Unambiguous transformation of eremanthin into (−)-estafiatin

A stereoselective synthesis of (?)-estafiatin ($\text{2a}$) from eremanthin ($\text{1}$) has been achieved. The α configuration of the epoxide in the former is confirmed by chemical shift correlation with related oxides.     

Insights into the Structure of Large-Ring Cyclodextrins through Molecular Dynamics Simulations in Solution

Molecular dynamics simulations were carried out using the AMBER parm99 force field and explicit water molecules (TIP3P) to gain insight into the structural deformations and energetics of several large-ring cyclodextrins (with a degree of polymerization 26, 30, 55, 70, 85, and 100) in solution. The structures displayed by CD26 during the MD simulation (10.0 ns) did not correspond to the conformation in the crystalline state. The two "flips" present in the macroring of CD26 in the crystal state disappeared after 1.5 and 3.0 ns simulations, respectively. The larger CDs bear a considerable degree of flexibility. They display different modes of folding and cavity-like regions of different sizes and shapes: circular and elongated loops of variable size, orientated in different fashion; portions of a double helical strand with the two single helices parallel to each other (CD30, CD70); a helix of three turns and a serpentiform portion containing six loops (CD55); a cone-shaped spiral region (CD70); a rounded dendritic fold with several arbitrarily oriented small loops on the surface of the clustering (CD85); three spiral portions and a tendency for bending into two (CD100). These results support the hypothesis for the existence of more than one cavity in large-ring cyclodextrins.     

Organosolv-pulping III

Formic acid pretreatment onPinus radiata D. Don was studied in order to improve the cellulose hydrolysis by commercial cellulase. The formic acid treatment effectively removed the lignin. A low substitution (formylation) and a crystallinity decrease of the cellulose in the pulp were observed. As consequence of these parameter changes, owing to the formic acid pretreatment on sawdust, a higher saccharification value was observed. The degree of saccharification increased when the degree of substitution (measured by titration) decreased and the portion of amorphous cellulose (measured via an X-ray technique) increased.Trichoderma reesei cellulase hydrolyzed the untreated and pretreated Pinus sawdust with formic acid in 25% and 56% of saccharification, respectively.     

Experimental (NMR) and Theoretical (MD Simulations) Studies on the Conformational Preference of Three Cycloalkanols when Included in β-Cyclodextrin

The inclusion complexes between three cycloalkanols (cyclopentanol, cyclohexanol and cycloheptanol) and β-cyclodextrin (β-CD) have been studied by NMR experiments, and by molecular dynamics (MD) simulations. Complexes present medium to small association constants. All experimental data show the equatorial conformer as the most stable after complexation because no changes were detected in the coupling constants of the H1 protons. Intermolecular ROE experiments suggest that while cyclopentanol is deeply included into the β-CD cavity, cyclohexanol and cycloheptanol occupy mainly the wider entrance. The MD simulations agree with the experimental data (equatorial conformers are always the most stable), and average geometries coincide with those deduced from the ROE experiments.     

The reaction of eremanthin and isoeremanthin with bromine. Unprecedented simultaneous addition of Br2 to two isolated but topographically related double bonds.

The reaction of $\text{1}$ with Br₂ in CHCl₃ proceeds with the simultaneous and stereospecific incorporation of two moles of Br₂ to give $\text{2}$ thus providing strong experimental evidence for the existence of Br₃.     

Tuning secondary structure and self-assembly of amphiphilic peptides

Self-assembly is one of nature's mechanisms by which higher order structures are obtained. Two of the main driving forces for self-assembly, hydrophobic interactions and hydrogen bonding, are both present within amphiphilic peptides. Here, it is demonstrated how the intricately interconnected folding and assembly behavior of an N-terminally acylated peptide, with the sequence GANPNAAG, has been tuned by varying its hydrophobic tail and thermal history. The change in interplay between hydrophobic forces and peptide folding allowed the occurrence of different types of aggregation, from soluble peptides with a random coil conformation to aggregated peptides arranged in a beta-sheet assembly, which form helically twisted bilayer ribbons.     

Crystal structures and magnetic properties of 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz)-containing copper(II) complexes

Four new copper(II) complexes of formula [Cu(2)(tppz)(dca)(3)(H(2)O)].dca.3H(2)O (1), [Cu(5)(tppz)(N(3))(10)](n)() (2), [[Cu(2)(tppz)(N(3))(2)][Cu(2)(N(3))(6)]](n)() (3), and [Cu(tppz)(N(3))(2)].0.33H(2)O (4) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine and dca = dicyanamide anion] have been synthesized and structurally characterized by X-ray diffraction methods. The structure of complex 1 is made up of dinuclear tppz-bridged [Cu(2)(tppz)(dca)(3)(H(2)O)](+) cations, uncoordinated dca anions, and crystallization water molecules. The copper-copper separation across bis-terdentate tppz is 6.5318(11) A. Complex 2 is a sheetlike polymer whose asymmetric unit contains five crystallographically independent copper(II) ions. These units are building blocks in double chains in which the central part consists of a zigzag string of copper atoms bridged by double end-on azido bridges, and the outer parts are formed by dinuclear tppz-bridged entities which are bound to the central part through single end-on azido bridges. The chains are furthermore connected through weak, double out-of-plane end-on azido bridges, yielding a sheet structure. The intrachain copper-copper separations in 2 are 6.5610(6) A across bis-terdentate tppz, 3.7174(5) and 3.8477(5) A across single end-on azido bridges, and from 3.0955(5) to 3.2047(7) A across double end-on azido bridges. The double dca bridge linking the chains into sheets yields a copper-copper separation of 3.5984(7) A. The structure of 3 consists of centrosymmetric [Cu(2)(tppz)(N(3))(2)](2+) and [Cu(2)(N(3))(6)](2)(-) units which are linked through axial Cu.N(azido) (single end-on and double end-to-end coordination modes) type interactions to afford a neutral two-dimensional network. The copper-copper separations within the cation and anion are is 6.5579(5) A (across the bis-terdentate tppz ligand) and 3.1034(6) A (across the double end-on azido bridges), whereas those between the units are 3.6652(4) A (through the single end-on azido group) and 5.3508(4) A (through the double end-to-end azido bridges). The structure of complex 4 is built of neutral [Cu(tppz)(N(3))(2)] mononuclear units and uncoordinated water molecules. The mononuclear units are grouped by pairs to give a rather short copper-copper separation of 3.9031(15) A. The magnetic properties of 1-4 have been investigated in the temperature range 1.9-300 K. The magnetic behavior of complexes 1 and 4 is that of antiferromagnetically coupled copper(II) dimers with J = -43.7 (1) and -2.1 cm(-)(1) (4) (the Hamiltonian being H = -JS(A).S(B)). An overall ferromagnetic behavior is observed for complexes 2 and 3. Despite the structural complexity of 2, its magnetic properties correspond to those of magnetically isolated tppz-bridged dinuclear copper(II) units with an intermediate antiferromagnetic coupling (J = -37.5 cm(-)(1)) plus a ferromagnetic chain of hexanuclear double azido-bridged copper(II) units (the values of the magnetic coupling within and between the hexameric units being +61.1 and +0.0062 cm(-)(1), respectively). Finally, the magnetic properties of 3 were successfully analyzed through a model of a copper(II) chain with regular alternating of three ferromagnetic interactions, J(1) = +69.4 (across the double end-on azido bridges in the equatorial plane), J(2) = +11.2 (through the tppz bridge), and J(3) = +3.4 cm(-)(1) (across the single end-on azido bridge).     

Volumetric Properties of Toluene with Ethyl Acrylate, Butyl Acrylate, Methyl Methacrylate, and Styrene at 25°C

Densities of the binary systems of toluene with ethyl acrylate, butyl acrylate, methyl methacrylate, and styrene have been measured as a function of the composition, at 25°C and atmospheric pressure, using an Anton Paar DMA 5000 oscillating U-tube densitometer. The calculated excess volumes were correlated with the Redlich–Kister equation and with a series of Legendre polynomials. The excess volumes are negative for systems toluene + (ethyl acrylate, butyl acrylate, and styrene) and positive for the system toluene + methyl methacrylate.     

Rhenium(IV)-copper(II) heterobimetallic complexes with a bridge malonato ligand. Synthesis, crystal structure, and magnetic properties

The Re(IV) complex [ReCl4(mal)]2-, in the form of two slightly different salts, (AsPh4)1.5(HNEt3)0.5[ReCl4(mal)] (1a) and (AsPh4)(HNEt3)[ReCl4(mal)] (1b), and the Re(IV)-Cu(II) bimetallic complexes [ReCl4(mu-mal)Cu(phen)2].CH3CN (2), [ReCl4(mu-mal)Cu(bpy)2] (3), and [ReCl4(mu-mal)Cu(terpy)] (4) (mal=malonate dianion, AsPh4=tetraphenylarsonium cation, HNEt3=triethylammonium cation, phen=1,10-phenanthroline, bpy=2,2'-bipyridine and terpy=2,2':6',2' '-terpyridine) have been synthesized and the structures of 1a, 1b, 2, and 3 determined by single-crystal X-ray diffraction. The structures of 1a and 1b are made up of discrete [ReCl4(mal)]2- anions and AsPh4+ and HNEt3+ cations, held together by electrostatic forces and hydrogen bonds. The Re(IV) atom is surrounded by four chloride anions and a bidentate malonate group, in a distorted octahedral environment. The structure of 2 consist of neutral dinuclear units [ReCl4(mu-mal)Cu(phen)2], with the metal ions united through a bridge carboxilato. The environment of Re(IV) is nearly identical to that in the mononuclear complex, and Cu(II) is five coordinate, being surrounded by four nitrogen atoms of two bidentate phen ligands and one oxygen atom of the malonato ligand. In 3, there are also dinuclear units, [ReCl4(mu-mal)Cu(bpy)2], but the Cu(II) ions complete a distorted octahedral coordination by binding with the free malonato oxygen atom of a neighbor unit, resulting in an infinite chain. The magnetic properties of 1-4 were also investigated in the temperature range 2.0-300 K. The magnetic behavior of 1a and 1b is as expected for a Re(IV) complex with a large value of the zero-field splitting (2D ca. 110 cm(-1)). For the bimetallic complexes, the magnetic coupling between Re(IV) and Cu(II) is antiferromagnetic in 2 (J=-0.39 cm(-1)), ferromagnetic in 4 (J=+1.51 cm(-1)), and nearly negligible in 3 (J=-0.09 cm(-1)).     

Heterobimetallic oxalato-bridged M(II)Re(IV) complexes (M = Mn,Fe, Co,Ni): synthesis,crystal structure,and magnetic properties

Four rhenium(IV)-M(II) bimetallic complexes of formula [ReCl(4)(mu-ox)M(dmphen)(2)].CH(3)CN with M = Mn (1), Fe (2), Co (3), and Ni (4) (ox = oxalate anion, dmphen = 2,9-dimethyl-1,10-phenanthroline) have been synthesized and the crystal structures of 1 and 3 determined by single-crystal X-ray diffraction. 1 and 3 are isostructural and crystallize in the monoclinic system, space group P2(1)/c, with a = 16.008(4) A, b = 12.729(2) A, c = 18.909(5) A, beta = 112.70(2) degrees, and Z = 4 for 1 and a = 15.998(4) A, b = 12.665(2) A, c = 18.693(5) A, beta = 112.33(2) degrees, and Z = 4, for 3. The structure of 1 and 3 is made up of neutral [ReCl(4)(mu-ox)M(dmphen)(2)] bimetallic units (M = Mn (1), Co (3)) and acetonitrile molecules of crystallization. M(II) and Re(IV) metal ions exhibit distorted octahedral coordination geometries being bridged by a bis(bidentate) oxalato ligand. The magnetic behavior of 1-4 has been investigated over the temperature range 2.0-300 K. A very weak antiferromagnetic coupling between Re(IV) and Mn(II) occurs in 1 (J = -0.1 cm(-)(1)), whereas a significant ferromagnetic interaction between Re(IV) and M(II) is observed in 2-4 [J = +2.8 (2), +5.2 (3), and +5.9 cm(-)(1) (4)].