An L-shaped conformation is responsible for the sweet taste of aspartame (1) and other dipeptides, according to NMR studies and computer simulations of 1 and twelve analogues. Since the conformations of the compounds are dominated by packing forces in the solid state, the X-ray crystal structures do not reflect the bioactive conformations.

An efficient method for producing tasty beer: characteristic bittering agents such as trans-isohumulone (1) can be obtained readily by photolysis of the hops extract in supercritical CO₂ in an established industrial process without large losses incurred on boiling. Nontoxic, environmentally friendly, cheap, nonflammable, and applicable on a large scale—these assets of supercritical CO₂ have long been recognized for extractions; this highlight shows that supercritical CO₂ is also advantageous for many reactions.

Mission Impossible? This is hardly true for the quest for small-ring alkynes. Maier et al. (Angew. Chem. Int. Ed. Engl. 1994, 33, 1248) combined a new method of gas-phase pyrolysis with matrix isolation techniques to synthesize a three-membered ring compound with a CC bond, the CH₂Si isomer 3, from disilane 1 [Eq. (a)] and characterize it by spectroscopy. More information on the exact bonding and the anticipated reactivity of this highly strained compound are eagerly awaited.

Model compounds for the study of the primary processes in photosynthesis continue to spur chemists on to elaborate syntheses. Latest efforts have resulted in the triple-decker compounds (“triads”) 1 and 2, in which two quinone and one porphyrin, and one quinone and two porphyrin units, respectively, are linked in rigid arrangements.

Model compounds for the study of the primary processes in photosynthesis continue to spur chemists on to elaborate syntheses. Latest efforts have resulted in the triple-decker compounds (“triads”) 1 and 2, in which two quinone and one porphyrin, and one quinone and two porphyrin units, respectively, are linked in rigid arrangements.

Minimal use of protecting groups is required in the synthesis of the branched hexa-saccharide Lewis^b 1. Thus, donor and acceptor glycosyl units were linked stereo-selectively to the polymer-bound glycal in such a way that an oligosaccharide recognition element is formed which can be bound to a carrier protein.

Linked through sodium ions to form a tetranuclear complex, the neutral unit [MoL(NO)(O)(OH)] of the title compound displays two types of bonding between the Na+ ions and NO (shown on the right) in the crystalline state. The bonding in this and similar monomeric nitrosyloxo complexes can be described by a synergistic bonding model. L = 1,4,7-triiso-propyl-1,4,7-triazacyclononane.

Even this simple helical molecule, the benzyl-protected pentamer 1, has a few surprises in store. Two independent conformers of 1 with the same absolute configuration are found in the unit cell which demonstrate an unprecedented form of isomerism. The helicity of the oligomeric formaldehyde is definitely a consequence of stereoelectronic effects.

The dark red Fe and colorless ZrCl₂ complexes 2 are easily accessible from the doubly bridged biscyclopentadienes 1. The substantial torsion of the two eclipsing decks of the conformationally rigid ligand framework of 2 exposes the metal center and points to the potential of such complexes as catalysts for the stereoselective polymerization of alkenes.

Readily accessible from bis(propynyl)zirconocene, the organometallic betaines like 1 form by an intramolecular alkyne insertion and trapping reaction with tBuNC from an intermediate that has a hexadiyne ligand unsymmetrically coordinated to the Zr and B centers. This diyne complex is obtained in a rapid CC coupling reaction reaction between the starting material and B(C₆F₅)₃.

An unusually short SiSi distance of 2.518(3) Å despite an antibonding interaction between these centers and a small endocyclic angle of 67° at phosphorus, these are the surprising structural features of the title compound 1, which forms by cyclocondensation of tBuSiFCl₂ with CyPHLi in 33% yield. The structure of 1 reflects a tug-of-war between the three SiPSi and the six PSiP angles, Cy = cyclohexyl.

Hyperconjugation is the reason for the different structures of 1 and 2: in 1 the electron deficiency of the boracyclohexadiene leads to strong hyperconjugation with the CB bond in the spiro-linked three-membered ring, and between the thus more positive boron atom and a CSi bond. In 2, in contrast, the electron deficiency of the boron atom of the four-membered ring is reduced by formation of a 2π electron homoarene by using the n electrons of the CC double bond. Dur = 2,3,5,6-tetramethylphenyl, R = SiMe₃.

Significant downfield shifts in the NMR spectrum are displayed by the central Si atoms in compounds 1 relative to those of disilenes not having four silyl substituents. The extraordinary color change of 1c on dissolving in hexane (yellow → deep red) is particularly impressive and indicates a conformational change caused by reduction in steric strain.

Depending on the solvent, two completely different products are obtained from the title reaction: the cage compound 1 (91%) is formed from 2 and AlEt₃ (mol ratio 3:2) in n-hexane, whereas the polycycle 3 (74%) results from 2 and AlEt₃ (mol ratio 4:1) in diethyl ether.

A door to the underdeveloped organocalcium chemistry has possibly been opened with the synthesis of the title compound. Long SnC bonds and small CSnC angles are found, which result from the predominantly ionic CaSn bonding. The centrosymmetric Me₃SnCaSnMe₃ fragment has D₃₆ symmetry.

What happens when protozoa are injected onto an HPLC column? Because they do not have a rigid cell wall, they instantaneously release their contents, which can thus be analyzed without loss or opportunity for artefact formation (“ex vivo HPLC”). This method applied to the example, Pseudokeronopsis rubra (picture below), showed that the organism stores 1 in the form of the sulfate ester (R¹ = SO₃Na), which only on destruction of the cells is converted enzymatically into the more toxic pyrone 1 (R¹ = H), R² = H, Br.

Traces of nitrate in the starting material FeCl₃ lead to the formation of the analogous nitrosyliron(I) complex 2 during the synthesis of the tris(dithiocarbamato)iron complex 1. EPR studies on ⁵⁷Fe-labeled 2 show that this complex gives rise to the triplet signal in the EPR spin-crossover studies on 1 at g = 2.0 that has been discussed for a long time.

In two steps and 85% yield the enol triflate pyranose 1 was prepared. This compound has proved to be well suited for the synthesis of pyranoses with conjugated elements.

Kinetic stability in water, good protection of the metal center, and strong luminescence of the metal ion make lanthanide complexes of the ligand 1 ([M⊂1]³⁺, M = Eu, Tb) promising as luminescent labels in fluoroimmuno-assays.

The clearing temperature is unexpectedly higher for the liquid crystalline phases of the macrocycle 1 (209°C) than for comparable acyclic biphenyls. The coupling of two appropriate rigid units through flexible terminal chains is apparently a highly effective method of inducing and stabilizing the mesophase.

Exceptionally high selectivity for cesium ions (α(Cs/Na) > 33000) is displayed by calix[4]arenecrowns-6 in the 1,3-alternate conformation. They also remove ¹³⁷Cs quantitatively (>96%) from radioactive waste that is 1 M in HNO₃. The complexation properties result from the simultaneous operation of several effects: the size of the crown ether ring, the polarity of the calix conformation, and the strength of the cation/π-electron interactions. The latter interaction was evident in the X-ray crystal structure of the cesium complex (shown on the right).

40π-Electrons and ten pyrrole units are found in macrocycle 1, the largest expanded porphyrin reported thus far. The tetrahydrochloride salt of 1 was characterized in solution and in the solid state. These investigations indicate that the macrocycle exists as a pair of enantiomers whose chirality arises from a twist of the ring.

The outer complex initially formed on electrophilic addition has been shown to have the structure 1 in the prototypical reaction of ethene with BrCl. The rotational spectra of two isotopomers of a weakly bound complex were observed by Fourier transform microwave spectroscopy with a fast-mixing nozzle. This result is important for the understanding of the mechanism of electrophilic addition, a fundamental reaction in organic chemistry.

Up to 50 um long, stable, cylindrical aggregates (shown on the right) are formed spontaneously from dispersions of perfluoroalkylated, single-chain, nonchiral amphiphiles derived from dimorpho-linophosphoramidate and phosphocholine. Depending on the polar head group, rigid hollow tubules or thinner, longer, flexible fibers are obtained. Thus, the strong hydrophobic effect of the fluorinated chains alone appears to be sufficient to promote the formation of assemblies more stable than vesicles.

Silver(I) ions are bound selectively and with high cooperativity to the peptide westiellamide obtained from Lisso-clinum. The complex contains a unique [Ag4]⁴⁺ cluster sandwiched between two neutral macrocyclic ligands (see structure on the right). The importance of metal chelation for the biological activity is currently under investigation.

Cytostatically active titanocene dichloride reacts with α-amino acids to give titanium(IV) amino acid complexes of the type [Cp₂Ti(aa)₂]²⁺ [Cl⁻]₂ (aa = α-amino acid). The structure of the cation (1) in the corresponding 2-methylalanine complex is shown on the right.