Erkki Kolehmainen

Recent advances in steroidal supramolecular gels

During the last decade or two the interest towards small molecules capable of self-assembly leading to gelation has increased intensively. The investigation of these supramolecular gels aims not only at understanding the fundamental processes underlying gel formation but also at development of new materials with a myriad of applications. Steroids are widely-spread natural products with a large and rigid steroidal nucleus combined with derivatizable functional groups leading to an adjustable polarity profile, which makes them attractive building blocks when designing novel low molecular weight gelators. Due to their unique properties, steroid-based supramolecular gels may find use in applications ranging from materials science and nanoelectronics to their application as reaction media or as sensing and responsive materials. Moreover, biomaterials based on steroidal gels may find use in biomedicine, drug delivery, regenerative medicine, and tissue engineering. This article summarizes the most recent advances in the field of steroidal supramolecular gels in terms of steroid-derived hydro- and organogels, metallogels, two-component gels, and stimuli-responsive gels. Furthermore, the potential applications of the systems are discussed.

Bile Acids as Building Blocks of Supramolecular Hosts

A review of the use of bile acid-based compounds as building blocks for designing novel supramolecular hosts for molecular recognition is presented. Pharmacological applications and the newest spectroscopic and computational studies of bile acid derivatives are also shortly considered.

15N NMR Spectroscopy in Structural Analysis: An Update(2001-2005)

Since our previous review article (Curr. Org. Chem. 2002, 6, 35), significant improvements and an array of 15N NMR applications in structural analysis have been published. This report aims to update coverage of improvements in methodology and various types of applications published over the period 2001 - 2005. Substantial progress in cryogenic probe technology and the commercial availability of cryoprobes have facilitated the measurement of 15N NMR parameters. The number of solid-state applications has increased significantly during the past few years. In contrast to our previous review, this article covers 15N solid-state studies. The 15N NMR chemical shifts of organic molecules are routinely measured by using cross-polarization magic-angle spinning (CP/MAS) techniques. The principal values of the chemical shift tensors can also be determined. 1H-15N and 2H-15N distance measurements made by means of 1H detection are currently used in NMR crystallography. User friendly quantum chemical programs allow for the routine calculation of 15N chemical shielding and indirect spin-spin coupling constants, especially using density functional theory (DFT). Applications of 15N NMR spectroscopy in various fields of chemistry are summarized here. Major sections represent tautomerism, complexation, protonation, and hydrogen bonding. The other topics comprise N-alkylation, N-oxidation, regioisomerism, and changes in configuration or conformation.

Anti-leukaemic compounds derived from stilbenes in Picea abies bark

Abstract Resveratrol was identified and isolated as a minor stilbene derivative from the bark of Picea abies . Three derivatives of resveratrol and three additional derivatives of the primary stilbenes from P. abies bark were prepared. Improvement over piceatannol in antileukaemic activity was observed in a preliminary test.

First Crystallographic Investigation of Complexes of cis‐VO2+, cis‐MoO22+, and trans‐UO22+ Species with Schiff‐Base Molecules Derived from 4,6‐O‐Ethylidene‐β‐D‐glucopyranosylamine

The interaction of Schiff-base ligands derived from 4,6-O-ethylidene-β-D-glucopyranosylamine with cis-VO 2 + , cis-MoO 2 2+ , and trans-UO 2 2+ species have been studied by isolating and characterizing the corresponding products. The structures of one complex of each type of species have been established by single-crystal X-ray diffraction analysis. In all the complexes, the saccharide moiety adopts a chair conformation and has a β-anomeric form. A gradual increase in coordination number (5, 6, and 7) and a gradual variation in the geometry (distorted trigonal-bipyramidal, distorted octahedral, and pentagonal-bipyramidal) are observed on going from the complexes of cis-VO 2 + (mononuclear) to cis-MoO 2 2+ (mononuclear) to trans-UO 2 2+ (dinuclear). A variation in the binding mode of the ligand towards these dioxometal species is also observed. Intermolecular interactions of the type O-H···O, C-H···O, and N-H···O present in the lattices of these complexes lead to the formation of interesting structures.

Self-Organization of 2-Acylaminopyridines in the Solid State and in Solution

Aggregation of 2-acylaminopyridines and their 6-methyl derivatives in chloroform solution was studied by (1)H, (13)C, and (15)N NMR spectroscopies. The results were compared with (13)C and (15)N CPMAS NMR and IR spectral as well as with X-ray structural data. Intermolecular interactions in solution and in solid state were found to have a similar nature. Relatively strong N(amide)-H···N(pyridine) intermolecular hydrogen bonds enable dimerization to take place. Steric interactions in N-pivaloyl- and N-1-adamantylcarbonyl as well as that caused by the 6-methyl group hinder formation of the dimeric aggregates stabilized by the N(amide)-H···N(pyridine) intermolecular hydrogen bonds. In general, the DFT optimized geometries of the aggregates in chloroform solution are in agreement with the X-ray crystal structures. Wavenumbers of the stretching vibration band of the C═O group were also found indicative of the type of hydrogen bond present in the solid state.

Transition to Reinforced State by Percolating Domains of Intercalated Brush-Modified Cellulose Nanocrystals and Poly(butadiene) in Cross-Linked Composites Based on Thiol–ene Click Chemistry

The classic nanocomposite approach aims at percolation of low fraction of exfoliated individual reinforcing nanoscale elements within a polymeric matrix. By contrast, many of the mechanically excellent biological nanocomposites involve self-assembled and space-filled structures of hard reinforcing and soft toughening domains, with high weight fraction of reinforcements. Here we inspect a new concept toward mimicking such structures by studying whether percolation of intercalated domains consisting of alternating rigid and reinforcing, and soft rubbery domains could allow a transition to a reinforced state. Toward that, we present the functionalization of rigid native cellulose nanocrystals (CNCs) by esterification with a dense hydrocarbon chain brush containing cross-linkable double bonds. Composite films with 0-80 wt % of such modified CNCs (mCNCs) within a poly(butadiene) (PBD) rubber matrix were prepared via cross-linking by UV-light initiated thiol-ene click reaction. Transmission electron microscopy showed structures at two length scales, where the mCNCs and PBD form domains having internal aligned self-assemblies of alternating hard mCNCs and soft PBD with periodicity of ca. 40 nm, and where additional PBD connects such domains. Increasing the weight fraction of mCNCs causes an uncommon abrupt transition from PBD-dominated soft materials to significantly reinforced mCNC-dominated mechanical properties, suggesting that the intercalated self-assembled mCNC/PBD domains percolate in PBD upon passing 30-35 wt % of mCNCs. Maximum stress of 16 MPa at mCNC fraction of 80 wt % was obtained. The mechanical properties of the composites show exceptional insensitivity to air humidity. The shown simple concept of percolative intercalated nanocomposites suggests searching for more general biomimetic compositions involving several deformation mechanisms for improved mechanical properties.

Resorcarenes in the boat conformation as building blocks for hydrogen-bonded assemblies including two ammonium cations.

Crystal structures are reported for various co-crystals of rccc-resorcarenes with triethylammonium chloride. Usually, two molecules of a C2v-symmetric tetraester 2 in the boat conformation are linked through four hydrogen-bonded chloride anions to give dimeric assemblies. Two of the chloride anions may be replaced by four hydrogen-bonded ethanol molecules in an otherwise similar structure. These assemblies, which consist of six or eight components, posses voluminous, negatively charged chambers in which two triethylammionium cations, 3+, are included as guests by strong electrostatic and hydrogen-bonding interactions. The host-guest N-H...Cl hydrogen bonds were clearly detected at 173 K. These are the first examples of hydrogen-bonded, solid-state capsules trapping two ions of the same charge in close proximity. In the 1:2 complex with 3+ Cl-, the molecule of the parent resorcarene 1 also adopts a boat conformation whose cavity is considerably extended by four hydrogen-bonded chloride anions. The pocket formed in this way again includes two 3+ ions as a result of electrostatic and hydrogen bonding host-guest interactions. All these structures show that the boat conformers of resorcarenes can be used as a novel motif for the construction of hydrogen-bonded assemblies capable of molecular inclusion and encapsulation.

Subcomponent Self‐Assembly: A Quick Way to New Metallogels

1-2-3 gel! Subcomponent self-assembly is introduced as a new design route towards multistimuli-responsive metallogels. It offers a rapid and facile access to supramolecular gels and allows to design smart materials with diverse functional and structural properties by simply exchanging one (or more) of the components. Herein, the exchange of the metal ions is emphasized (see scheme).

Novel one-pot synthesis of quaternary ammonium halides: new route to ionic liquids

Treatment of an amide with an alkyl or substituted alkyl halide in the presence of a weak base in a one-pot reaction leads to crystalline quaternary ammonium halides with reasonable chemical yields; some of the compounds show low melting points and a liquid range of over 50–100 °C before decomposition.