Enantioselective synthesis of bicyclo[3.1.0]hexane carbocyclic nucleosides via a lipase-catalyzed asymmetric acetylation. Characterization of an unusual acetal byproduct

The bicyclo[3.1.0]hexane scaffold can lock the conformation of a carbocyclic nucleoside into one of the two antipodal (north or south) conformations typical of conventional nucleosides that normally exist in a rapid, two-state equilibrium in solution. In a recent brief communication, we reported a practical method to access the requisite bicyclo[3.1.0]hexane pseudosugar for the north antipode via an intramolecular olefin-ketocarbene cycloaddition. The most attractive features of this synthesis was that a relatively complex synthon was obtained from simple and inexpensive starting materials and that the resulting racemic mixtures of purine nucleosides could be successfully resolved by adenosine deaminase (ADA) hydrolysis. In this work, we describe the development of a more general, lipase-catalyzed double-acetylation reaction, which could successfully resolve an earlier precursor, 4-(tert-butyldiphenylsilamethoxy)-1-(hydroxymethyl)bicyclo[3.1.0]hexan-2-ol [(+/-)-7], into enantiomerically pure (+)-diacetate 8 and (-)-monoacetate 9. The former diacetate was converted to the conformationally locked (north)-carbocyclic guanosine (+)-17 identical to the one obtained previously by ADA resolution. The present method represents a more general and efficient process applicable to the synthesis of all classes of (north) bicyclo[3.1.0]hexane nucleosides, including pyrimidine analogues. During the lipase-catalyzed resolution, we were able to demonstrate the presence of an unusual acetal-forming reaction that consumed small amounts of the unreactive monoacetate (-)-9. This side reaction was also enzyme-catalyzed and was triggered by the byproduct acetaldehyde generated during the reaction.     

Synthesis and self-assembly of nearly monodisperse nanoparticles of a naturally occurring polymer

Nearly monodisperse nanoparticles have been synthesized based on a naturally occurring polymer of hydropropylcellulose (HPC) using precipitation polymerization method. It is found that when the polydispersity index value of the HPC nanoparticles is less than 1.10, the HPC particles can self-assemble into an ordered structure, displaying bright colors. UV-visible spectroscopy reveals that the color shifts to a lower wavelength as the interparticle distance decreases following the Bragg diffraction equation. The HPC nanoparticle assembly in water has been further stabilized by covalently bonding neighboring particles to form a three-dimensional network. This network contains a large amount of water similar to a conventional bulk gel but displays colors as a result of its ordered structure.     

Cover Picture

The cover picture shows in the center an electron-density map of the molecular container compound cucurbit[7]uril. The polarizability inside the cavity of this host can be explored through solvatochromic effects on the absorption spectra (bottom right) when probed by the inclusion of an azo chromophore guest (top left). This novel method allows insight into the inner phase of supermolecules. More details about this process is described by Marquiz and Nau on pp. 4387 ff.     

Rapid direct nanowriting of conductive polymer via electrochemical oxidative nanolithography

Conductive polymer nanolines having widths as small as 45 nm were obtained on glass using a novel scanning probe lithographic (SPL) technique at writing speeds of >5 mum/s. Herein we demonstrate that our nanowriting is >1500 times faster than current SPL nanoscale writing of conductive polymers. The lack of a specific restriction on the choice of substrates and the ability to write within a polymer matrix to provide a continuous film that is either 2-D or 2.5-D provide tremendous potential for our SPL technique in nanotechnology and plastic electronics applications.     

Stereocontrolled total synthesis of (-)-callipeltoside A

[structure: see text] A highly stereocontrolled total synthesis of the cytotoxic macrolide (-)-callipeltoside A has been achieved in 23 steps (4.8% overall). Notable features include a novel asymmetric vinylogous aldol reaction to install the C13 stereocenter and (E)-trisubstituted alkene, an anti-selective aldol addition, a Sonogashira coupling, and, last, a Schmidt-type glycosylation to attach the sugar unit.     

A DFT theoretical analysis of aldehyde condensation pathways onto methyllithium, lithium dimethylamide, and their aggregates

A DFT analysis of the condensation of monomeric methyllithium and lithium dimethylamide (LMA), as well as their homo and hetero dimers, on formaldehyde and acetaldehyde is reported. A stable complex, exhibiting a directional interaction between a lone pair of the oxygen on the aldehyde and a lithium, is first found. At this stage, the aldehyde carbonyl and the Li-X (X = C or N) bonds lie in the same plane. To proceed, the condensation reaction has to go through a transition state that mainly consists of a rotation of the aldehyde plane, placing it perpendicular to the C-C or C-N forming bond. The reaction then leads, in a strongly exothermic final step, to the addition product that is a lithium alcoholate or alpha-amino alcoholate, associating into an hetero-aggregate with the remaining moiety of the initial dimer. From the relative heights of the activation barriers, it appears that, for the heterodimer MeLi-LMA, the formation of the C-N bond should be kinetically favored over the C-C one, while the lithium ethylate resulting from the C-C binding is the thermodynamic product. A decomposition of the activation energy barriers has been carried out in order to determine the physicochemical forces responsible for the variation of the condensation activation barriers with the structure of the final species formed. The results obtained are discussed in relation with corresponding experimental data.     

Simultaneous determination of amino acids and neurotransmitters in plasma samples from schizophrenic patients by hydrophilic interaction liquid chromatography with tandem mass spectrometry

A sensitive, reproducible, and rapid method was developed for the simultaneous determination of underivatized amino acids (aspartate, serine, glycine, alanine, methionine, leucine, tyrosine, and tryptophan) and neurotransmitters (glutamate and γ‐aminobutyric acid) in plasma samples using hydrophilic interaction liquid chromatography coupled to triple quadrupole tandem mass spectrometry. The plasma concentrations of amino acids and neurotransmitters obtained from 35 schizophrenic patients in treatment with clozapine (27 patients) and olanzapine (eight patients) were compared with those obtained from 38 healthy volunteers to monitor the effectiveness of treatment. The chromatographic conditions separated ten target compounds within 3 min. This method presented linear ranges that varied from (lower limit of quantification: 9.7–13.3 nmol/mL) to (upper limit of quantification: 19.4–800 nmol/mL), intra‐ and interassay precision with coefficients of variation lower than 10%, and relative standard error values of the accuracy ranged from –2.1 to 9.9%. The proposed method appropriately determines amino acids and neurotransmitters in plasma from schizophrenic patients. Compared with the control group (healthy volunteers), the plasma levels of methionine in schizophrenic patients treated with olanzapine are statistically significantly higher. Moreover, schizophrenic patients treated with clozapine tend to have increased plasma levels of glutamate.     

Exploiting Atropisomerism to Increase the Target Selectivity of Kinase Inhibitors

Many biologically active molecules exist as rapidly interconverting atropisomeric mixtures. Whereas one atropisomer inhibits the desired target, the other can lead to off‐target effects. Herein, we study atropisomerism as a possibility to improve the selectivities of kinase inhibitors through the synthesis of conformationally stable pyrrolopyrimidines. Each atropisomer was isolated by HPLC on a chiral stationary phase and subjected to inhibitor profiling across a panel of 18 tyrosine kinases. Notably different selectivity patterns between atropisomers were observed, as well as improved selectivity compared to a rapidly interconverting parent molecule. Computational docking studies then provided insights into the structure‐based origins of these effects. This study is one of the first examples of the intentional preorganization of a promiscuous scaffold along an atropisomeric axis to increase target selectivity, and provides fundamental insights that may be applied to other atropisomeric target scaffolds.