Stereochemistry of intramolecular Diels-Alder furan (IMDAF) reactions of furyl-substituted chiral ethanolamides

This paper describes the stereoselective outcome of the intramolecular Diels-Alder furan (IMDAF) reaction of substituted (2S,3S)-ethanolamides 9-13, which were synthesised from a furyl substituted cyanohydrin. The latter was obtained from 2-furaldehyde with high enantioselectivity by an oxynitrilase catalysed addition of hydrogen cyanide. The stereochemistry of the IMDAF products was shown to be dependent on the size of the ethanolamide substituents R. Small substituents (H, Me, CN) gave exclusively exo-cycloaddition, whereas more bulky ones (Ph, Et) gave both evo- and endo-addition, the larger phenyl substituent giving a high endo-exo-ratio.     

Biosynthesis of anthecotuloide,an irregular sesquiterpene lactone from Anthemis cotula L. (Asteraceae) via a non-farnesyl diphosphate route

Retrobiosynthetic analysis of the allergenic sesquiterpene lactone, anthecotuloide, suggested that this natural product could be formed either by head to head condensation of geranyl diphosphate with dimethylallyl diphosphate, or from farnesyl diphosphate (FPP), the accepted regular sesquiterpene precursor via the rearrangement of a germacranolide precursor. Isotopic labelling of anthecotuloide has now been achieved by feeding [1-(13)C]-glucose, [U-13C6]-glucose and [6,6-(2)H2]-glucose to aseptically grown plantlets of Anthemis cotula(family Asteraceae). Analysis of labelling patterns and absolute 13C abundances using quantitative 13C NMR spectroscopy showed that the isoprene building blocks of this sesquiterpene are formed exclusively via the MEP terpene biosynthetic pathway. This was supported by results from an experiment using [U-13C6]-glucose. A deuterium labelling experiment using [6,6-(2)H2]-glucose supported the original proposal and showed that anthecotuloide is formed from a non FPP precursor. Isotope ratio mass spectrometry suggested that there were two pathways for sesquiterpene biosynthesis in A. cotula.     

Alcoholysis of acylpalladium(II) complexes relevant to the alternating copolymerization of ethene and carbon monoxide and the alkoxycarbonylation of alkenes: the importance of Cis-coordinating phosphines

The mechanism and kinetics of the solvolysis of complexes of the type [(L-L)Pd(C(O)CH(3))(S)](+)[CF(3)SO(3)](-) (L-L = diphosphine ligand, S = solvent, CO, or donor atom in the ligand backbone) was studied by NMR and UV-vis spectroscopy with the use of the ligands a-j: SPANphos (a), dtbpf (b), Xantphos (c), dippf (d), DPEphos (e), dtbpx (f), dppf (g), dppp (h), calix-6-diphosphite (j). Acetyl palladium complexes containing trans-coordinating ligands that resist cis coordination (SPANphos, dtbpf) showed no methanolysis. Trans complexes that can undergo isomerization to the cis analogue (Xantphos, dippf, DPEphos) showed methanolyis of the acyl group at a moderate rate. The reaction of [trans-(DPEphos)Pd(C(O)CH(3))](+)[CF(3)SO(3)](-) (2e) with methanol shows a large negative entropy of activation. Cis complexes underwent competing decarbonylation and methanolysis with the exception of 2j, [cis-(calix-diphosphite)Pd(C(O)CH(3))(CD(3)OD)](+)[CF(3)SO(3)](-). The calix-6-diphosphite complex showed a large positive entropy of activation. It is concluded that ester elimination from acylpalladium complexes with alcohols requires cis geometry of the acyl group and coordinating alcohol. The reductive elimination of methyl acetate is described as a migratory elimination or a 1,2-shift of the alkoxy group from palladium to the acyl carbon atom. Cis complexes with bulky ligands such as dtbpx undergo an extremely fast methanolysis. An increasing steric bulk of the ligand favors the formation of methyl propanoate relative to the insertion of ethene leading to formation of oligomers or polymers in the catalytic reaction of ethene, carbon monoxide, and methanol.     

1H detection in MAS solid-state NMR spectroscopy of biomacromolecules employing pulsed field gradients for residual solvent suppression

In this communication, we demonstrate the feasibility of 1H detection in MAS solid-state NMR for a microcrystalline, uniformly 2H,15N-labeled sample of a SH3 domain of chicken alpha-spectrin, using pulsed field gradients for suppression of water magnetization. Today, B0 gradients are employed routinely in solution-state NMR for coherence order selection and solvent suppression. We suggest to use gradients to purge water magnetization which cannot be suppressed using conventional water suppression schemes. The achievable gain in sensitivity for 1H detection is in the order of 5 compared to the 15N detected version of the experiment (at a MAS rotation frequency of 13.5 kHz). We expect that this labeling concept which achieves high sensitivity due to 1H detection, in combination with the possibility to measure long range 1H-1H distances as we have shown previously, to be a useful tool for the determination of protein structures in the solid state.     

Hybrid genetic algorithm–tabu search approach for optimising multilayer optical coatings

Constructing multilayer optical coatings (MOCs) is a difficult large-scale optimisation problem due to the enormous size of the search space. In the present paper, a new approach for designing MOCs is presented using genetic algorithms (GAs) and tabu search (TS). In this approach, it is not necessary to specify how many layers will be present in a design, only a maximum needs to be defined. As it is generally recognised that the existence of specific repeating blocks is beneficial for a design, a specific GA representation of a design is used which promotes the occurrence of repeating blocks. Solutions found by GAs are improved by a new refinement method, based on TS, a global optimisation method which is loosely based on artificial intelligence. The improvements are demonstrated by creating a visible transmitting/infrared reflecting filter with a wide variety of materials.     

Highly oriented self-assembled monolayers as templates for epitaxial calcite growth

The lateral alignment of [012] habit-modified calcite crystals with respect to a carboxylic acid terminated self-assembled monolayer (SAM) of thiols has been determined. The crystals were grown from a Kitano solution (pH 5.6-6.0), and the samples were investigated with scanning electron microscopy, X-ray diffraction, and polarization microscopy. For the first time, a lattice match in one direction, which is the nearest neighbor direction of the SAM and the calcite <100> direction, has been experimentally shown. The experimental results are in good agreement with the theoretical models proposed in previous work, and it is expected that this method can be applied to similar systems where inorganic crystals nucleate with a preferred orientation to a SAM.     

Electron delocalization in cross-conjugated p-phenylenevinylidene oligomers

The synthesis, structure, and electronic properties of a series of cross-conjugated p-phenylenevinylidene oligomers with one to four double bonds are reported. The X-ray crystal structure of the compound with two double bonds reveals a nonplanar conformation with torsion angles about the C(phenylene)-C(vinylidene) and C(phenyl)-C(vinylidene) formal single bonds of 39.5(2) degrees and 30.5(2) degrees, respectively. Admixture of quinoid character in the ground state is observed. Infrared and Raman spectroscopy do not provide a clear picture of the degree of electron delocalization in the series, since the C=C stretching mode does not adequately reflect the C=C bond order and has a local nature. In contrast, electronic spectra and electrochemical data, as well as AM1 and PPP/SCF calculations, reveal that the cross-conjugated compounds basically behave as linearly pi-conjugated systems in the sense that molecular orbitals are delocalized over the entire structure and systematically change in energy. The electronic interaction between the repeating units is, however, not very strong, which has the consequence that spatial extension of the molecular orbitals does not lead to a red shift of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) electronic transition. This is related to the feature that the modest narrowing of the HOMO-LUMO gap with the chain length is accompanied by a relatively large reduction of electron repulsion. This finding implies that care should be taken in the use of electronic spectra for the evaluation of conjugation phenomena.     

Palladium-catalyzed synthesis of novel optically active tryptophan analogues

[reaction: see text] Both unsaturated proline derivatives and optically active tryptophan analogues have been obtained via Pd-catalyzed cyclization of aniline-containing acetylenic amino acids. The side chain length of the cyclization precursor determines which one of the two possible products will be formed.     

Ph2SO/Tf2O: a powerful promotor system in chemoselective glycosylations using thioglycosides

Diphenylsulfoxide in combination with triflic anhydride provides a very potent thiophilic glycosylation promotor system, capable of activating disarmed thioglycosides. The usefulness of this novel thiophilic activator is illustrated in a successful chemoselective glycosylation sequence in which the donor thioglycoside in the first condensation step may be either armed or disarmed. [reaction: see text]