlorobenzene
Authors: Manion Jeffrey A. , Dijks Johannes H. M. , Mulder Peter , Louw Robert
Published: September 2010

The gas‐phase thermolysis of chlorobenzene 1 and o‐dichlorobenzene 6 in H2 at atmospheric pressure between 773‐1280 K, τ = 3‐9 s, has been carried out in tubular and tank flow reactors. On a per‐chlorine basis, 1 and 6 react at almost identical rates; the major end products are HCl and benzene. Small amounts of biphenyl (3) and, at incomplete dechlorination levels, chlorobiphenyls are also formed.

Dechlorination levels of 99.95% have been achieved. In an excess of H2, the pseudo‐first‐order rate constant for HCl production follows approximately log(k/s−1) ⋍ 10.9 ‐ 241.kJ/mol/(2.3. R. T) between 928 and 1155 K.

The mechanism involves hydrogen atoms, which dechlorinate 1 and 6 via both ipso substitution and chlorine abstraction. At temperatures ≥ 900 K, atomic hydrogen is almost in equilibrium with H2, while at lower temperatures, production of H• can be chemically induced, leading to chain reactions and a steady‐state concentration of H• greater than that for equilibrium.
New base‐labile amino‐protective groups for peptide synthesis
Authors: Verhart C.G.J. , Tesser G.I.
Published: September 2010

Modification of the methyl group in 2‐(methylsulphonyl)ethanol gave alcohols which could be used to introduce amino‐protective groups of the urethane type, which are labile in alkaline media. The cleavage involves β‐elimination.

Exchange of the methyl group by a phenyl group bearing a function with a negative inductive effect (NO2 or MeSO2) afforded protecting groups with a higher sensitivity to base. The recommended function is a disulphone: 2‐[4‐(methylsulphonyl)phenylsulphonyl]ethanol. The function has a somewhat better stability than the Fmoc group, resists catalytic hydrogenolysis, is highly stable in acidic medium and its elimination product does not polymerize spontaneously. The suggested designation of the new protective group is Mpc.
Conformational analysis of a biologically active heparin‐like compound, which contains an open chain fragment
Authors: van Boeckel C.A.A. , Wagenaars G.N. , Mellema J.‐R.
Published: September 2010

Conformational analysis of a biologically active heparin like oligosaccharide, which contains a glyceric acid‐O‐CH2‐O moiety instead of iduronic acid, was performed by high field (600 MHz) 1H‐NMR spectroscopy. Most remarkably, the glyceric acid ‐O‐CH2‐O moiety adopts a linear conformation in which the anomeric effect does not operate. Most likely, this phenomenon should be ascribed to electrostatic repulsions among the charged sulphate and carboxylate groups.
The elusive zirconium(III)
Authors: Wielstra Ytsen , Gambarotta Sandro , Chiang Michael Y.
Published: September 2010

The chemistry of zirconium(III) is comprehensively reviewed (95 references) up to April 1988 with emphasis being placed on synthetic methods and structural features.
Acetalation studies. Part VII. Some new aspects of the isopropylidenation of galactitol
Authors: Chittenden Gordon J. F.
Published: September 2010

Reaction of galactitol in dimethyl sulphoxide, with 2,2,‐dimethoxypropane‐pyridinium p‐toluenesulphonate has led to the isolation of some new isopropylidene acetals. Their structures have been established spectroscopically. Attempts have been made to assess their possible role in the conventional acid‐catalysed isopropylidenation of this hexitol.
Hybridization of phosphate‐methylated DNA and natural oligonucleotides. Implications for protein‐induced DNA duplex destabilization
Authors: van Genderen Marcel H. P. , Koole Leo H. , Buck Henk M.
Published: September 2010

Duplex stabilities have been determined for several hybrids of phosphate‐methylated oligonucleotides and natural DNA or RNA using UV hyperchromicity experiments. These hybrids have a higher stability than the corresponding natural duplexes, due to the absence of interstrand electrostatic repulsions. Comparison with stability data for hybrids of other neutral oligonucleotides (with phosphate‐ethylated and methyl phosphonate linkages) and natural DNA or RNA revealed that differences in stability could be attributed mainly to steric and stereoelectronic factors. For phosphate‐ethylated oligonucleotides, hybridization with a natural strand is strongly influenced by steric interactions of the ethyl group. Hybridization with RNA, which requires a tight A‐type conformation, is therefore difficult and the ethyl orientation (inward or outward, depending on the phosphorus configuration) determines the strength of the association with natural DNA. In methyl phosphonate systems, it appears that the presence of a PC bond disturbs the helix conformation for stereoelectronic reasons. This leads to a weaker hybridization with DNA and RNA for longer strands. Phosphate‐methylated oligonucleotides are found to have an optimal combination of steric and stereoelectronic factors and form the strongest hybrids with natural DNA. Absence of intrastrand phosphate‐phosphate repulsions causes a slightly different conformation for phosphate‐methylated DNA, which is evident in the cooperative character of the hybridization with natural DNA. A thermodynamic model for this cooperativity is presented and the model studies are extended to protein‐DNA complexes in which phosphate charges are also shielded. The preliminary results suggest that protein association can destabilize a DNA duplex, thus providing a possible mechanism for the action of DNA unwinding enzymes.
α‐Amino ketones derived from L‐proline, as precursors for isomitosanes and mitosenes
Authors: Orlemans E. O. M. , Verboom W. , Lammerink B. H. M. , Reinhoudt D. N.
Published: September 2010

2‐Benzoyl‐ and 2‐(2‐naphthoyl)pyrrolidines which can be viewed as aryl α‐amino ketones, have been prepared from L‐proline via Grignard reactions, bromo‐to‐lithium exchange, or by Friedel‐Crafts acylations. It was impossible to convert the ketone function of these aryl α‐amino ketones into a tert‐alcohol group. The BOC‐protected (S)‐2‐acetyl‐l‐pyrrolidine (15), obtained by reaction of BOC‐proline 2‐pyridinethiol ester (2) with LiMe2Cu, could be converted in two steps into the amino alcohol 17, which contains a tert‐alcohol function. Upon treatment of 17 with BBr3, followed by air oxidation, mitosene 20 was obtained in a low yield. The Friedel‐Crafts reaction products 23 of 1,4‐dimethoxynaphthalene, or 2,5‐dimethoxytoluene and (S)‐l‐(trifluoroacetyl)–2‐pyrrolidinecarbonyl chloride, could be converted in two steps into the isomitosanes 25. The isomitosanes 25 could easily be dehydrated, using acid, to the corresponding mitosenes 26. Upon reduction of the isomitosanes 25 with Na2S,2O4 to the corresponding hydroquinones 28, spontaneous dehydration occurred.
The use of a double Wolff‐Kishner reduction in the preparation of aceanthrene and aceanthrylene
Authors: Boerrigter Jos C. Olde , Mulder Patrick P. J. , van der Gen Arne , Cornelisse Jan , Lugtenburg Johan , Mohn Georges R.
Published: September 2010

Aceanthrene (2) and aceanthrylene (1) are easily prepared in high yield from aceanthrenequinone (3) by means of a double Wolff‐Kishner reduction.
Reactions of 11C atoms with benzene. II. Formation of 11C‐labelled toluene and cycloheptatriene
Authors: Kuipers P. , van Halteren B. W. , Lindner L. , Brinkman G. A.
Published: September 2010

The reactions of recoil 11C atoms with liquid C6H6 result in the production of 11C‐labelled cycloheptatriene (CHT) and toluene in equal amounts of approximately 3%. From the effect of O2 on the product yields it is concluded that recoil labelling proceeds via both singlet and triplet 11CH2 intermediates, whereas 11CH may also contribute specifically to the formation of C6H5CH3. Addition of H2S increases the [11C]C6H5CH3 yield by ca. 5%, probably due to reactions of triplet phenyl‐carbene (C6H5CH). The reactions of singlet CH2, produced via the photolysis of CH2N2, with liquid C6H6 were also investigated in the presence of N2, O2 and I2.
Proton NMR study of alkali cation‐cyclogentiotetraose peracetate complexation
Authors: Bonas Georges , Vignon Michel
Published: September 2010

Complexation studies of alkali cations with a cyclic tetrasaccharide made up of (1 → 6)‐β‐D‐gluco‐pyranosyl residues, cyclogentiotetraose peracetate (CGD4Ac) by proton n. m. r. spectroscopy in deuterated acetone and nitromethane are described.

The ligand affinity decreases in both solvent, from cesium to lithium as follows: Cs+ ∼ Rb+ > K+ > Na+ > Li+, in agreement with the ligand cavity size (3.3 Å) calculated by molecular modelling.

During complexation, a conformational change of the CGD4Ac ligand was observed, the gg conformer being predominent at the complexed state.
The “tert‐amino effect” in heterocyclic chemistry: Synthesis of tetra‐ and pentacyclic compounds
Authors: Nijhuis W. H. N. , Leus G. R. B. , Egberink R. J. M. , Verboom W. , Reinhoudt D. N.
Published: September 2010

A new and convenient route for the synthesis of several heterotetra‐ and pentacyclic compounds is presented. Heating the 2‐vinyl‐N,N‐dialkylanilines 3a‐h, synthesized from the aldehydes 2a‐f,h or the acetophenone 2g and malononitrile or ethyl cyanoacetate, in refluxing 1‐butanol yields selectively the tetra‐ and pentacyclic compounds 4a‐h. This cyclization proceeds via a 1,5 hydrogen shift, followed by an addition of the carbanion to the iminium double bond. The benzo[f]thieno[3,2‐a]quinolizine 4c and the benzo[f]thieno[2,3‐a]quinolizine 4d represent new classes of heterocycles.

The first example of formation of a six‐membered ring via carboncarbon bond formation, starting from 2‐vinyl‐substituted enamines, was the conversion of 5a and 5b into the benzo[h]pyrrolo[1,2‐a]‐quinoline 6a and the naphtho[2,1‐c]quinolizine 6b, respectively. Most probably, the cyclization of 5a and 5b proceeds in the same way as does the cyclization of the 2‐vinyl N,N‐disubstituted aniline derivatives.

Heating the tricyano compound 5c in refluxing 1‐butanol gave both the benzo[g]pyrrolo[1,2‐a]indole 7 and malononitrile. This cyclization proceeds probably via a 1,6 hydrogen shift, followed by a 1,5 electrocyclization, and a subsequent fast elimination of malononitrile.
Synthesis of retinals labelled with 13C in the cyclohexene ring
Authors: Gebhard R. , Courtin J. M. L. , Shadid J. B. , van Haveren J. , van Haeringen C. J. , Lugtenburg J.
Published: September 2010

In this paper we report the synthesis of five retinals, specifically mono‐labelled with 13C at the positions 1, 2, 3 and 4 and di‐labelled with 13C at the positions 16 and 17, with more than 98% chemical purity and high 13C incorporation. (1‐13C)‐ and (16, 17‐13C2)retinal were obtained in 20% overall yield from (2‐13C)‐ and (1,3‐13C2)acetone. (2‐13C)‐ and (3‐13C)retinal were prepared in 10% overall yield from (1‐13C)‐ and (2‐13C)acetonitrile, while (4‐13C)retinal was synthesized in 17% overall yield starting from (2‐13C)acetonitrile.
Synthesis of fragments of the capsular polysaccharide of Haemophilus influenzae type b: Part IIII‐3. A solid‐phase synthesis of a spacer‐containing ribosylribitol phosphate hexamer
Authors: Elie C. J. J. , Muntendam H. J. , van den Elst H. , van der Marel G. A. , van Boom J. H. , Hoogerhout P.
Published: September 2010

A solid‐phase synthesis of an aminohexyl‐containing hexameric fragment of the H. influenzae type b capsular polysaccharide, using a phosphoramidite approach and controlled‐pore glass as the solid support, is reported.
On the synthesis of bis{2‐(dimethylarsino)ethyl} amine
Authors: Edwards Peter G. , Wills A. Richard
Published: September 2010

The previously reported syntheses of HN(CH2CH2AsMe2)2 are found to be irreproducible. Two by‐products, the compound ClCH2CH2N(H)CH2CH2AsMe2 and the salt {H2N(CH2CH2AsMe2)2}+ I−, have been identified.
Sucrose‐derived dialdehydes: The course of the periodate oxidation as studied by HPLC
Authors: de Wit D. , van Rantwijk F. , Maat L. , Kieboom A. P. G.
Published: September 2010

Sucrose was oxidized with sodium periodate to di‐ and tetraaldehyde compounds. A fast and reliable analytical HPLC method was developed to follow the course of the reaction as a function of the amount of periodate added.

Apart from the sucrose tetraaldehyde, i.e. the final oxidation product, only two out of the four possible sucrose dialdehyde products were formed: one with the glucose ring twice oxidized (GOO‐F) and one with an oxidized fructose ring (G‐FO). Both dialdehydes were formed in equal amounts when low amounts of periodate were added, whereas GOO‐F was preferentially formed when higher amounts of periodate were added.

The course of the reaction is discussed in terms of internal hemiacetal formation of the sucrose dialdehydes.
Preparation and calcium complexation of oxidized polysaccharides. II: Hydrogen peroxide as co‐reactant in the chlorite oxidation of dialdehyde glucans
Authors: Floor M. , Hofsteede L. P. M. , Kieboom A. P. G. , van Bekkum H. , Greenland W. P. T. , Verhaar L. A. Th.
Published: September 2010

A series of dialdehyde glucans (obtained from starch, cellulose and dextran by periodate glycol cleavage) were oxidized to the corresponding dicarboxy polysaccharides by the combined use of sodium chlorite and hydrogen peroxide (two moles of each per mole dialdehyde moieties). As compared to the conventional method (using six moles of chlorite), this new procedure (i) reduces oxidant costs by a factor of 2.5, (ii) avoids the evolution of toxic chlorine dioxide and (iii) more selectively yields dicarboxy polysaccharides of high molecular weight with excellent calcium complexing properties. The products thus obtained are potentially attractive co‐builders in phosphate‐free detergents.
Sulfonation of 1, 2‐(methylenedioxy)benzene, 1, 2‐(ethylenedioxy)benzene, 2‐methylanisole, 2, 3‐dihydrobenzofuran, chromane, benzo‐15‐crown‐5 and dibenzo‐30‐crown‐10 in concentrated sulfuric acid and with sulfur trioxide
Authors: Ansink Harold R. W. , Cerfontain Hans
Published: September 2010

The sulfonation of 1, 2‐(methylenedioxy)benzene (2), 1,2‐(ethylenedioxy)benzene (3), benzo‐15‐crown‐5 (4), 2‐methylanisole (5), 2,3‐dihydrobenzofuran (7), 2, 3‐dihydrobenzopyran (chromane, 8) and dibenzo‐30‐crown‐10 (9) in concentrated sulfuric acid and with sulfur trioxide mainly in nitromethane has been studied. Upon dissolution in sulfuric acid, 3 yields a mixture of 3‐3‐sulfonic acid (3‐3‐S) and 3–4‐S in a ratio of 0.8, independent of the H2SO4 concentration. In sulfuric acid with a concentration greater than 83% H2SO4, the two monosulfonic acids both eventually yield 3‐3,5‐S2. The ratio of rate coefficients of sulfonation of 3‐3‐S and 3‐4‐S is approximately 70. The reaction of the crown ethers 4 and 9 in sulfuric acid leads to formation of 4‐4‐S and to an equimolar mixture of 9‐4,4′‐S2 and 9‐4,5′‐S2, respectively.

Sulfonation of 5 in 85 and 98% H2SO4 yields a mixture of 5–4‐S and 5‐5‐S in a ratio of approximately 7:1 and 11:1, respectively; in 98% H2SO4, these products are converted into 5‐4,6‐S2 and 5‐3,5‐S2, respectively. On dissolving 5 in 105% H2SO4, the initially observed product is 5–4,6‐S2, which exhibits O demethylation.

Sulfonation of both 7 and 8 in 85% H2SO4 yields a mixture of 4‐S and 6‐S in an initial ratio of 1.4. These acids eventually yield 7‐4,6‐S2 and 8‐4,6‐S2. The rate coefficients of sulfonation of 7‐6‐S and 8‐6‐S are both approximately 50 times those of the corresponding 4‐sulfonic acid isomers. Upon sulfonation with 1.0 equiv of SO3 in nitromethane, all the substrates studied yield the corresponding 4‐sulfonic acid. Upon reaction with 2.0‐4.0 equiv of SO3, 2, 3, 7 and 8 all yield the 4,6‐S2 almost quantitatively; 4 does not yield any disulfonic acid. Reaction of 9 with ≥2.0 equiv of SO3 yields a 1:1 mixture of 4,4′‐S2 and 4,5′‐S2.

The routes of the sulfonation of 1,2‐(methylenedioxy)‐ (2) and 1, 2‐(ethylenedioxy)benzene (3) and their monosulfonic acids have been compared with those of 1,2‐dimethoxybenzene (1) and its 4‐sulfonic acid and the differences are discussed in terms of the Curtin‐Hammett principle. The differences observed between the mono‐ and disulfonation of 2,3‐dihydroxybenzofuran (7) and 2,3‐dihydrobenzopyran (8) on the one hand and those of 2‐methylanisole (5) on the other are discussed along similar lines.
Glucose 1‐phosphate oxidation on platinum‐on‐carbon catalysts: Side‐reactions and effects of catalyst structure on selectivity (Pt/C oxidation catalysts, part 7)
Authors: van Dam H. E. , Kieboom A. P. G. , van Bekkum H.
Published: September 2010

The selectivity aspects of the platinum‐catalyzed oxidation of glucose 1‐phosphate into glucuronic acid 1‐phosphate have been studied. A partial loss of starting material occurs through side‐reactions starting with oxidation of secondary hydroxyl groups, followed by CC bond cleavage. The ring‐CHOH oxidation is retarded by the substituent at C(5); its protecting ability follows the order COO− > CH2OH > H. The maximum selectivity (∼ 70%) is independent of the usual reaction variables (e.g. pH, metal dispersion), but can be increased upto 85% by an oxidative pretreatment of the activated carbon used as catalyst carrier.
A structural study of phosphate‐methylated d(CpG)n and d(GpC)n DNA oligomers. Implications of phosphate shielding for the isomerisation of B‐DNA into Z‐DNA
Authors: Quaedflieg Peter J. L. M. , Koole Leo H. , van Genderen Marcel H. P. , Buck Henk M.
Published: September 2010

It is shown that the phosphate‐methylated DNA miniduplex d(CpG)2 adopts a left‐handed Z‐type structure, whereas the phosphate‐methylated miniduplex d(GpC)2 is right‐handed. Furthermore, it is shown that elongation of the systems to the tetramer level results in right‐handed duplexes with rather low stability. The phosphate‐methylated hexamer d(CpGpCpGpCpG) is present in the single‐strand form. It is suggested that these results are of interest in the understanding of the exact role of salt cations in stabilizing the Z‐DNA structure of natural duplexes d(CpG…CpG)2 in high salt solution, since the methylated phosphate groups in fact mimic the situation of complete phosphate‐charge shielding. Therefore, it is concluded that exclusive cation‐phosphate complexation in the d(CpG)2 segments stabilizes the Z structure. 31P‐NMR experiments on the natural duplex d(CpGpCpGpCpG)2 at different concentrations of Mg2+ provided independent proof for the proposed model.
Synthesis of 3‐methyl‐, 4‐methyl‐ and 3,4‐dimethylcyclopenta[cd]pyrenes
Authors: Kellenbach E. R. , Lugtenburg J. , Cornelisse J.
Published: September 2010

A convenient synthesis of three cyclopenta[ad]pyrene derivatives methylated on the five‐membered ring is described. The starting materials were easily available ketones which were either Wolff‐Kishner‐reduced or treated with methyllithium followed by dehydration with p‐toluene‐sulfonic acid to give the partially saturated derivatives. Careful aromatization with DDQ gave the novel methylated compounds in reasonable yields.
Substituent effects on the photocycloaddition reactions of anisoles to acrylonitriles
Authors: Al‐Jalal Nader , Gilbert Andrew
Published: September 2010

Methyl‐, ethoxy‐, and phenyl‐substituted acrylonitriles undergo efficient 1,2‐photocyclo‐addition to 4‐cyanoanisole, but the azocine formation of the parent addend is inhibited by these groups. The regiochemistry, selectivity and efficiency of the cycloaddition of acrylonitrile to cyanomethoxyanisoles vary markedly with the position of the arene substituents and reflect competition between the influences of the cyanoanisole and dimethoxybenzene moieties.
Models for the adsorption of dioxygen on metal chelates
Authors: Elzing A. , van der Putten A. , Visscher W. , Barendrecht E.
Published: September 2010

Metal chelates are suitable electrocatalysts for the reduction of dioxygen. The selectivity of the reduction process (H2O2 or H2O as end product) depends on which catalyst is used. Comparable compounds, such as cobalt and iron phthalocyanine (CoPc and FePc), show different selectivities. CoPc catalyzes the dioxygen reduction pathway with H2O2 as the end product, whereas FePc gives H2O. The interaction between the catalyst and dioxygen is crucial for the selectivity of the reduction process. In this review, the most important models for the adsorption of dioxygen on metal chelates are discussed.

Most of these models predict that dioxygen is adsorbed in a so‐called “end‐on (bent)” configuration on to the metal phthalocyanines. The models also predict that, for the dioxygen adduct of CoPc, the oxygen fragment is more negatively charged than for the adduct of FePc, and that this may be the reason for the observed difference in selectivity. Alternatively, dioxygen can bind in between two metal phthalocyanine molecules. For these dioxygen‐bridged dimers, as can be derived from adsorption (bonding) models, the cleavage of the oxygen‐oxygen bond occurs more easily in the case of the FePc dioxygen dimer than in the corresponding Co complex.
Acid‐catalyzed transfer sulfonation of anisole using overcrowded polymethylbenzenesulfonic acids
Authors: Koeberg‐Telder Ankie , Cerfontain Hans
Published: September 2010

Transfer sulfonations of anisole using various polymethylbenzenesulfonic acids (PoMBR's) catalyzed by trifluoromethanesulfonic acid at 25 °C are described. The observed reactivity order for transfer sulfonation increases in the order 2,4‐DiMBS, 2,6‐DiMBS, 2,3,4,5‐TeMBS (all non‐reactive) < 2,3,5,6‐TeMBS ≈︁ 2,4,6‐TrMBS < 2,3,4,6‐TeMBS < PeMBS. This order illustrates that the main factor in determining the transfer sulfonation reactivity is ‐ in addition to the electronic activation by the 4‐methyl ‐ the degree of overcrowding around the sulfonic acid group of the PoMBS. A mechanism is suggested in which the essential step involves the transfer of SO3H+ from the 1‐arenonium‐1‐sulfonic acid ion to the anisole.
On Shustorovich's Bond‐Order conservation method as applied to chemisorption
Authors: van Santen R. A.
Published: September 2010

The Bond‐Order Conservation principle is shown to be rigorously true within the Valence‐Bond approximation excluding ionogenic terms. After a short discussion of Shustorovich's adaptation of this method to chemisorption on metals, a new improved expression for the activation energy of dissociation is derived, based on a transition‐state model derived from Extended Hückel calculations.

It appears to be essential to account for the weakening of the surface‐surface metal bonds.

New expressions for atomic and molecular adsorption energies are derived.

A major result of this work is a significant reduction in the activation energy for dissociation due to loss of surface metal‐metal atom bonding upon chemisorption.
Kinetics and mechanism of the gas‐phase oxidation of styrene to benzaldehyde, acetophenone and trans‐cinnamaldehyde over a heterogeneous surface‐vanadate Wacker catalyst
Authors: van der Heide Evert , Schenk John , Gerritsen Albert W. , Scholten Joseph J. F.
Published: September 2010

A study has been made of the kinetics and mechanism of the oxidation of styrene over a heterogeneous vanadate‐monolayer Li2PdCl4 Wacker catalyst in the temperature range 348 K to 453 K at 0.11 MPa total pressure. The kinetics of the reaction, with respect to the dependence of the reaction rate on the partial pressures of styrene and water, as well as on the surface LiCl concentration, appear to be logically related to the kinetics of the homogeneous Wacker process.

In addition to acetophenone (initial selectivity 45 mol %), benzaldehyde, probably originating from phenylacetaldehyde, is formed with gradually increasing selectivity (initial selectivity 55 mol %). Formation of benzoic acid can sometimes be detected but in all cases with a selectivity lower than 5 mol %. Initially no other products are detected but, after a short period, trans‐cinnamaldehyde is formed with increasing selectivity (15 mol % after three days).

Deactivation of the catalyst is due to carbonaceous deposits and removal of these deposits by oxidation with air at 473 K leads to partial regeneration.
Basket‐shaped hosts with semi‐flexible handles
Authors: Smeets J. W. H. , Visser H. C. , Kaats‐Richters V. E. M. , Nolte R. J. M.
Published: September 2010

Basket‐shaped hosts were synthesized starting from the concave building block 4

magnified image To this end, oligo(oxyethylene) bridges, containing different aromatic rings, were attached to the 3,6‐ and 3′,6′‐positions of the xylylene rings of 4. These bridges include 5,6‐benzo‐1,4,7,10‐tetraoxadec‐5‐ene, 5,6,7‐benzo‐1,4,8,11‐tetraoxaundeca‐5,6‐diene, 5,6‐(2,3‐naphtho)‐1,4,7,10‐tetraoxa‐dec‐5‐ene, 8,9‐benzo‐1,4,7,10,13,16‐hexaoxahexadec‐8‐ene and 8,9‐(2,3‐naph‐tho)‐1,4,7,10,13,16‐hexaoxahexadec‐8‐ene chains. Alkali‐metal ions and protonated aliphatic or aromatic diamines are bound in these baskets in a 1:1 host/guest ratio. For the metal ions, a clamshel‐llike or a sandwich‐like complex is proposed. Aliphatic diammonium guests are bound in such a way that the methylene chains are wedged in between the o‐xylylene units of the host, as concluded from the upheld shifts observed in the 1H NMR spectra of the guest CH2 protons. Free energies of binding of the new hosts with nineteen guests were measured using the picrate extraction technique. The highest ‐ ΔG° values were obtained for the complexes with the diammonium salts (‐ ΔG° values up to 13.4 kcal/mol).
Cation complexation with functionalized 9‐arylacridinium ions. Part II: Conformational and spectral response upon metal‐ion complexation
Authors: Jonker S. A. , Verhoeven J. W. , Reiss C. A. , Goubitz K. , Heijdenrijk D.
Published: September 2010

The chromoionophoric behaviour of compound 1 in solution was studied in relation to the X‐ray structure of its complexes with Na+, Ba2+ and Ag+. Interaction between the metal ion and the “soft” nitrogen atom, which forms part of the aza crown ether of 1, is concluded to be of crucial importance. It is shown that in solution a soft cation such as Ag+ induces a greater special spectral shift than do hard cations of equal charge and almost the same size, such as Na+ and K+. Solid‐state structures of the Ag+ and Na+ complexes of 1 show a relatively short Ag+ ‐N distance and a relatively long Na+ ‐ N distance.

Netherlands Organization for Chemical Research (SON) with financial aid from the Netherlands Organization for Scientific Research (NWO).
Synthesis of specifically deuteriated 9‐ and 13‐demethylretinals
Authors: van den Berg Ellen M. M. , van der Bent Arie , Lugtenburg Johan
Published: September 2010

(13–2H) 13‐Demethylretinal, (11,12,13‐2H,) 13‐demethylretinal, (9‐2H)9‐demethylretinal and (9,10‐2H2)9‐demethylretinal were prepared in all‐E, 9Z, 11Z and 13Z isomeric form with high deuterium incorporation. In the synthesis of (9,10‐2H2)9‐demethylretinal, a Horner‐Emmons reaction with deuteriated diethyl (cyanomethyl)phosphonate was used. This synthon was subsequently used for the synthesis of (10‐2H)‐ and (14‐2H)retinal in high yield and with high deuterium incorporation. From the 13C NMR spectra of the demethylretinals (all‐E, 9Z and 13Z), he effect of a methyl group on the 13CNMR parameters of polyene‐chain carbons was determined.
X‐Ray‐excited luminescence of organic gadolinium compounds
Authors: Blasse G. , Brixner L. H.
Published: September 2010

The X‐ray excited emission spectra of gadolinium acetate tetrahydrate, [Gd ⊂ 2.2.1]Cl3·2H2O and Gd[C3F7COCHCOC(CH3)3]3 (or Gd[FOD3), are reported. The first two compounds show strong Gd3+ emission (6P←8S) at room temperature, while the third compound gives rise to strong ligand emission, but only at very low temperature. Eu(FOD)3, however, shows intense Eu3+ emission upon X‐ray excitation. The Gd3+ emission spectra show many vibronic lines and, although their intensity is weak, it is much higher than generally accepted. These vibronic lines indicate which groups coordinate the Gd3+ ion and it is shown that their intensity increase as the ligand polarizability increases.
Rate‐limiting deprotonation of (arylsulfonyl)methyl perchlorates in dichloromethane. Evidence for proton tunneling and the high dynamic basicity of HMPA in a nonaqueous solvent
Authors: van Andel Maarten A. , Engberts Jan B. F. N.
Published: September 2010

Kinetic data are reported for the water‐catalyzed hydrolysis of several (arylsulfonyl)‐methyl perchlorates in water and for the HMPA‐catalyzed decomposition of the same substrates in dichloromethane. Under both conditions, the reaction exhibits general base catalysis and the covalent perchlorates react via rate‐limiting deprotonation at the α‐sulfonyl methylene moiety. Primary kinetic deuterium isotope effects (KDIE's) in water are indicative for a complete loss of zero‐point energy in the transition state. The high symmetry of the transition state is supported by the Brönsted β value of ca. 0.5.

HMPA was found to be an effective general base catalyst in dichloromethane as the solvent. This reaction shows primary KDIE's (kH/kD = 9.3 at 25°C) which are about 1.5 times higher than the (semi)classical limit. In addition, the ratio of the pre‐exponential Arrhenius factors is abnormally small (AH/AD = 0.14). These classically anomalous results are interpreted in terms of a contribution of quantum‐mechanical tunneling to the transition state for HMPA‐induced deprotonation. Possible reasons for the different kinetic characteristics of the deprotonation reaction in the aqueous and nonaqueous medium are given.
Transduction of host‐guest complexation into electronic signals: Favoured complexation of potassium ions by synthetic macrocyclic polyethers using membrane‐modified, ion‐sensitive field‐effect transistors (ISFETs)
Authors: Sudhölter Ernst J. R. , van der Wal Peter D. , Skowronska‐Ptasinska Maria , van den Berg Albert , Reinhoudt David N. , Bergveld Piet
Published: September 2010

The selective complexation of K+ and Na+ salts with a series of synthetic macrocyclic polyethers has been studied on the SiO2 gate surface of an Ion‐Sensitive Field‐Effect Transistor (ISFET). The synthetic macrocycles were incorporated in a PVC membrane directly attached to the SiO2 gate surface. The selectivities were measured either with tetraethylammonium chloride or lithium chloride as background electrolyte or by direct comparison. The slope of the response curves generally shows near‐Nernstian behaviour (53‐56 mV/pK+). Hemispherands 2 and 3, which have molecular cavities composed of 21 ring atoms and 1,3‐dimethyl‐p‐(tert‐butyl)calix[4]arene crown‐5 (11), show high selectivities of K+ over Na+ (log K(K+/Na+)= −2.9 to 3.2). The (substituted) benzo‐27‐crown‐9 derivatives (5‐7) show moderate selectivities (log K(K+/Na+) ≃ −2). The 18‐membered pyrido hemispherand hardly discriminates. Our results correlate well with reported data of picrate extraction experiments and with the relative‐association constants in methanol.