Crystal structure of supramolecular complexes formed by 18-crown-6 andcis-anti-cis-dicyclohexano-18-crown-6 (host) with 3,4-diaminofurazane (guest)

An X-ray—diffraction study is reported for two molecular complexes containing 3,4-diamino-1,2,5-oxadiazole as guest (G) with 18-crown-6 (18-C-6) andcis-anti-cis-dicyclohexano-18-crown-6 (DCH-6B) as host. Both complexes are of the polymeric-chain structure with the guest molecule bridging two crown neighbours. ComplexI: [18-C-6*G*H₂O], 111, monoclinic,P2₁/n,a=8.171(1),b=15.042(2),c=16.209(6) Å, =101.15(2)°, finalR-factor 0.068. ComplexII: [DCH-6B*G], 11, monoclinicC2/c,a=21.212(4),b=9.380(2),c=13.049(3) Å, =108.61(3)°, finalR 0.047.     

Synthesis and structure–activity relationship study of cytotoxic lupane-type 3β-O-monodesmosidic saponins with an extended C-28 side chain

A concise synthesis of lupane triterpenes with an elongated carbon chain at the C-28 position, as well as saponins containing d-mannose, l-arabinose, and l-rhamnose moieties at the C-3 position is described. The overall synthesis of the new triterpenes involved seven linear steps starting from natural betulin: selective protection of a hydroxyl group, oxidation, elongation of the carbon chain by Grignard reaction, and deoxygenation. O-Glycosides were obtained by glycosylation of triterpenes with classical Schmidt's donors. Additionally, all new compounds were evaluated in vitro for their cytotoxic activities. Several triterpenes and the corresponding saponins exhibited an interesting cytotoxic activity profile against human cancer cell lines. The therapeutical index of active triterpenes is very high, since almost none of them were cytotoxic for normal BJ fibroblasts. These results open the way to the synthesis of various lupane-type saponin derivatives as potentially bioactive compounds.     

Crown Inclusion Compounds with 3,4-Diamino-1,2,5-Oxadiazole. X-Ray Structures of Its 1:1 Inclusion Complexes with 15-Crown-5 and Benzo-15-crown-5

The X-ray crystal structures of two closely related molecular complexes of 15-crown-5 and benzo-15-crown-5 with 3,4-diamino-1,2,5-oxadiazole are reported (I and II). Both complexes are of 1:1 stoichiometry with the host–guest alternation in the infinitechains formed due to the unsymmetrical H-bonding patterns between the components. Crystals of I are monoclinic, P2_1/c, a = 7.856(3), b = 12.994(1), c = 16.033(1) , = 94.79(2)°, Z = 4, final R-factor is 0.0488. Crystals of II are orthorhombic, P2₁2₁2₁, a = 8.260(4), b = 15.692(5),c = 13.955(7) , Z = 4, final R-factor is 0.0522.     

Ring-expanded bicyclic β-lactams: a structure-chiroptical properties relationship investigation by experiment and calculations

In the present work, the validity of the helicity rule relating the absolute configuration of the bridgehead carbon atom in bicyclic β-lactams to the sign of the 220 nm band observed in their electronic circular dichroism (ECD) spectra is examined for ring-expanded cephalosporin analogues. To this end, a series of model compounds with a seven-membered ring condensed with the β-lactam unit was synthesized. A key step of their synthesis was either the ring-closing metathesis (RCM) or the free radical cyclization leading to the seven-membered ring with an S, O, or C atom at the 6 position in the bicyclic skeleton. To investigate the scope and limitations of the simple, empirically established helicity rule, a combination of ECD spectroscopy, variable-temperature ECD measurements, X-ray analysis, and time-dependent density functional theory (TD-DFT) calculations was used. A comparison of the experimental ECD spectra with the spectra simulated by TD-DFT calculations gives a reasonable interpretation of the Cotton effects observed in the 240-215 nm spectral range. The results suggest that the helicity rule does not apply to the investigated compounds because of the planarity of their amide chromophore. Thus, these compounds do not constitute an exception to the rule that was established for bi- and polycyclic β-lactams with the nonplanar amide chromophore only.     

Glucofuranose derivatives as a library for designing and investigating low molecular mass organogelators

We designed and synthesized a class of saccharide-based gelators having three free OH groups in the glucofuranose fragment. The gelating abilities of fourteen compounds were examined to systematically study the influence of the hydrophobic fragment connected to the C2′ carbon. Also the correlation between the saccharide crystal structure and its gelating properties was examined, showing limited usefulness in this particular case. SEM observations were carried out in order to investigate the hierarchical structure of xerogels and changes depending on different gel concentration.     

X-ray structure of 2,6-dimethoxy-7-methylpurine

The title compound (C₈H₁₀N₄O₂) is monoclinic, with a = 7.740(2), b = 17.044(7), c = 6.992(3) Å, = 100.60(1)°, and space group P2₁/c. Two O-methyl groups are coplanar with the pyrimidine ring. Whereas, the O(6)-methyl group is directed away from the imidazole ring toward the N(1) atom, the O(2)-methyl is pointed away from the N(1) atom toward the N(3) atom. Two intermolecular hydrogen bonds H(8)···N(1) and H(711)···O(2) of 2.48(2) and 2.58(3) Å make a linear arrangement of the molecules. The conformation of the O-methyl groups explains some results of thermal rearrangement of 2,6-dialkoxy-7-methylpurines and differences in alkylations of 2,4-dialkoxypyrimidines and 2,6-dialkoxy-7-methylpurines.     

Thermal properties of the gel made by low molecular weight gelator 1,2-O-(1-ethylpropylidene)-alpha-D-glucofuranose with toluene and molecular dynamics of solvent

The studies of the gel-to-sol phase transition by the Raman, FT-IR, and 1H NMR methods of the gel made by low molecular weight organogelator 1,2-O-(1-ethylpropylidene)-alpha-D-glucofuranose with toluene as the solvent are reported. The FT-IR spectra revealed the existence of a hydrogen bond network formed by gelator molecules in the crystalline and gel phase. In both phases, the network formation is dominated by the gelator self-interaction. Upon gelation, only one stretching band of infrared absorption modes nualpha, assigned to the O(6)H hydroxyl protons of gelator, is shifted by Deltaupsilonalpha = 25 cm-1, which indicates the involvement of this proton in the interaction with the solvent molecules. The phase transition measurements performed as a function of gelator concentration allowed the calculation of the energy correlated with the transition from gel to solution phase. The obtained value of 72 kJ/mol is the largest one reported up until now for monosaccharide-based gels. The analysis of the temperature measurements of the toluene 1H NMR spectra provides evidence for a different chemical environment of toluene molecules in the gel. The toluene spin-lattice relaxation in bulk and gel indicate that the viscosity is most likely the main factor that influences the dynamics of toluene.     

On the relation between the solvent parameters and the physical properties of methyl-4,6-O-benzylidene-α-d-glucopyranoside organogels

This paper reports the mechanisms of gel formation, the thermal properties and the microstructures of the networks of the gels composed of methyl-4,6-O-benzylidene-α-d-glucopyranoside and selected organic solvents: p-xylene, benzene, toluene, diphenyl ether and tetraethoxysilane. The Fourier transform infrared measurements together with simulation spectra, the air bath method and Polarized Optical Microscopy were employed in our studies. The experimental data show that the solvent has an influence on the microstructure of the gel network but there is no predictable influence of the solvent polarity on the shape of the formed gelator aggregates and correspondingly on the fibrous assemblies as revealed by the different microstructure of the gel network. Independently of the solvent polarity, the studied gelator, like other methyl-4,6-O-benzylidene derivatives of monosaccharides, formed gels through the formation of a hydrogen-bond network. The solvent parameters, such as the dielectric constant, Hildebrand solubility parameter, the polarity scale E_T and the Kamlet–Taft parameters were considered to quantify solvent effects on the gelation. The conclusions about the correlations are of interest but only to this particular sugar based gels.     

Electronic states of cyclophanes with small bridges

Electronic absorption and magnetic circular dichroism were recorded for five cyclophanes with ethano bridges: [2.2]paracyclophane, (1,2,4)[2.2.2]cyclophane, (1,2,4;1,2,5)[2.2.2]cyclophane, (1,2,3,4,5,6)(1,2,3,4,5,6)cyclophane, and trans-[2.2]metacyclophane. Spectral and structural analyses were based on geometry optimization and calculations of transition energies, carried out using density functional theory methods. The assignments have been proposed for several electronic transitions observed in the region below 52,000 cm(-1). The observation of transitions which should be forbidden in the high D(2h) symmetry [2.2]paracyclophane suggests a twisted ground state structure of D(2) symmetry, although the former structure with large amplitude vibrations at room temperature cannot be excluded. The PBE0 functional turned out to appropriately reproduce the inter-ring distances and electronic transition energies.