In the first carbotetracation, 1, the charged groups are tetrahedrally arranged on an adamantane cage. Carbocations are no longer a domain of physical organic chemistry. They also play a role in organometallic chemistry and biochemistry, as a new samarocene complex containing a norbornadienyl cation fragment and the newly coined term “Olah enzyme” testify.

In the design of peptide-based drugs the β turn has been promoted to a design principle in recent years. In research on antiadhesive RGD mimetics, receptor selectivity has been shown to be encoded in the β-turn type within cyclic peptides of type 1 having an identical lead sequence. Switching the turn type from βII′ to βI inverts the selectivity profile so that a compound originally with antithrombotic activity becomes an antimetastatic agent.

One-electron reduction of the rhodium complex 1 has resulted in the first successful conversion of a discrete metal–metal bonded “dimer” into a one-dimensional polymer (2). The doped material 2 (depicted on the right) differs significantly from the well-known tetracyanoplatinates in that the metal backbone is cationic. In addition, there is the possibility of fine-tuning the material properties by varying the organic group.

Three solid-state techniques—atomic force microscopy, X-ray structure analysis, and diffuse reflectance spectroscopy—were combined to discover why E/Z isomerizations in the crystal, for example 1 → 2, are possible despite their large spatial demands. These studies revealed, for instance, that on irradiation with wavelengths corresponding to the absorption tail of the crystals, anisotropic surface features appear.

Templates for the arrangement of guest molecules—that is the role played by microporous crystalline SiO₂ modifications in the insertion compounds presented. The electronic interactions between the guest species can be controlled by the dimensionalities of the void systems. When iodine is the guest component, this is directly apparent from the colors of the insertion compounds, which for the host structures shown below range from violet to red-brown.

Not interpenetrated, but filled with solvent molecules, the large cavities of the polymer [Co(NO₃)₂(4,4′-bpy)_1.5]_n form different structures depending on the solvent system from which it crystallizes. One of the two possible structures (section of structure shown on the right) exhibits hydrophobic cavities about 11 × 11 Å in size. The solvent molecules can be readily removed, but the process has not yet been proven reversible.

Anion complexation drives the self-assembly of appropriately designed macrocycles (sapphyrin and/or calix[4]pyrrole) containing pendant carboxylic arms (black in the schematic representation on the right). The self-assembly process can be disrupted by addition of F⁻ ions (sapphyrin) or in the case of calix[4]pyrrole, polar solvents.

An increased stability of duplexes between amide-modified oligonucleotides (depicted on the right) and RNA was achieved by the introduction of 2′-OMe groups. Since the modified oligonucleotides display a very high affinity for their RNA complement and are more stable towards nucleases, they can be considered as promising antisense agents. X = H, OH, OMe; Y = H, OMe.

Monodisperse, chiral dendrimers from trimesic acid and (R)-3-hydroxybutanoic acid were synthesized and characterized. Preliminary investigations have shown that these biodegradable dendrimers (depicted on the right) are substrates for hydrolases.

In calichearubicin B (1) the aglycon of daunorubicin and the carbohydrate domain of calicheamicin γ are linked by a spacer (calichearubicin A lacks this spacer). The aglycone serves as an anthracycline-like intercalator, while the carbohydrate portion binds to the minor groove of DNA. Thus calichearubicin B exhibits properties of both parent compounds.

An audacious macroaldolization is the key step in the first total synthesis of the cytotoxic epothilone A, which starts with the units 1, 2, and 3. This condensation closes the ring to form the macrolide, whose structure was determined and published (also in Angewandte Chemie) as recently as July 1996.

1,2-Addition of the surface silanol groups on SiO₂ to the nitridomolybdenum fragment of the title compound and subsequent α-elimination of neopentane forms the catalytically active surface species I. This is the first case in which an alkylated complex is converted into a heterogeneous catalyst in a well-defined way. The immobilized compound is significantly more active in ring-opening metathesis polymerization than the molecular precursor.

The ee values of the hydrosilylation of acetophenone with the rhodium catalyst 1 are only small; nevertheless, the robustness of the catalyst is impressive: The chiral carbene ligand, which is readily available and can be easily varied, remains bound to the metal in solution to over 100°C. Thus, no excess of ligand is necessary in the catalytic reactions.

In a one-pot reaction from 15 components, a tetra-hedral host-guest compound forms by self-assembly. Its tripod–iron(II) groups are the corners of a tetrahedron (depicted on the right). The edges of the tetrahedron are bridged by 1,2-dinitrile ligands, and a BF ion is encapsulated in the tetrahedral cavity.

All of the selectivity criteria for catalytic asymmetric aminohydroxylation with sulfonamides “Eq. (a)” are improved when the original nitrogen source, Chloramine-T, is replaced by its methyl analogue, Chloramine-M (1). This newly introduced reagent exhibits substantially higher ligand dependence, and for most substrates the desirable phenomenon of ligand-accelerated catalysis was observed. DHQ-H = dihydroquinine, PHAL = 1,3-phthalazinediyl.

Better enantioselectivities and convenient deprotection make the sodium salts of N-chlorocarbamates the oxidant/nitrogen source of choice for the osmium/cinchona alkaloid catalyzed asymmetric aminohydroxylation (AA) of olefins [e.g. Eq. (a)]. This methodology simplifies the synthesis of a vast array of compounds such as unnatural amino acids and other pharmacologically important compounds. DHQH = dihydroquinine, PHAL = 1,3-phthalazinediyl, Z = benzyloxycarbonyl.

A difference of at least 35 kcalmol⁻¹ between the activation barriers results from ab initio calculations for the competing [2 + 2] and [3 + 2] additions of OsO₄ to ethylene, clearly favoring the latter pathway (the two transition state structures are pictured on the right). The relative barrier heights for the proposed reaction paths hardly change in the presence of NH₃, used to model the base-catalyzed reaction. These theoretical findings on asymmetric dihydroxylation contradict the conclusions drawn from kinetic studies by Sharpless et al., but support the recent kinetic investigations by Corey et al.

An exo:endo ratio of 98:2! Thermochemistry would have predicted the opposite, but in the rearrangements of 5-hexenyl radicals the exo pathway is preferred due to kinetic reaction control. Selected ab initio methods are able to properly describe cyclizations of the 4-penten-1-oxyl radical (see sketch).

The secondary structure is of primary importance! The D and L forms of α-helical poly(glutamic acid) (PGA) are complexed selectively by the (R) and (S) isomers, respectively, of the porphyrin-based, chiral receptor 1, which has a xylylenediamide strap. Helical polypeptides can thus be separated by complexation with enantiomerically pure 1.

Anchoring gives stability. The recently synthesized anionic tungsten alkylidynes fixed over a planar tetraoxo surface of calix[4]arene (see picture) can undergo reversible protonation and metalation by carbophilic metals without disturbing the calixarene ligand. R = Ph, nPr, SiMe₃.

Although it was once deemed a nearly impossible task (since Cu_II centers are generally readily reduced by thiolates), authentic Cu_II and Cu_III complexes with aliphatic thiolates in the metal coordination sphere have been synthesized. The unusual pair of squareplanar complexes (NEt₄)₂[Cu(phmi)] · H₂O and (PPh₄)[Cu(phmi)] (the anion is depicted on the right) displays the lowest known redox potential ever recorded for a Cu^III/Cu^II redox couple and has been completely characterized. H₄phmi = N, N′-1,2-phenylenebis(2-mercapto-2-methylpropionamide).

Conjugate addition of a vinyl cuprate to the sterically and electronically very demanding enone 1 is the key step of a short asymmetric total synthesis of (—)-dihydro-codeinone (2), an immediate precursor of (-)-morphine.

Trinuclear gold(I) pyrazolate complexes of type 1 show liquid crystalline phases at room temperature. The molecules are arranged in a hexagonal columnar structure. The mesogenic behavior of these compounds is strongly related to the molecular symmetry, which is determined by the substituents R and R′. R = 3,4-(H₂₁C₁₀O)₂C₆H₃, R′ = R, 3,4,5-(H₂₁C₁₀O)₃C₆H₂.

A reversibly coordinating auxiliary is the key to the first highly diastereo-selective hydroformylation of acyclic methallylic alcohols (syn:anti up to 96:4; in the representation on the right the auxiliary is shown as a sphere). The reaction can be used for the efficient construction of stereotriades, the central structural units of polyketide natural products. R is, for example, Ph, CO₂Me, Me, CH₂Ph.

A novel “copper crown” complexes UO preferentially (see picture), as competition experiments with Ca²⁺ and Cu²⁺ showed. This ligand thus provides an alternative to expanded porphyrins for the complexation of actinides and lanthanides by high denticity, planar ligands.

Alternating phenylene units and methylsulfonium groups make up polymer 1, which is formed by polymerizing benzene with sodium methanesulfinate. This reaction is conducted in trifluoromethane-sulfonate, and xylene and diphenyl ether are also suitable monomers.

Many different trisaccharides (theoretically 256) can in principle be constructed from only four protected building blocks by following the strategy described for the synthesis of trisaccharide libraries. The method relies on latent-active glycosylation. The formula below gives an idea of the compounds in these libraries.

Steric strain shifts the equilibrium between the two isomeric allyltitanium compounds and isomeric allenyl- and propargyltitanium compounds containing cyclopropane units. This leads to unusual regioselectivity in their additions to electrophilic carbonyl compounds (shown on the right) and thus provides easy access to alkylidenecyclopropane derivatives.

The length of the linear hydrocarbon chain in the solvent is a key factor in the facial diastereoselectivity of the nucleophilic addition of n-butyllithium to 2-phenyl-propanal [Eq. (a)]. The reaction in these solvents shows inversion temperatures that correlate with the melting point of the solvent.

The first metalated germanide ion and only the second example of a cluster of the rare nido-10 (iv+iv) structural type, the anion 1, was isolated from the reaction of [Ni(CO)₂(PPh₃)₂], K₄Ge₉, and “2.2.2”cryptand in ethylenediamine (en). However, the structure of the Ge₁₀Ni cluster (see picture) is distinctly distorted relative to the ideal nido-10 (iv+iv) geometry.

Re₆ octahedra inside Te₈ cubes is a common feature of both title compounds. The picture on the right shows a section of the structure of Re₆Te₁₆Cl₁₈, in which the Re₆ octahedron and the binding of the cluster to the [Te₈Cl₁₈]²⁻ ligand are apparent. Different binding to ligands results in a three-dimensional structure for Re₆Te₁₆Cl₁₈, and a two-dimensional structure for Re₆Te₁₆Cl₆.

Only 10¹² atoms of an element are required for the determination of its first ionization potential by resonance ionization with laser light in the presence of an electric field and subsequent mass spectrometry. The figure on the right shows the ionization thresholds W_th of berkelium and californium and the extrapolation to the external field strength E = 0, which yields the ionization potentials I (49989(2) and 50665(2) cm⁻¹, respectively).

Twofold, transannular cyclization of the bis(methylene)bisseco intermediates B, obtained from the pagodane diester A, provides access to the parent nonpentagonal bis(homo)dodecahedrane C and pairwise symmetrically functionalized derivatives. Some of these products are important model compounds for quantifying the factors that determine the transannular electronic interactions in cage compounds of this type.

Self-assembly of functionally “inverse” zinc chlorins—3¹-hydroxy and 13¹keto groups, which are functionally essential in the bacteriochlorophylls c, d, and e, have been interchanged—is possible in nonpolar solvents (see schematic representation below). This result sets structural conditions for the formation of large aggregates similar to those formed by bacteriochlorophylls in vitro and in light-harvesting antenna systems of photosynthetic bacteria.

Metal halide complexes and functionalized amines react to form imidobridged transition metal clusters. This method can be used to synthesize imido clusters of electron-rich transition metals such as [Li(thf)₄]₄-“Cu₂₄(μ₃-NPh)₈ μ₄-NPh)₆” 1 (the structure of the anion of 1 is depicted on the right).

The missing link in the series clathrate hydrates, group silicate hydrates—layer silicate hydrates—porous framework silicas is provided by RUB-15, a new layer silicate hydrate with a three-dimensional bonding network composed of layerlike silicate ions and water molecules linked by hydrogen bonds. A section of the structure of is shown on the right. RUB-15 further illustrates the close relationship between crystalline compounds of water, silica, and mixtures thereof.

Monomeric in solution, dimeric in the crystal is the dicarbonyl(pentaisopropyl-cyclopentadienyl)iron(I) radical 2, which is formed from the high-spin iron(II) complex 1 by carbonylation and reduction.

The bonding in heteropolyhedral compounds of the closo-borane type is discussed for “As₂(AlCp^*)₃” clusters, whose molecular structure has been characterized by X-ray crystallography (framework shown on the right). Quantum chemical calculations on several trigonal-bipyramidal EE structures revealed the mode of bonding for a variety of ligands (Cp^* = C₅Me₅; E^III: Group 13 element, E^V: Group 15 element).

Two important aspects, the synthesis of imido complexes of radioactive metal isotopes and the diversity that can be incorporated with this ligand, distinguish this communication. The multiply bonded organoimide ligand provides a new and powerful means of conjugating suitable radioisotopes of rhenium to biologically relevant molecules in the design of radiopharmaceuticals. Here stable isotope and ¹⁸⁸Re-labeled analogues (1) of the anticancer drug chlorambucil were synthesized.

A bond stretch isomerism predicted over 30 years ago could exist in the organic cations 2⁺ and 3⁺, which arise from the protonation of 1 and have been characterized by X-ray crystallography. The two isomeric cations can be interconverted by changing the solvent polarity.