Angewandte Chemie International Edition

Chemical raw materials from the farmer? Thanks to modern plant breeding and gene technology, oils and fats are so chemically pure—the “new sunflower” produces 83 % oleic acid—that they have become attractive for synthesis. In recent years, through the use of modern synthetic methods including enzymatic and microbiological methods, hundreds of novel fatty compounds with potentially interesting properties have been made available.

Nonnatural media such as organic solvents have been shown to be suitable environments for enzyme-catalyzed reactions. In these media, enzymes often display improved stability, different selectivity, and the ability to catalyze reactions that are impossible or difficult in water. This is demonstrated by examples in the preparation of key intermediates for the pharmaceutical industry, the modification of fats, and the synthesis of polymers. The hydrolytic selectivity of a Pseudomonas lipase towards the enantiotopic alkoxycarbonyl groups of dihydropyridine diester 1 (R=R¹=POM) can be switched, giving rise to exclusively the S enantiomer (R=POM, R¹=H) of the product in diisopropyl ether or to the R enantiomer (R=H, R¹=POM) in cyclohexane. POM=CH₂OCOtBu.

The course of scientific research can be described in terms of the action of paradigms on the conduct of normal, everyday science and on puzzle generation and solution (see diagram). When certain research appears to create anomalies and attracts the attention of the scientific community, a scientific crisis results which may lead to a true scientific revolution and paradigm shift or to pathological science which is then ignored by the community.

Colon cancer is widespread in western populations. In many cases, the proteins β-catenin and APC play a decisive role. New research results give an insight into the molecular connections which eventually cause the disease, and suggest new ways not only for prevention but also for the improvement of existing drugs and the development of new therapies.

Small organic gelling agents very efficiently self-assemble into an entangled network in a wide variety of organic solvents, thereby turning the liquid into a gel. Recent progress in the design of organogelators led to tailor-made gelling agents for supercritical CO₂. Furthermore, such agents can be used as templates for the construction of novel materials and as responsive gel systems. The picture shows switchable organogels and indicates schematically how viscoelastic properties can be affected.

That flexible side groups pointing to the outside are not essential for the formation of discotic liquid crystals is displayed by the structure of 1, in which the rigid core functions as the periphery and the alkyl chains point toward the inside.

How can secondary building units (SBUs) be assembled periodically in three dimensions? An answer is provided by the procedure presented here, in which virtual libraries of inorganic architectures are generated by simulations. The method uses tailored “sticky-atom” potentials, and a simulated annealing–minimization cluster analysis cascade, and yields a library of candidate structures (see example), ranked by cost, and sorted according to their space group, unit cell, and atomic positions.

With high asymmetric induction (up to 96 % de) TADDOL-dimethacrylate can be cyclized to an 11-membered monocycle, or obtained as a dimer, a trimer, or a polymer. Enantiomerically pure, isotactic methylmethacrylate oligomers and polymers can be prepared from these by hydrolysis [Eq. (1), R=CHPh₂, CPh₃, fluorenyl, PhNHCH₂CH₂NPh, E=Me]. X-ray crystal structure analyses show that the TADDOL-containing polymers occur as M helices with three monomer units per turn.

Despite a flexible linking unit between the base and backbone, bicyclo[3.2.1]DNA preserves a DNA-like base-pairing behavior system, such as selective Watson–Crick base-pair formation (see picture) with an antiparallel alignment of strands in the duplex.

Amidotriphosphate and diamidophosphate are reagents for the regioselective α-phosphorylation of aldoses. The mechanism involves the carbonyl addition product (see scheme); the hydroxy groups are phosphorylated by an intramolecular phosphate group transfer with formation of a cyclic intermediate, which is then hydrolyzed to the O-phosphorylated product.

With a d⁸configuration equivalent to palladium(II), gold(III) also catalyzes a number of interesting and effective reactions. The cycloisomerization of allenyl ketones with 1 mol % of catalyst requires about one hour with [PdCl₂(MeCN)₂] but only a minute with AuCl₃; a selective cross coupling of these substrates with Michael acceptors (see scheme) is possible only with the gold catalyst.

And they are stable! Rhodium-catalyzed transformations of α-trialkylsilyl-α-diazoacetates with amidic reactants afford chiral α-silyl-substituted amino acid derivatives 1 in good yields. The enantiomers can be separated and are configurationally stable. Deprotection affords free α-trialkylsilyl-α-amino acetic acids. Incorporation of the latter into peptidic structures such as 2 is possible under standard conditions for peptide coupling. PG=protecting group.

A convergent synthetic strategy has been used to accomplish the first total synthesis of the title compound 1, a γ-lactone-containing member of the antineoplastic marine macrolide family. The C1−C16 and C17−C27 fragments were combined by the Julia–Lythgoe olefination and the Yamaguchi macrolactonization. This was followed by the stereoselective introduction of a methoxycarbonylmethylene group, and final hydrolysis of the acetal provided access to the target molecule 1.

Configurationally stable secondary diorganozinc reagents with three adjacent stereocenters can be prepared with high diastereoselectivity by a reaction sequence comprising a substrate-controlled hydroboration and a boron–zinc exchange (see scheme). After a second transmetalation numerous cyclic products are accessible by reaction with various electrophiles.

CdSe quantum dots, in the form of colloidal nanocrystals, are bound to the surface of a gold electrode by bisulfide spacer molecules. The distance dependance of the electron transfer rate between the CdSe and the electrode was investigated by a time-resolved optoelectronic method and related to the length r of the spacer molecule 1–3 [Eq. (1)]. For β, a value of 0.50±0.05 Å⁻¹ was found.

k=k₀ e^−βr (1)

In an oxygen-dependant reaction, tryptophan 7-halogenase (PrnA) regioselectively chlorinates tryptophan (1) at the 7-position to yield 2. During purification of the halogenase, a flavin reductase, as second protein component, was detected in addition to NADH and FAD; this second protein is also required for the reaction.

Hydrogen bonds facilitate the coordination of prochiral 2-quinolones, such as 1, to the chiral host 2. As a result, the intramolecular [2+2]-photocycloaddition reaction which yields cyclobutanes, such as 3, proceeds with high enantioselectivity (up to 93 % ee). The host can be separated from the products and is recovered almost quantitatively.

The bis(butadiene)rhodium(I) complexes [Rh(s-cis-η⁴-C₄H₆)₂(PR₃)]A readily convert into the isomeric octadienediyl derivatives [Rh(η³:η³-C₈H₁₂)(PR₃)]A , which are apparently intermediates in the rhodium-catalyzed cyclotetramerization reaction of butadiene. R=iPr, Cy; A=OSO₂CF₃, B[3,5-(F₃C)₂C₆H₃]₄; the structure of the complex with iPr and OSO₂CF₃ is shown.

In a one-pot synthesis, the phosphorus hetereocycle 1 is formed quantitatively by reaction of an annelated nucleophilic carbene with PCN(iPr)₂ produces the 1,2,3-triphosphetene 2, which contains two double bonds with an N,N,N,P-substitution pattern not previously reported.

A critical dependance upon molecular constraints is found for the aerial, free radical, shape-selective oxidation of cyclohexane (see picture). The oxidation occurs at the Fe^III sites inside the framework of FeAlPO molecular-sieve catalysts. Oxidation of cyclohexane with the catalyst FeAlPO-5 produces predominantly cyclohexanol and cyclohexanone, but with FeAlPO-31 adipic acid is produced.

The aerobic oxyfunctionalization of both (methyl) ends of n-hexane by a MAlPO-18 catalyst requires a significant amount of higher valent framework cobalt ions (M=Co^III) located on the inner walls of each cage of the AlPO-18 structure (Co:P ratio ca. 0.10; see picture). The two tetrahedrally coordinated Co^III framework ions per cage are separated by about 7–8 Å. The Co^III ions function as centers for the regioselective oxidation of both ends of the n-hexane molecule to produce adipic acid.

The new angular bridging ligand 2,4′-(1,4-phenylene)bispyridine toposelectively forms dinuclear metallacycles with Cd(NO₃)₂ and Zn(NO₃)₂. The Cd^II helicates associate into an unprecedented three-dimensional zeolite-like helical coordination polymer with hexagonal hydrophobic channels with a diameter of about 10 Å (see scheme). These channels, initially occupied by uncoordinated ligand molecules, can be emptied to afford a guest-free coordination polymer.

Two symmetry-related hexameric units that are linked to each other form the basis of the structure of a novel dodecanuclear copper cluster (one hexameric unit is shown). The cluster is assembled in a one-pot synthesis from copper(II) chloride, tert-butylphosphonic acid, 3,5-dimethylpyrazole, and triethylamine. The core of the cluster is encased in a lipophilic sheath.

Local order in crystals can be manifested by phenyl/perfluorophenyl interactions. A cooperative network of such attractions can induce a specific stacking order, as observed in complex 1. These structures provide striking examples of a simple principle in crystal engineering and supramolecular synthesis.

Both left-handed and right-handed helical chains of [(VO₂)(HPO₄)]_∞, which are linked by [M(4,4′-bpy)₂] to form three-dimensional framework structures (see picture), are present in the structures of two novel inorganic–organic hybrid materials, [M(4,4′-bpy)₂(VO₂)₂(HPO₄)₄]. These were prepared from hydrothermal systems and characterized by single-crystal X-ray diffraction. M=Co or Ni; bpy=bipyridine.

To be syn or to be anti? This is an “open” matter! A good degree of syn simple diastereoselection (up to 83:17) and high enantioselectivity (ee values up to 90 %) were obtained with the addition of crotyl bromide to aromatic aldehydes promoted by the [Cr(salen)]/Mn/Me₃SiCl catalytic system (see scheme, path B). The remarkable degree of syn simple diastereoselection obtained with organochromium reagents can be explained by an open transition state in a chromium-mediated process.

Designer hairpins: 12-Residue peptide 1, containing D-proline, folds to form a two-stranded antiparallel β-sheet in aqueous solution, which causes clustering of the side chains of Trp 2, Tyr 4, Phe 11, and Val 13 (highlighted in the drawing). A similar cluster occurs in the folded state of the protein GB1. Thermodynamic analysis suggests that both the hydrophobic effect and intrinsic interstrand attractions contribute to β-hairpin stability.

The remarkable phase behavior for binary mixtures of large-core polynuclear aromatic species and small-core discogens cannot be explained simply in terms of net quadrupolar or charge-transfer interactions. The stable π stacking in these mixtures can instead be rationalized by invoking a complimentary polytopic interaction. The picture shows the stacking (in elevation and plan views) between such species.

Unprecedented dirhodium(II) complexes with tBuNC and bridging tBu₂P ligands were obtained by treating [{Rh(μ-PtBu₂)(CNtBu)₂}₂] with chlorinated hydrocarbons. Interestingly, they have coplanar square-planar environments with a Rh−Rh bond, as opposed to the usual face-to-face structures, and a low valence electron count of 30. The picture shows the crystal structure of trans-[{Rh(μ-PtBu₂)(Cl)(CNtBu)}₂].

Conversion of a racemic mixture of a chiral allene into a single enantiomer requires selective inversion of the absolute configuration of one species. This can be accomplished as shown in Equation (1): The enantiopure imido complex 1 reacts more rapidly with one enantiomer of chiral 1,3-disubstituted allenes, and more slowly with the other, so a deficiency of 1 gives an efficient kinetic resolution. For equimolar amounts of 1 and a 1,3-disubstituted allene, both allene enantiomers are transformed into the same metallacycle. The enantioenriched allene, which now has the same configuration as the fast reacting enantiomer, is released in high yield and high ee recovery upon treatment of the metallacycle with 1,2-propadiene.

Hydrogen-atom abstraction by hydroxyl radicals takes place to generate both tert- and sec-adamantyl radicals in Gif-type oxygenation of adamantane by H₂O₂ in pyridine/trifluoroacetic acid when the reaction is mediated by [Fe(O₂CCF₃)₂(py)₄] or [Fe₂O(O₂CCF₃)₄(py)₆], which are formed by dissociation of [Fe₃O(O₂CCF₃)₆(L)₃] in pyridine (L=H₂O, DMSO; see scheme).

Over 30 years since the first tridirectional zeolite: The new ITQ-7 material is catalytically active, high in silica, and has large pore sizes. More significantly, T-ITQ-7 material can be prepared by a new synthetic process which allows atomic substitution of the Si in the framework for T atoms boron, aluminum, or germanium. The Ge-ITQ-7 variant shows a markedly different mean crystallite size. The catalytic activity of these materials is demonstated by cracking n-decane.

The thinnest possible boron polymer strand is found in LiB_x (0.82<x<1.0, see picture) and can be described by the two mesomeric forms =[B⁻=]_nB⁻= or −[B⁻. The compound LiB_x consists of an aligned array of such boron wires embedded in a lithium matrix. Furthermore, each boron atom yields one negative charge and, thus, becomes isoelectronic to carbon.

A precious metal cartwheel is one way to describe the novel heteronuclear compound [Au₆Ag(μ-C₆H₂iPr₃)₆]CF₃SO₃ (see picture, isopropyl groups are omitted) which displays an unusual planar hexacoordination for the silver(I) center. A convenient and simple synthesis of new gold heterometallic clusters is presented.

Two DNA molecules of predetermined lengths are selectively produced from one template DNA. The template-dependent T7 DNA polymerase reaction can be photoregulated with an oligonucleotide bearing an azobenzene as a modulator (see scheme).

Alkynes are the substrates in a dimethyl sulfide catalyzed chalcogeno-Baylis–Hillman reaction involving titanium halides (see scheme). α-Halomethylene aldols or β-halo-α-(hydroxyalkyl)acrylates can be prepared readily and diastereoselectively (E selectivity for the former and Z selectivity for the latter).

Polynucleobase strands, isolated from reactions of copper(II) with diamine-tethered derivatives of guanine and adenine, have been structurally characterized (see picture). These strands exhibit markedly different topologies resulting from a base-specific minor groove site binding. The absence of this interaction for guanine generates a novel cationic polyguaninyl analogue.

A bidentate bridging coordination of molecular diiodine has been found for the first time in [Rh₂(O₂CCF₃)₄I₂]⋅I₂, which was obtained by deposition from the vapor phase. An X-ray structure reveals a zigzag chain polymer (shown in the picture) formed by alternating [Rh₂(O₂CCF₃)₄] and I₂ molecules; another diiodine molecule is clathrated within the channels of the network.

Small but perfectly formed: A three-component copolymer can be transformed into a physically cross-linked gel on heating, which can be moulded into the shape of the vessel. This gel collapses on further heating to give a shrunken, shape-remembered gel (see picture), which is mechanically strong enough to be transported to another vessel. The gel is formed thermoreversibly and the miniaturized version of the original gel in the vessel can be resolubilized simply by cooling and the polymer recast into another form.