If all atoms of the starting materials are found in the product and only catalytic amounts of other reagents are needed, a reaction may be defined as ideal. A promising route to this ideal state is approached by the use of transition metal complexes as catalysts for addition and isomerization reactions. In this way, for instance, seven rings have been formed very efficiently from an acyclic precursor in a single step according to Equation (a).

The analysis of the relation between configuration and action of steroids at the receptor level has led to the discovery that–contrary to the ruling dogma–C/D-trans steroids 1 of hormone type can improve cardiac contractility like the therapeutically applied C/D-cis steroids 2 of digitalis type, and that the former may thus be used as lead structures for the design of novel cardioactive drugs.

By which route is monensin A biosynthesized? Is an (E.E.E.) triene first epoxidized and the polyepoxide then transformed into the natural product in a cascade of ring openings and ring closures? Or does a (Z.Z.Z.) triene undergo successive oxidative cyclizations to give monensin A directly? New results from McDonald and Towne, who carried out oxidations of (Z) dienes with pyridinium chlorochromate, support the second route.

Alkyltrimethoxysilanes yield much better host matrices for lipases than tetramethoxysilane (see picture). This is revealed by the esterifications discussed herein catalyzed by lipases that are immobilized in RSi(OMe)₃ gels. These heterogeneous biocatalysts are readily reusable and should also be suitable for continuous processes.

Sixteen thiophene units make up the longest oligothiophene to date, the 64 Å-long oligomer 4. Like its shorter homologues 1–3, 4 forms molecular structures on surfaces, which can be examined by scanning tunneling microscopy with submolecular resolution. Oligomers 1-4 can be considered components of “molecular wires” and serve as model compounds for the study of charge transport in electrically conducting polymers.

A twelve-membered ring made up of Fe²⁺ ions is present in the cluster framework of 1 (shown on the right without phenyl groups; Fe atoms are white, Se atoms shaded). The Fe²⁺ ions are located in the centers of edge-sharing Se₄ tetrahedra, which in turn form a ring. The shiny black needles of 1 are prepared from FeCl₃, PPh₃, and PhSeSiMe₃, and decompose readily upon heating.

A new structural type for polyrotaxanes (shown schematically on the right) results from the condensation of an amino-substituted cyclodextrin semirotaxane with the free acid groups of a dicarboxylic acid group in the side chain of a polymethacrylate. The structure of the novel polyrotaxane was determined unequivocally by NMR spectroscopic studies and by comparisons with a cyclodextrin-free model compound.

Diacylated flavonol glycosides are the response of Scots pine seedlings to UV-B irradiation. The two induced compounds 1 and 2 both have their absorption maximum at 315 nm (ϵ = 50300) in the range of UV-B radiation (290–320 nm), are compartmented in the epidermis, and offer much better protection than the monoacylated analogues at the same molarity.

Laser ablation of Zn(CN)₂ and Cd(CN)₂ gives rise to a series of anions [M_x(CN)_2x+1]⁻, which have helical structures according to density functional and force-field calculations. M atoms with trigonal-planar coordination and each bearing a terminal CN ligand are linked by linear MCNM bridges to form helices. The supramolecular helices are stabilized by interstrand coulombic interactions. The calculated structure of [Zn₂₇(CN)₅₅]⁻ is shown on the right = Zn, = C, = (N)

Clusters from lithium or sodium, phosphorus, and silicon form the frameworks of the title compounds 1–4. Compound 2 consists of two clusters 1, which are “glued together” by a (LiCI)₂ ring. Compounds 3 and 4 form under the same conditions; 4 relieves the ring strain by opening the polyhedron. Two Na centers in 4 are each η²-coordinated to one toluene (L) molecule. SiR₂ = SitBuIs, Is = 2,4,6-iPr₃C₆H₂; PR = PSiiPr₃.

The paramagnetic hydroxo-bridged diplatinum complex 1 (X = CI) results from the reaction of the title compound with O₂ and triphenylphosphane. The ferrocene unit in 1 not only fixes the two Pt atoms at an appropriate distance but also functions as an electron pool, as reactions with CO and pyridine show.

A cavity can be made to fit an aromatic guest molecule in certain polyazacyclophanes by contraction of the inner volume, which is initiated by the binding of metal cations. The anticipated cooperative effects can be quantified by fluorescene spectroscopy with suitable indicators, which opens new possibilities for the detection of, for instance, zinc ions in aqueous solution. The ternary complex formed from ligand, metal ion M, and guest G is depicted on the right.

Sterically demanding and carbanion-stabilizing substituents like SiMe₃ raise the barrier to the configurational inversion of α-thioalkyllithium compounds. With such substituents these compounds can undergo, for example, electrophilic substitution [Eq. (a)] without changes in configuation. Experiments with deuterated compounds proved that alkyllithium intermediates from primary S-alkylthiocarbamates are typically not configurationally stable.

The surprising thermal stability of the first monomeric, homoleptic diaryliron(II) complex 1 arises because its Fe center is very well shielded by the sterically demanding ligands. The magnetic moment of 4.77 μ_B indicates a high-spin state of the highly soluble, air- and moisture-sensitive complex.

Electropositive elements are prerequisites for the formation of anti van't Hoff/Le Bel geometries, as evident in compounds 1, which contain planar tetracoordinate boron atoms in the CB₂H framework. These compounds extend the range of molecules of this structural type to include those without metal atoms. R = CH(SiMe₃)₂, R′ = 2,3,5,6-Me₄C₆H.

s-Indacenes reveal the limits of the Hückel rules: These 12π systems are formally antiaromatic, but the results of more recent quantum mechanical calculations indicate π-electron delocalization. The synthesis and spectroscopic investigation of the non-electronically stabilized 4-hetero-s-indacenes 1 and 2 have now been achieved for the first time.

A very short GaGa bond, stabilized by carborane moieties, is present in the digallane 1, which is air- and moisture-sensitive, and soluble in both polar and nonpolar organic solvents. The coordination of each Ga atom to the respective C₂B₃ carborane face is “slip distorted”. • = H, □ = SiMe₃.

IGLO computations reveal the aromatic character of the transition structures 1 a and 1 b for the Cope rearrangement of 1,5-hexadiene. In addition, high-level ab initio calculations favor a synchronous concerted mechanism via intermediates not having sigificant diradicaloid character.

Energetic, geometric, and magnetic criteria yield quantitatively the same order of aromaticity (antiaromaticity) for the five-membered ring systems 1: the aromatic 6π electron systems are stabilized, their bond lengths are equalized, and the magnetic susceptibility exaltations are diamagnetic. In contrast, the antiaromatic 4π electron systems are destabilized, double bonds are localized, and susceptibility exaltations are paramagnetic.

The potential of rigid macromolecules as building blocks of supramolecular architectures is shown here by the reactions in LB layers from cellulose derivatives. With an appropriate sequence of layers, a two-dimensional network is accessible by Diels-Alder reaction between the side chains in the zones of overlap shown schematically on the right.

The Mn^IVO unit in the five-coordinate oxo(porphyrinato)manganese(IV) complex 2, which is formed by oxidation of the bromo(porphyrinato)manganese(III) complex 1 with potassium peroxycarbonate, was characterized by X-ray absorption spectroscopy. The relatively long MnO bond (1.69±0.03 Å) is probably attributable to the high-spin state and the low polarizability of the half-filled t subshell of high-spin Mn^IV.

Twelve electrons more than the starting material, but the same cluster framework—this modification was achieved for the first time in the synthesis of the Pt₃Re clusters 1 (66e) from [Pt₃{Re(CO)₃}(μ-dppm)₃]⁺ (54e) and H₂O₂ or propene sulfide. The metal-oxygen units in 1 could serve as models for the interactions between oxo ligands and metal clusters, which are important in oxide-supported PtRe bimetallic catalysts.

The reversible photoisomerization of the helical thioxanthenes 1 and 2 leads to photostationary states with large diastereomeric excesses. These compounds are promising starting materials for optical data storage devices.

Anti-AIDS deoxynucleosides like stavudine (d4T, 3) are now accessible in a short (five steps!) asymmetric synthesis. The “missing link” between the acyclic precursors and the product is a Mo(CO)₅-catalyzed cycloisomerization. It yields the ether 2, the saccharide building block for many nucleosides, from 1 in 80% yield. R = tBuCO.