Angewandte Chemie International Edition in English

The cover picture shows schematically how leukocytes from the blood stream bind to activated endothelium during inflammation processes and are finally extravasated into the surrounding tissue. The first step is the interaction of glycoproteins on leukocytes with adhesion molecules on the endothelium (P-, E-selectin). The tetrasaccharide sialyl Lewis^x has been identified as the binding epitope in complex glycoproteins, which are bound through carbohydrates to the selectins. Trivalent glycopeptides like that in the middle of the picture, which present three sialyl Lewis^x units, were synthesized by H. Kunz et al. with the goal of achieving improved receptor affinity. The authors describe the results of the adhesion tests and the synthesis of the glycopeptides on pages 321 – 324.

Still more can be expected from the Horner–Wittig reaction. Not only regioselective but also stereoselective Horner–Wittig additions and eliminations are possible with the bulky, anion-stabilizing Ph₂PO group. For example, alkenes having chiral centers on opposite ends of stereochemically defined double bonds can be constructed. Decisive advantages in these syntheses: Ph₂PO compounds are simple to purify, and diastereomeric β-hydroxy phosphane oxide adducts are easily separated.

Cycloadditions and cyclooligomerizations are two of the reaction types that are possible with metallophosphaalkenes. The five modes of metalation in these compounds can be described by IV. Syntheses, structures, and reactivity of these molecules, which are also versatile ligands, are presented herein.

Not only synthetic chemists, but also nature makes use of the spontaneous Diels-Alder reaction. Now there is also increasing evidence for a Diels–Alder-ase, for example in the biosynthesis of solanapyrones. The search for an enzyme-catalyzed [4 + 2]cycloaddition in nature will therefore certainly continue.

New impetus both for the organometallic chemistry of alkali metals as well as for organoboron chemistry should be stimulated by the synthesis of 1 by Fu et al. Though this compound is the second complex containing a hydrogen atom on the boron atom of boratabenzene, it is the first that has been characterized by X-ray structural analysis.

Tetrafluoride 1, pentafluoride 2, and fluoride dioxide 3 can be obtained from [Cp^*WCl₄] and Me₃SnF according to the reaction scheme shown below. Complexes of type 2 were hitherto not obtainable with chromium, molybdenum, and tungsten. The structure analysis of 2 revealed that the W center is in a square-bipyramidal environment and that the compound exists as a monomer.

The connectivity of a finite fragment of the prototypical silicon network of the intermetallic compound α-ThSi₂ is represented by the carbon-rich macrotricycle 1. The compound has a cavity with dimensions of 36 × 12 × 12 ų. Calculations indicate that concerted torsional rotation around the parallel lateral bonds results in collapse to a nearly planar, folded structure. The flexibility of compounds like 1 single them out as promising building blocks for the preparation of auxetic materials.

[C(OR)₃]⁺ ions are stabilized differently than [C(XR)₃]⁺ ions (X = S, Se, Te). While the stability of the RO-substituted carbenium ions can be attributed to the high polarity of the C^δ+ -O^δ− bond, the polarity of the C-X bond in the homologous S-, Se-, and Te-containing ions is reversed, and the stabilization is achieved by the σ-and π-dative effect of the chalcogen centers. These results were obtained by ab initio calculations on [C(XH)₃]⁺ ions and by experimental studies on the new compounds [C(XR)₃][PF₆] (X = S, Se, Te; R = 2,4,6-iPr₃C₆H₂).

NMR experiments provide evidence that C- and O-glycosides such as 1 and 2, respectively, are not necessarily bound in the same conformation by a protein. A projection of the conformations of 1 and 2 recognized by ricin B is shown on the lower right.

Intramolecular double bifurcated hydrogen bonding is responsible for the preferred conformation I of the catenane shown below in the solid state and in nonpolar solvents; in polar solvents competitive hydrogen bonding and polarphobic forces leads to conformation II.

A free tricarbene is most likely a long-lived intermediate in the formation of the first hexacarbene iron complex 1 (R = Me) from trimethylated tris(1-imidazolyl)borate, n-butyllithium, iron chloride, and tetraphenylborate. This tripodal C,C′,C″-chelating ligand, for which the authors predict a brilliant future, is an isomer of Trofimenko's tris(1-pyra-zolyl)borate and represents a novel class of negatively charged six-electron donors with the greatest possible similarity to the cyclopentadienyl anion.

Three two-coordinate phosphorus atoms are present in the W-shaped 2,3,4-triphos-phapentadienide 3, which is obtained as a lithium salt from the reaction of the phosphaalkene 1 with 2 (R = SiMe₃). Heterocycles and heterobicycles containing three phosphorus atoms are accessible from 3.

The selectivity for addition of the BB bond of Cat'BBCat' (Cat' = 4-tBuC₆H₃O₂) to [Cp₂W] even in the presence of reactive CH bonds in the solvent benzene is remarkable. In contrast to facile BB cleavage, BH bond formation occurred upon photolysis of [Cp₂W(H)(BCat)] (Cat = C₆H₄O₂). This facile cleavage of BB bonds and the preferred formation of BH bonds contrasts with the typical selectivities for CC and CH activation reactions. The formation and some reactions of tungstenocene are shown on the right.

A novel class of chelator lipids combines the fundamental properties of self-assembly with protein engineering techniques, leading to well-organized biofunctionalized interfaces and two-dimensional protein arrays. Reflection FT-IR spectroscopy at the air - water interface was used to monitor the formation of a metal-sensitive lipid film as well as the reversible and specific docking (represented on the right) of a fusion protein (B) to this chelator lipid layer through a histidine tag (A).

Potentially explosive difluoroamine, HNF₂, which is gaseous under normal conditions, is associated in the solid state through NHċN hydrogen bondsand not NHċF bondsto form infinite zigzag chains. The results of the X-ray structure analysis are consistent with those of earlier spectroscopic findings and theoretical calculations.

Threefold condensation of the sialyl Lewis^x amine derivative 1 with cyclopeptides leads to multivalent glycopeptides with repetitive saccharide units. A trivalent sialyl Lewis^x N-glycopeptide proved to be a potent antagonist of E-selectin.

Chair inversions and FSi migrations can explain why in solution the trisilacyclohexane anion 1 provides only one ¹⁹F and one ²⁹Si NMR signal. The ion shows fluxional behavior in the crystal as well, as is confirmed by the ²⁹Si CP/MAS NMR spectrum at 298 K, which contains only one signal.

Two well-defined, interconvertible helical states are distinguishable by CD and fluorescence spectroscopy in a synthetic peptide containing both electron donor (D) and acceptor (A) units (shown schematically below). This provides a suitable model system for a molecular switch, regulated by conformational changes, with potential applications in molecular electronics.

The first stable, highly unsaturated fat! Previous attempts at the synthesis of these compounds had led to rather unstable compounds without practical application; however, the use of a carotenoid as fatty acid has resulted in stable, conjugated polyene glycerides such as 1.

A sufficiently long spacer between the antithrombin- and thrombin-binding domains (ABD and TBD, respectively) is necessary to impart antithrombin activity to a synthetic glycoconjugate (shown schematically below). The spacer need not consist of multiple sugar units as in heparin; a simple chain about 50 atoms in length suffices. In addition, varying the charge density of the TBD affects the inhibitor activity, and symmetric glycoconjugates (TBD = ABD) also display high antithrombin activities.

Structural modification of the prostacyclin agonist isocarbacyclin resulted in the stable, highly selective ligand 1, which displays high binding affinity for a prostacyclin receptor in the central nervous system. The binding profile of 1 thus contrasts markedly with those of cicaprost and iloprost.

In three ways at once the macrocyclic polyamine [24]aneN₆O₂ promotes the H/D exchange at the α position in malonate ions: The quadruply protonated species in neutral pH range forms a quite stable dicationic complex (1) with the malonate dianion, which facilitates the approach of a base and assists the enolization through the acidic NH⁺ centers. The acceleration factor for the fourfold protonated polyamine [24]aneN₆O₂ is k_cat/k_uncat ≈ 1.4 × 10³.

Pronounced bond length alternation in the central aromatic ring is the eye-catching structural feature of triepoxytriphenylene 1. By contrast, no significant bond length alternation is observed in the linearly bisannelated compounds 2 and 3. The key difference is ascribed to the position of annelations with the bicyclic units.

The asymmetric catalytic Fe(CO)₃ complexation of a prochiral ligand is achieved for the first time. The preparation of different enantiopure 1-aza-1,3-butadiene catalysts and their application to the enantioselective synthesis of optically active, planar, chiral transition metal π complexes is reported [Eq. (a)].