2,5-Diformylbenzene-1,4-diol: A Versatile Building Block for the Synthesis of Ditopic Redox-Active Schiff Base Ligands

2,5-Diformylbenzene-1,4-diol (5) is a well-suited starting compound for the preparation of ditopic hydroquinone-based ligands. Here, we report an optimized synthesis of 5 which improves the overall yield from published 7% to 42 %. Three new ditopic Schiff base ligands, 2,5-[iPr2N(CH2)2N=CH]2 - 1,4-(OH)2-C6H2 (8), 2,5-(pyCH2N=CH)2-1,4-(OH)2-C6H2 (9), and 2,5-[py(CH2)2N=CH]2-1,4- (OH)2-C6H2 (10), have been synthesized from 5 and structurally characterized by X-ray crystal structure analysis (py = 2-pyridyl).

Studies of the solvatochromic emission properties of N-aroylurea derivatives II: influence of hydrogen-bonding interactions

The solvatochromic emission properties of five naphthoylurea derivatives with different substitution patterns at the naphthoylurea functionality were investigated, with a particular focus on the influence of inter- and intramolecular H-bonding interactions. The bathochromic shifts of the emission maxima correlate well with the acceptor number or Catalán's acidity of the solvent (Δλ = 47-86 nm), indicating an excited species with a pronounced negative charge that is stabilized by H-bond donating (HBD) solvents. In media with restricted free volume the formation of the charged species is not favored, because the required conformational change to establish an intramolecular charge transfer (ICT) between the fluorophore and the acylurea substituent is hindered, and the emission mainly originates from the locally excited state. This relationship between the alignment of the naphthoyl carbonyl functionality relative to the naphthyl ring and the spectroscopic shift was confirmed by the comparison of the ground state conformation and the emission spectra of the naphthoylurea derivatives in the solid state. Time-resolved experiments revealed different excited entities, whose lifetimes are significantly influenced by the HBD properties and the temperature of the environment. With few exceptions the naphthoylurea derivatives exhibit only two emissive species in the nanosecond range. All experimental data point to conformational relaxation and solvent reorganization leading to the cis and trans isomers of one preferential conformer with respect to the acylurea unit. The structure of the preferred conformation is mainly determined by the possible inter- or intramolecular H-bonds and is therefore also strongly influenced by the HBD and H-bond accepting (HBA) properties of the polar solvents. As the NH groups of the acylurea functionality contribute mainly to the entire inter- and intramolecular H-bond arrangement the variation of the substitution pattern of the urea unit, specifically the presence and position of the NH groups, leads to derivatives with significantly different steady-state and time-resolved emission properties.

Diastereo- and enantioselective synthesis of alpha,beta-disubstituted gamma-nitro methyl sulfonates

A novel asymmetric synthesis of highly enantioenriched homotaurine precursors has been developed via diastereo-selective Michael addition of lithiated enantiopure sulfonates to nitroalkenes by using 1,2:5,6-di-O-isopropylidene-alpha-D-allofuranose as chiral auxiliary.

Gold catalysis: on the phenol synthesis

[reaction--see text] For the gold-catalyzed phenol synthesis an intramolecular migration of the oxygen atom was proven. Several other late transition metals with d(8) configuration allow this conversion, providing evidence for Au(III) being a catalytically active species. On the other hand gold was still the most active catalyst and gives the cleanest conversion. With additional alkynyl substituents another ring could be closed to provide doubly annellated arenes.

Electronic communication in oligometallic complexes with ferrocene-based tris(1-pyrazolyl)borate ligands

Ferrocene-based tris(1-pyrazolyl)borate ligands 1R-Li and 1R-Tl have been synthesized and used to generate a variety of heterotrinuclear transition metal complexes, 3R-M [R = H, SiMe(3), cyclohexyl, (cyclohexyl)methyl, phenyl; M(II) = Mn, Fe, Co, Ni, Cu, Zn]. The poor solubility of 3H-M is greatly enhanced by the introduction of large organic substituents into the 4-positions of all pyrazolyl rings. The unsubstituted ligand 1H-Li and the trinuclear complex 3Cym-Cu [Cym = (cyclohexyl)methyl] have been investigated by X-ray crystallography. 1H-Li, which represents the first example of a structurally characterized lithium tris(1-pyrazolyl)borate, forms centrosymmetric dimers in the solid state. A severe Jahn-Teller distortion was observed for the (Bpz(3))(2)Cu fragment in 3Cym-Cu. Compared to the parent compounds [(HBpz(3))(2)M], the presence of uncharged ferrocenyl substituents in 3R-M tends to shift the M(2+)/M(3+) redox potential to significantly more cathodic values. The opposite is true if the ferrocenyl fragments are in their cationic state, which results in an anodic shift of the M(2+)/M(3+) transition. Most interestingly, the two ferrocenyl fragments in 3R-Cu appear to be electronically communicating.

Two isomorphous benzenesulfonamide crystal structures determined by intermolecular C-H...O, C-H...pi and C-H...Cl interactions

The title compounds, N-[5-(4-chlorophenyl)furan-2-ylmethyl]-4-methyl-N-(prop-2-ynyl)benzenesulfonamide, (Ia), and N-[5-(2-chlorophenyl)furan-2-ylmethyl]-4-methyl-N-(prop-2-ynyl)benzenesulfonamide, (Ib), both C(21)H(18)ClNO(3)S, have isomorphous crystal structures. The crystal packing is mainly determined by intermolecular C-H...O and C-H...pi interactions. These interactions are very similar in (Ia) and (Ib). Additional intermolecular C-H...Cl interactions appear less important and are different in (Ia) and (Ib). The different positions of the Cl atoms result in small variations of the crystal packing of the two compounds.

Pseudosymmetry and phase transition in dimethyl 2,3-bis(tricyclo[3.3.1.13,7]dec-2-ylidene)butanedioate

The crystal structure of the title compound, C(26)H(34)O(4), shows a reversible phase transition at about 178 K. The structure of the high-temperature phase contains two independent molecules related by pseudosymmetry elements. Cooling through the phase-transition temperature results in a doubling of the c axis. The low-temperature structure contains four independent molecules related by pseudosymmetry elements. The phase transition results in a rearrangement of some weak intermolecular C-H.O interactions. The number of very weak C-H.O interactions, with H.O distances between 2.8 and 2.9 A, is increased in the low-temperature structure.

Dimethyl dispiro[tricyclo[3.3.1.1(3,7)]-decane-1,2'-bicyclo[1.1.0]butane-4',1"-tricyclo[3.3.1.1(3,7)]decane-1',3'-dicarboxylate, a strain-enhanced bicyclo[1.1.0]butane with a very short intramolecular C--H...H--C contact

The title molecule, C(26)H(34)O(4), shows a very short repulsive intramolecular C-H.H-C contact, with an H.H distance of only 1.71 A. The flap angle varphi of the bicyclo[1.1.0]butane group is widened to 131.23 (8) degrees. Both methylene C atoms show tilt angles of 6.1 degrees. Bond lengths in the fused cyclopropane rings range from 1.501 (1) to 1.542 (1) A and are influenced by pi interactions with the carboxylate substituents.

Planar chirality: cycloaddition and transannular reactions of optically active azoninones that contain (E)-olefins

Unsaturated nine-membered ring lactams that contain (E)-olefins within the ring are characterized by planar chiral properties. Thus, selective conversions of the double bond allowed a complete transfer of the planar chiral information into new stereogenic centers The basis of the transformations was the high activation barrier that prevented efficient flipping of the double bond at room temperature (epimerization pR <=> pS) with respect to the ring. Cycloadditions led diastereoselectively to cyclopropano, epimino, epoxy, and dihydroxy azonanones under mild conditions with moderately high yields. The epoxy azonanones were subjected to regio- and diastereoselective transannular epoxide opening/ring contraction sequences to give hydroxy indolizidinones. The regiochemical and stereochemical outcome strongly depends on the configuration of the oxirane and the chiral information of the lactam unit. The so-formed optically active bicycles with defined substitution patterns should serve as versatile building blocks in alkaloid synthesis.

Solid-state transformation of 4, 2'-anhydro-5-(beta-D-arabinofuranosyl)uracil dihydrate to 4, 2'-anhydro-5-(beta-D-arabinofuranosyl)uracil monohydrate

Crystals of 4,2'-anhydro-5-(beta-D-arabinofuranosyl)uracil, (I), obtained from an aqueous solution, were characterized as the dihydrate, C(9)H(10)N(2)O(5).2H(2)O, (Ia). In air, these crystals slowly transform to the monohydrate, C(9)H(10)N(2)O(5).H(2)O, (Ib), but remain crystalline. The solid-state transformation proceeds with the loss of one water molecule and a rearrangement of hydrogen-bonded layers of molecules. The furanose ring in (I) has an approximate C4'-exo,O4'-endo twist conformation. The central five-membered ring is slightly puckered. The uracil group is planar within experimental uncertainty.

A modular approach to structurally diverse bidentate chelate ligands for transition metal catalysis

A modular approach to a new class of structurally diverse bidentate P/N, P/P, P/S, and P/Se chelate ligands has been developed. Starting from hydroquinone, various ligands were synthesized in a divergent manner via orthogonally bis-protected bromohydroquinones as the central building block. The first donor functionality (L1) is introduced to the aromatic (hydroquinone) ligand backbone either by Pd-catalyzed cross-coupling (Suzuki coupling) with hetero-aryl bromides, by Pd-catalyzed amination, or by lithiation and subsequent treatment with electrophiles (e.g., chlorophosphanes, disulfides, diselenides, or carbamoyl chlorides). After selective deprotection, the second ligand tooth (L2) is attached by reaction of the phenolic OH functionality with a chlorophosphane, a chlorophosphite, or a related reagent. Some of the resulting chelate ligands were converted into the respective PdX2 complexes (X = Cl, I), two of which were characterized by X-ray crystallography. The methodology developed opens an access to a broad variety of new chiral and achiral transition metal complexes and is generally suited for the solid-phase synthesis of combinatorial libraries, as will be reported separately.

First and Highly Diastereoselective Synthesis of Palladepanes

Unexpected stability is observed for the palladacycloheptane rac-2, which is obtained as major product and single diastereomer from the cyclization of 1 and Pd(0). Up to a temperature of 50 degrees C 2 shows neither reductive elimination nor intramolecular insertion of another olefin.

Conformational analysis of cyclic peptides in solution

The strategy and tactics of conformational analysis of cyclic peptides in solution is demonstrated by the example of cyclo(-D-Pro-Phe-Thr-Phe-Trp-Phe-). Spin-locked experiments like rotating frame nuclear Overhauser enhancement spectroscopy (ROESY), ROTO, and TOCSY are successfully applied to assign all proton signals and to obtain distance information. A crude conformational model was built using the nmr data. This starting model was refined by restrained molecular dynamics (MD) calculations using ROE derived distances and fixed bond angles as determined from homo- and heteronuclear coupling constants. To mimic the solvent and to reduce artifacts in an in vacuo calculation the charges of the solvent-exposed NH protons were gradually reduced according to the temperature gradients. The thus obtained "conformation" (mean of a 40 ps MD trajectory) shows very close similarity to x-ray structures in an orthorhombic and in two monoclinic crystal modifications of the same compound. The main difference is the breaking of an intermolecular hydrogen bond of the threonine hydroxyl group on dissolution of the crystal and forming an intramolecular hydrogen bond in solution.