Although discotic liquid crystals are attractive functional materials, their use in electronic devices is often restricted by high melting and clearing points. Among the promising candidates for applications are [15]crown-5 ether-based liquid crystals with peripheral n-alkoxy side chains, which, however, still have melting points above room temperature. To overcome this problem, a series of o-terphenyl and triphenylene [15]crown-5 ether derivatives was prepared in which δ-methyl-branched alkoxy side chains of varying lengths substitute the peripheral linear alkoxy chains. The mesomorphic properties of the novel crown ethers were studied by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction.

δ-Methyl branching indeed lowers melting points resulting in room-temperature hexagonal columnar mesophases. The mesophase widths, which ranged from 87 to 30 K for o-terphenyls, significantly increased to 106–147 K for the triphenylenes depending on the chain lengths, revealing the beneficial effect of a flat mesogen, due to improved π–π interactions.

A systematic study on ring-closing metathesis with Grubbs II catalyst to cembranoid macrocycles is described. Acyclic terpenoids with a functional group X in the homoallylic position relative to an RCM active terminus and substituents R, R¹ directly attached to the other terminal double bond were prepared from geraniol derived trienes and fragments that are based on bromoalkenes and dimethyl malonate. Such terpenoids were suitable precursors, despite the presence of competing double bonds in their framework. The size of R and R¹ is crucial for successful macrocyclization. Whereas small alkyl substituents at the double bond directed the RCM towards six-membered ring formation, cross metathesis leading to dimers dominated for bulkier alkyl groups. A similar result was obtained for precursors without functional group X. In the case of unsymmetrically substituted terpenoid precursor (R=Et, R¹=Me) with homoallylic OTBS or OMe group, the RCM could be controlled towards formation of macrocyclic cembranoids, which were isolated with excellent E-selectivity. The role of the substituents was further studied by quantum chemical calculations of simplified model substrates. Based on these results a mechanistic rationale is proposed.

The (E)- and (Z)-terpene-based aldehydes 6b and 6c with a silyl ether function in the γ-position were prepared and investigated in boron-mediated asymmetric Evans aldol reactions. Screening experiments of chiral N-acylated oxazolidinones 7, which are conveniently accessible from 5-methyl-5-hexenoic acid and Evans oxazolidinone auxiliaries, with various boron triflates and terpenoid neral (Z)-6a as aldehyde component, provided conditions in which highly selective formation of syn-aldol adduct 5a occurred and competing C=C double bond isomerization to 10 was completely suppressed. Applying the optimized conditions to O-silylated aldehydes 6b and 6c and N-acyloxazolidinone derivative (R)-7a confirmed the syn-selectivity and gave the appropriate products syn-5b,c and syn-21b,c in good yields. In the case of neral-derived syn adduct 5a, the configuration of the new stereogenic centers C-2/C-3 could be assigned as (2R,3S).

Invited for this month’s cover are the three collaborating groups from the University of Stuttgart and the Fraunhofer Institute for Applied Polymer Research in Potsdam, both from Germany as well as the University of Windsor, Canada. The cover picture shows the shift of frontier orbital energies and symmetries upon excitation with light for three different chromophore lengths. Read the full text of the article at 10.1002/cplu.201300308

Three series of rigidified tri-, penta- and heptamethine merocyanine dyes were synthesised. A piperidyl moiety was chosen as the electron-donating substituent while the electron-accepting group was varied from ketones to malononitriles and cyanoacetates. The structures of the compounds in the solid state and in solution were elucidated by X-ray diffraction and NMR spectroscopy, respectively, while optical properties were investigated by absorption and emission spectroscopy. As a general trend, the acceptor properties decrease in the series malononitrile>cyanoacetate>ketone based on the analysis of their solvatochromic behaviour. The experimental results were further supported by calculations at the B3LYP 6-311+G(d) level of theory.

Tropane-derived phosphorus–olefin hybrid ligands bearing various combinations of P-units (chiral BINOL-derived units/achiral PPh₂-units) with a tropene skeleton (chiral/achiral) have been synthesized and used in Cu-catalysed conjugate 1,4-additions of diethylzinc to cyclic enones and in Rh-catalysed 1,4-additions of phenylboronic acid (Hayashi–Miyaura reaction) with cyclic and acyclic enones. Whereas in Cu-catalysed reactions only moderate yields and ee values (up to 49 % ee) were obtained, up to 97 % ee was observed in the Hayashi–Miyaura reaction with various substrates. The most promising results in both reactions were achieved with ligands 11 and 15, which contain a chiral alkene/achiral P-unit or an achiral alkene/chiral P moiety, respectively, and where the metal is complexed on the exo side of the tropene skeleton. In contrast, ligands with an endo coordination mode showed only low (13) or no activity (12) in all of the 1,4-additions. NMR and ESI-MS experiments revealed the presence of two cationic [Rh^I(15)₂]⁺ species in solution.

The silyl-protected allylhydropentalenone derivative 9, derived from the Weiss diketone, was functionalized by enantioselective deprotonation in the presence of lithium bis(1-phenylethyl)amide/LiCl as the chiral base, which was generated in situ from bis(1-phenylethyl)ammonium chloride [(R,R)- or (S,S)-14] and BuLi, and trapping of the resulting enolate with alkyl halide electrophiles to give pseudo-C₂ or -C_s-symmetrical bicyclo[3.3.0]octanones 10 and 11. The influence of the chiral base and electrophiles on the regioselectivity and double stereodifferentiation was investigated. Taking the double stereocontrol into account, a matched selectivity for the pseudo-C₂ pair (1R,4R)-10/(1S,4R)-10 and a mismatched selectivity for the pseudo-C_s pair (1S,6R)-11/(1R,6R)-11 were observed in the presence of (R,R)-14. The use of (S,S)-14, however, produced a mismatched selectivity for (1R,4R)-10/(1S,4R)-10 and a matched selectivity for (1S,6R)-11/(1R,6R)-11 with complementary diastereoselectivity depending on the electrophile. Furthermore, the hydropentalenone derivative 10a provided an alternative route to the bicyclo[3.3.0]octene core of the macrocyclic tetramic acid lactam, cylindramide (5).

Although catalytic reductions, cross-couplings, metathesis, and oxidation of CC double bonds are well established, the corresponding catalytic hydroxylations of CH bonds in alkanes, arenes, or benzylic (allylic) positions, particularly with O₂, the cheapest, “greenest”, and most abundant oxidant, are severely lacking. Certainly, some promising examples in homogenous and heterogenous catalysis exist, as well as enzymes that can perform catalytic aerobic oxidations on various substrates, but these have never achieved an industrial-scale, owing to a low space-time-yield and poor stability. This review illustrates recent advances in aerobic oxidation catalysis by discussing selected examples, and aims to stimulate further exciting work in this area. Theoretical work on catalyst precursors, resting states, and elementary steps, as well as model reactions complemented by spectroscopic studies provide detailed insight into the molecular mechanisms of oxidation catalyses and pave the way for preparative applications. However, O₂ also poses a safety hazard, especially when used for large scale reactions, therefore sophisticated methodologies have been developed to minimize these risks and to allow convenient transfer onto industrial scale.

Novel chiral amino acid derived ionic liquid crystals with amine and amide moieties as spacers between the imidazolium head group and the alkyl chain were synthesised. The key step in the synthesis utilised the relatively uncommon SO₃ leaving group in a microwave-assisted reaction. The mesomorphic properties of the mesogens were determined by differential scanning calorimetry (DSC), polarising optical microscopy (POM) and X-ray diffraction. All liquid crystalline salts exhibit a smectic A mesophase geometry with strongly interdigitated bilayer structures. An increase of the steric bulk of the stereogenic centre hindered the formation of mesophases. In case of phenylalanine-derived derivatives a mesomorphic behaviour was observed for shorter alkyl chains as compared to other amino acid derivatives indicating an additional stabilising effect by the phenyl moiety.

Taking the regio- and chemoselectivities of our iron complex precursors with pyridine core in aerobic oxidations into account, we envisioned a more effective influence on catalytic properties by the introduction of different substituents in 4-position of the pyridine moiety. The synthesis of these novel 4-substituted (pyridine-2,6-diyl)dipropanoic acids 4 is described. Analogously to the unsubstituted derivative, ligands 4 reacted with Fe(ClO₄)₃ to form trinuclear Fe₃(μ₃-O) complexes 3, which were tested in the aerobic Gif-type oxidation of α-pinene to myrtenol, verbenone, myrtenal, and pinene oxide. The electronic nature of the substituents was found to slightly effect the ratio of allylic oxidation/epoxidation, whereas its influence on the oxidation preference of secondary to primary CH bonds is negligible as compared to the unsubstituted complex.

Ionic liquid crystals are mesogenic compounds that consist of cations and anions, usually rod-like cations and spherical anions. Herein we report a new method for the synthesis of ionic liquid crystals by using cations and anions of the same molecular shape with oppositely charged head groups. Thus, 4-alkoxyphenylpentamethylguanidinium 4-alkoxyphenylsulfonate ion pairs have been synthesised. 4-Alkoxyphenylpentamethylguanidinium iodides were also prepared to determine the influence of congruently shaped anions, in comparison with their spherical counterparts, on mesophase behaviour, which was investigated by differential scanning calorimetry (DSC), polarising optical microscopy (POM) and X-ray diffraction (XRD). All the liquid crystalline salts exhibit smectic A mesophases with strongly interdigitated bilayer structures. The guanidinium sulfonate ion pairs show mesomorphic properties from shorter alkyl chain lengths (≥C₉) and lower melting points (≈10 K), whereas the corresponding guanidinium iodides are liquid crystalline for longer alkyl chain lengths (≥C₁₄). For chains with ≥C₁₈, however, the mesophase range decreases for the sulfonate ion pairs, but not for the iodide salts.

The ex-chiral pool synthesis of the Theodorakis C3–C11 fragment of borrelidin (1) by a chemoenzymatic approach in nine steps and 12 % overall yield starting from enantiopure methyl-branched preen gland wax esters is described. In the initial reaction, the wax ester was (ω-1)-hydroxylated regioselectively by using a mutant of the cytochrome P450 monooxygenase CYP102A1 as biocatalyst. The fourthstereocenter in the C3–C11 fragment was generated by chiral auxiliary mediated alkylation. In order to explain the experimental results of the alternative epimerization of an all-syn polypropionate, quantum chemical calculations were performed at the B3LYP/cc-pVDZ level.

Coupled to the development of a total synthesis of gephyronic acid, a series of diastereomeric analogues and their precursors have been prepared by employing complementary aldol strategies for the key coupling step of fragments 4 and 5. A biological evaluation revealed the importance of the epoxide for the cytotoxicity against L-929 (mouse fibroblast) and KB-3-1 (HeLa clone, human cervix carcinoma derived) cell lines. Moreover, variation of the configuration of the C3–C5 stereotriad and the C1 carboxylic acid were found to be important features. Improved activities compared with the natural product were observed when the carboxy terminus at C1 was replaced by a methyl ester or PMB-protected alcohol. Surprisingly, the derivatives (8R)-17d and (8S)-16a showed antibacterial activity against Pseudomonas aeruginosa.

New P-stereogenic phosphoramidites based on a (–)-pinane framework were synthesized and investigated by NMR spectroscopic methods and single-crystal X-ray structural analysis. The reaction between cis-pinandiol 2 and N,N-dialkylphosphoramidous dichlorides, followed by protection with theBH₃·THF adduct, afforded diastereomeric mixtures of P_S and P_R BH₃–phosphoramidite complexes. After separation by column chromatography, these stereoisomers were obtained in a pure form. The BH₃ group was further cleaved by treatment with Et₂NH at elevated temperature and the obtained free phosphoramidite ligands were used for catalytic enantioselective conjugate addition reactions. As substrates, a series of cyclic and acyclic enones was employed and addition was carried out with Et₂Zn in the presence of catalytic amounts of Cu^I thiophenecarboxylate (CuTC) and a phosphoramidite ligand. The results revealed that the configuration of the phosphorus center controls the configuration of the newly formed stereogenic center of the β-alkylated ketones.

In order to improve hydroperoxide formation from heterocyclic compounds relating to the formation rate and to allow a suitable choice of starting materials for autoxidation, theoretical studies on a set of different amino acid-derived diketopiperazines and pyrazinoquinazolines were carried out. To estimate their reactivity towards hydroperoxide formation, bond dissociation enthalpies (BDEs) of tertiary α-CH bonds as well as reaction enthalpies to the corresponding hydroperoxides were calculated at the B3LYP/TZVP and RMP2/aug-cc-pVTZ level of theory. The Evans–Polanyi relation was then used to correlate substrate reactivity with calculated BDEs. Thermal and zero point vibrational energy (ZPE) corrections were determined in the classical harmonic oscillator-rigid rotor-particle in a box model. While for the investigated set of diketopiperazines BDEs of 318.8–327.0 kJ mol⁻¹ were found, BDEs for pyrazinoquinazolines spread between 248.4 and 368.4 kJ mol⁻¹ at the B3LYP/TZVP level of theory. A selected subset of heterocycles was converted to the corresponding hydroperoxides and the diketopiperazines were obtained in up to 39% yield after 5–7 days, whereas the pyrazinoquinazoline hydroperoxides were isolated in up to 67% yield after 24 h. Thus, replacing an amido moiety in an N-aryl-imino moiety when using pyrazinoquinazolines instead of diketopiperazines leads indeed to an improved captodative stabilization of the radical intermediate. Furthermore the theoretical calculations allowed a distinctive forecast of the preferred regioisomeric hydroperoxide. Copyright © 2010 John Wiley & Sons, Ltd.

Conjugate additions of Gilman cyanocuprates to (S)-N-amino-2-(methoxymethyl)pyrrolidine (SAMP)-hydrazones 4, 5 derived from cyclic and acyclic α,β-unsaturated ketones were investigated. A protocol utilizing copper(II) sulfate/ammonium chloride was evolved, which allowed cleavage of SAMP (S)-1 under the hydrolysis and work-up conditions, followed by recovery of the auxiliary with ethylenediaminetetraacetic acid (EDTA). The enantioselectivity of cuprate additions was dominated for cyclic SAMP-hydrazones 4 by the cuprate alkyl substituent and the ring size, in the case of acyclic arylidene SAMP-hydrazones 5, however, by the nature of the aryl substituent. Electron-donating substituents gave poor enantiomeric excesses, whereas electron-withdrawing groups provided excellent ee values of 98–99%. The configuration of the new stereocenter was determined to be (R). Moreover, a reaction sequence was developed which integrates a tandem 1,4-addition/methylation and traceless hydrolytic cleavage of the auxiliary (S)-1 in a one-pot reaction, resulting in enantiomerically pure methyl ketones 11–13, each of them with>99% ee.

The functionalization of differently substituted hydropentalenone derivatives 9, derived from the Weiss diketone (8) by enantioselective deprotonation in the presence of lithium (R,R)-bis(1-phenylethyl)amide/LiCl (11·LiCl) as the chiral base is described. In the first route the resulting enolate was treated directly with alkyl halides as electrophiles to give the target α-alkylhydropentalenones 12, whereas in the second route the enolate was trapped as one of the triethylsilyl enol ethers 17, from which the enolate was regenerated by treatment with MeLi prior to alkylation with alkyl halides. The substituents on 9 seemed to influence which strategy is favored: for the OTBS-substituted hydropentalenone 9a the direct deprotonation/alkylation is preferred, whereas for the acetal-substituted hydropentalenone 9b the silyl enol ether route is more suitable. In all cases the α-alkylated hydropentalenones 12 and 15 were isolated with good diastereoselectivities.

The synthesis and electrochemical investigations of 9,10-diphenylphenanthrene 2a and its derivatives 2b–2e are reported. The cyclic voltammetry of derivatives 2a–2c and 2e in different solvent/Bu₄NPF₆ electrolyte systems reveals that the redox properties are dependent on solvent, temperature, and sweep rate. The oxidation of 9,10-diphenylphenanthrene 2a occurred as an irreversible process, while two fully reversible oxidation waves were observed for dimethoxy derivative 2c. The room-temperature oxidation of brominated compound 2b is reversible, whereas AcO-substituted phenanthrene 2e displayed a reversible oxidation peak only at low temperature. Furthermore, the electronic nature of the substituent affects the oxidation potentials. In the CH₂Cl₂-based electrolyte system, the first oxidation potentials increase in the order 2c<2e<2b.

The twisted lateral tetraalkyloxy ortho-terphenyl units in dibenzo[18]crown-6 ethers 1 a–f were readily converted into the flat tetraalkyloxytriphenylene systems 2 a–f by oxidative cyclization with FeCl₃ in nitromethane. Reactions of the latter with potassium salts gave complexes KX⋅2, which displayed mesomorphic properties. The aromatization increased both the clearing and melting points; the mesophase stabilities, however, were mainly influenced by the respective anions upon complexation with various potassium salts. In contrast, the alkyl chain lengths played only a secondary role. Among the potassium complexes of triphenylene-substituted crown ethers KX⋅2, only those with the soft anions I⁻ and SCN⁻ displayed mesophases with expanded phase temperature ranges of 93 °C and 132 °C (for KX⋅2 e), respectively, as compared to the corresponding o-terphenyl-substituted crown ether complexes KI⋅1 e (ΔT=51 °C) and KSCN⋅1 e (plastic crystal phase). Anions such as Br⁻, Cl⁻, and F⁻ decreased the mesophase stability, and PF₆⁻ led to complete loss of the mesomorphic properties of KPF₆⋅2 although not for KPF₆⋅1. For crown ether complexes KX⋅2 (X=F, Cl, Br, I, BF₄, and SCN), columnar rectangular mesophases of different symmetries (c2 mm, p2 mg, and p2 gg) were detected. In contrast to findings for the twisted o-terphenyl crown ether complexes KX⋅1, the complexation of the flat triphenylene crown ethers 2 with KX resulted in the formation of organogels. Characterization of the organogel of KI⋅2 e in CH₂Cl₂ revealed a network of fibers.

A series of N-4-(4′-alkoxybiphenyl)-N′,N′,N”,N“-tetramethylguanidinium salts was synthesized with varying alkoxy chain lengths and additional N-alkyl substituents, each with a number of different counterions. X-ray crystal-structure analyses of 1bI, 1bPF₆, 2aI, and 4aI reveal bilayer structures in the solid state and, for the 1b and 1b PF₆ salts, a hydrogen-bond-type connectivity between the guanidinium N-H group and the anion is found. For the N-alkyl homologues 2aI and 4aI the anion is still oriented close to the head group, although at a larger distance. Ion pairs are present also in solution, as demonstrated by ¹H NMR: the N-H chemical shift shows a good linear correlation with the radius, and hence the hardness, of the anion. The intramolecular conformational flexibility of 1bI, 2bI, 3bI, and 4bI was studied by temperature-dependent ¹H NMR spectroscopy and discrete activation barriers were determined for rotations about each of the three CN partial double bonds of the guanidinium core. The relative heights of the individual barriers change between the N-H and the N-alkylguanidinium salts. A fourth barrier is observed for the rotation about the Nbiphenyl bond. DFT calculations of charge densities show that the positive charge resides primarily on the central carbon atom. Rotational barriers were calculated for N′-substituted 2-amino-1,3-dimethylimidazolidinium cations as models, and are in qualitatively good agreement with the NMR data. Mesomorphic properties were studied by differential-scanning calorimetry, polarizing optical microscopy, and X-ray diffraction (WAXS/SAXS). All liquid-crystalline guanidinium salts exhibit smectic A mesophases. Clearing temperatures show a linear correlation with the anionic radius. Substitution of the N-H group with methyl, ethyl, or propyl results in decreasing mesophase widths and a concomitant shrinkage of the layer spacings.

A series of 2,6-diacylpyridine ligand precursors 5a–d·HCl with different tether lengths between the carboxyl and pyridine moiety was prepared and converted into the correspondig trinuclear Fe₃(μ₃-O) complexes 8a–d and 10. Under slow precipitation conditions a tetranuclear complex 9 was isolated instead of 8a. Single-crystal X-ray diffraction analyses were performed on ligands 5a–d and complexes 9 and 10. Characterization by X-ray absorption spectroscopy (XAS) proved a trinuclear Fe₃(μ₃-O) core for complexes 8a–d. When complex 8a was submitted to Gif-type oxidations (O₂, Zn, pyridine, HOAc), Mössbauer and nuclear inelastic scattering (NIS) suggested the formation of mononuclear species. The trinuclear ferric complex 10 has an isosceles molecular structure, which is manifested in the ⁵⁷Fe Mössbauer spectrum by two quadrupole doublets with a 2:1 intensity ratio. The magnetic measurements reveal two moderate antiferromagnetic exchange interactions of –22.1 and –33.8 cm^–1. Spin concentrations of complex 10 were determined by EPR spectroscopy, which supports the Mössbauer and magnetic studies. Complexes 8–10 were employed in catalytic aerobic oxidations of adamantane 11, cyclohexene 19, and α-pinene 23. For adamantane 11, the oxidation of secondary C–H bonds to the corresponding ketone 14 is favored. In addition, adamantylpyridines 15–18 were isolated, thus supporting a radical pathway. A strong preference of allylic oxidation versus epoxidation was found for cyclohexene 19 and α-pinene 23. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Novel calamitic N-methylimidazolium and pyridinium salts with modified citronellyl side-chains and various counterions 3–14 have been synthesised by utilising the commercially available (R)-citronellol (15) as the starting material. Differential scanning calorimetry (DSC), polarising optical microscopy (POM) and temperature-dependent X-ray diffraction studies revealed smectic A (SmA) mesophases for all the series. ¹H NMR studies of the N-methylimidazolium salts have shown that there is a correlation between the mesophase dependence on the counterion and the counterion-dependent chemical shifts of the acidic 2-H proton.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The three-component reaction between ylide Ph₃PCCO, amines and aldehydes is known to afford selectively (E)-α,β-unsaturated amides. We applied a variant of this methodology to the preparation of (2E,4Z)-dienamides 11 utilizing the phosphonium salt formation from ethyl 5-aminopentanoate hydrochloride and Ph₃PCCO followed by deprotonation with DBU and a Wittig olefination of the corresponding ylide with various (Z)-α,β-unsaturated aldehydes 10. The (2E,4Z)-dienamides 11 were isolated in yields of up to 80 %. The (Z)-configuration of the starting aldehydes 10 remained untouched during the reaction.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Three series of novel chiral tetraphenylethenes have been prepared: citronellyl-derived ethers 1a, b, lactate-derived ethers 2d, g, h, i and lactate-derived esters 3a–c, e–h. Helical twisting powers (HTPs) were determined for those derivatives of 1–3, which were sufficiently miscible with the nematic host 5CB 13 or the discotic nematic host hexayne 14. For binary solutions HTP values of 5.7–10.4 µm⁻¹ for 13/1, 12.8–16.5 µm⁻¹ for 13/2, 8.0–28.7 µm⁻¹ for 13/3 and 2.1–2.9 µm⁻¹ for hexayne 14/3 were determined, indicating a much stronger interaction between the C₄-symmetrical propeller-shaped tetraphenylethenes 1–3 with the calamitc host 5CB 13 than with the discotic C₆-symmetrical propeller-shaped host hexayne 14. Copyright © 2009 John Wiley & Sons, Ltd.

The [1,1′-biisoquinoline]-4,4′-diol (4a), which was obtained as hydrochloride 4a⋅2 HCl in two steps starting from the methoxymethyl (MOM)-protected 1-chloroisoquinoline 8 (Scheme 3), opens access to further O-functionalized biisoquinoline derivatives. Compound 4a⋅2 HCl was esterified with 4-(hexadecyloxy)benzoyl chloride (5b) to give the corresponding diester 3b (Scheme 4), which could not be obtained by Ni-mediated homocoupling of 6b (Scheme 2). The ether derivative 2b was accessible in good yield by reaction of 4a⋅2 HCl with the respective alkyl bromide 9 under the conditions of Williamson etherification (Scheme 4). Slightly modified conditions were applied to the esterification of 4a⋅2 HCl with galloyl chlorides 10a–h as well as etherification of 4a⋅2 HCl with 6-bromohexyl tris(alkyloxy)benzoates 11b,d–h and [(6-bromohexyl)oxy]-substituted pentakis(alkyloxy)triphenylenes 14a–c (Scheme 5). Despite the bulky substituents, the respective target 1,1′-biisoquinolines 12, 13, and 15 were isolated in 14–86% yield (Table).

Dibenzo[18]crown-6 derivatives 1 with two lateral tetraalkyloxy o-terphenyl units were prepared and converted to the corresponding complexes KX⋅1 (X=halide, BF₄, PF₆, SCN) and NH₄PF₆⋅1. Complexation was probed by MALDI-TOF spectrometry and NMR spectroscopy. Downfield shifts of ¹H NMR signals for complexes with soft anions Br, I, SCN, and PF₆ indicated the presence of tight ion pairs, whereas complexes with hard anions F, Cl, or BF₄ showed no or little shifts. In ¹³C NMR spectra, upfield shifts were detected for soft anions. The character of the anion also influenced the mesomorphic properties of complexes MX⋅1 (M=K, NH₄), which were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and XRD in comparison to neat 1. Hard anions slightly stabilize or even destabilize the mesophase. Soft anions, however, improve the mesomorphic properties yielding mesophases with up to 70 °C phase widths in the case of KI⋅1, KPF₆⋅1, and NH₄PF₆⋅1. For complexes KSCN⋅1 with a soft and bridging anion, the balance between mesophase stabilization and high order is shifted in favor of the plastic crystal phase.

To gain insight into the biological properties of tetramic acid lactam cylindramide 1, the analogues 4 a–d bearing a cyclopentane ring instead of the pentalene unit were prepared by tandem conjugate addition/enolate trapping of cyclopentenone 10; a Sonogashira or Stille coupling, followed by a Julia–Kocienski olefination, macrolactamisation and Lacey–Dieckmann cyclisation were the key steps. The previous NMR structure of cylindramide 1, which was based on NOE and J coupling restraints, could be refined by including residual dipolar coupling data measured for a sample of cylindramide that was aligned in polyacrylonitrile (18 %). Biological screening of cylindramide 1 and its analogues 2-epi-1, 20 and 4 revealed promising antiproliferative activity against several tumour cell lines. It turned out that the activity is strongly correlated to the functionalised pentalene system. The configuration of the cyclopentane ring and an intact tetramic acid lactam with the correct configuration seem to play an equal role in the cytotoxicity. The antiproliferative activity was found to be calcium dependent. Phenotypic characterisation of the mode of action showed vacuolisation and vesicle formation in the endoplasmic reticulum.