An efficient asymmetric synthesis of Fmoc-l-cyclopentylglycine via diastereoselective alkylation of glycine enolate equivalent

Stereoselective alkylation of the enolate derived from benzyl (2R,3S)-(−)-6-oxo-2,3-diphenyl-4-morpholinecarboxylate (1) with cyclopentyl iodide afforded anti-α-monosubstituted product, benzyl (2R,3S,5S)-(−)-6-oxo-2,3-diphenyl-5-cyclopentyl-4-morpholinecarboxylate (3) in 60% yield. Catalytic hydrogenolysis over PdCl₂ cleaved the auxiliary ring system to give l-cyclopentylglycine (4) in 84% yield. Subsequent protection of the α-amino function with Fmoc-OSu gave Fmoc-l-cyclopentylglycine (5) in high yield.     

Cu(I)/bis(azaferrocene)-catalyzed enantioselective synthesis of beta-lactams via couplings of alkynes with nitrones

As a consequence of the wide-ranging significance of beta-lactams (e.g., use as drugs and as chiral building blocks), a great deal of effort has been dedicated to the development of methods for their stereoselective synthesis. Although considerable progress has been achieved, nearly all of the approaches that have been described are based on the use of chiral precursors; direct catalytic enantioselective routes to beta-lactams are rare as well as limited in scope. In this communication, we establish that, using a new C2-symmetric planar-chiral bis(azaferrocene) ligand, we can generate beta-lactams with very good enantiomeric excess and cis diastereoselection via catalytic enantioselective Kinugasa reactions (couplings of alkynes with nitrones). Appealing attributes of this process include the ready availability of the starting materials, the functional-group tolerance of the reaction, and the convergency of the approach.     

Cationic fullerenes are effective and selective antimicrobial photosensitizers

Fullerenes are soccer ball-shaped molecules composed of carbon atoms, and, when derivatized with functional groups, they become soluble and can act as photosensitizers. Antimicrobial photodynamic therapy combines a nontoxic photosensitizer with harmless visible light to generate reactive oxygen species that kill microbial cells. We have compared the antimicrobial activity of six functionalized C(60) compounds with one, two, or three hydrophilic or cationic groups in combination with white light against gram-positive bacteria, gram-negative bacteria, and fungi. After a 10 min incubation, the bis- and tris-cationic fullerenes were highly active in killing all tested microbes (4-6 logs) under conditions in which mammalian cells were comparatively unharmed. These compounds performed significantly better than a widely used antimicrobial photosensitizer, toluidine blue O. The high selectivity and efficacy exhibited by these photosensitizers encourage further testing for antimicrobial applications.     

G-quadruplex formation of thrombin-binding aptamer detected by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry detected the formation of the G-quadruplex structure of the thrombin-binding aptamer, d(GGTTGGTGTGGTTGG), and established its specific interaction with metal ions. One piece of evidence that the bonding in the gas phase is via the G-quadruplex form is the enhanced binding, with respect to other metal ions, of the aptamer with Sr2+, Pb2+, Ba2+, and K+, which are of similar size. Another is the lack of specific binding with controls in which the G's are replaced with A's. The most convincing evidence is the extent of H/D exchange of the gas-phase aptamer as compared to that bound to K+ and Sr2+. The latter two complexes exchange six and nine fewer H's, indicating a significant increase in protection upon binding to the metals. Mass spectrometry will be an important tool in understanding G-quadruplexes, which are particularly important in DNA telomers.     

The Donor-Acceptor-Acceptor Purine Analog: Transformation of 5-aza-7-deaza-1H-isoguanine (=4-aminoimidazo-[1,2-a]-1,3,5-triazin-2(1H)-one) to 2′-deoxy-5-aza-7-deaza-isoguanosine using purine nucleoside phosphorylase

A new synthesis is reported for 4-aminoimidazo[1,2-a]-1,3,5-triazin-2(1H)-one ( =5-aza-7-deaza-isoguanosine; 8 ), a purine analog that, when incorporated into an oligonucleotide chain, presents a H-bond donor-acceptor-acceptor pattern to a complementary pyrimidine analog. A protected ribose derivative was coupled to 8 to yield 4-amino-8-(β-D -ribofuranosyl)imidazo[1,2-a]-1,3,5-triazin-2(8H)-one ( =5-aza-7-deaza-isoguanosine; 11 ) after deprotection, Alternatively, direct synthesis of both the ribo derivative 11 and the corresponding deoxyribo derivative 17 as the β-D -anomers was achieved using the enzyme purine nucleoside phosphorylase in a one-pot reaction. This adapts a known synthetic approach to yield a new strategy for obtaining diastereoisomerically pure deoxyribonucleoside analogs on 1-gram scales.     

A molecular tweezer for lysine and arginine

Lysine and arginine play a key role in numerous biological recognition processes controlling, inter alia, gene regulation, glycoprotein targeting and vesicle transport. They are also found in signaling peptide sequences responsible, e.g. for bacterial cell wall biosynthesis, Alzheimer peptide aggregation and skin regeneration. Almost none of all artificial receptor structures reported to date are selective and efficient for lysine residues in peptides or proteins. An artificial molecular tweezer is introduced which displays an exceptionally high affinity for lysine (K(a) approximately 5000 in neutral phosphate buffer). It features an electron-rich torus-shaped cavity adorned with two peripheral anionic phosphonate groups. Exquisite selectivity for arginine and lysine is achieved by threading the whole amino acid side chain through the cavity and subsequent locking by formation of a phosphonate-ammonium/guanidinium salt bridge. This pseudorotaxane-like geometry is also formed in small basic signaling peptides, which can be bound with unprecedented affinity in buffered aqueous solution. NMR titrations, NOESY and VT experiments as well as ITC measurements and Monte Carlo simulations unanimously point to an enthalpy-driven process utilizing a combination of van der Waals interactions and substantial electrostatic contributions for a conformational lock. Since DMSO and acetonitrile compete with the amino acid guest inside the cavity, a simple change in the cosolvent composition renders the whole complexation process reversible.     

Phosphine-mediated reductive condensation of gamma-acyloxy butynoates: a diversity oriented strategy for the construction of substituted furans

Exposure of gamma-acyloxy butynoates to stoichiometric quantities of triphenylphosphine results in reductive condensation to afford substituted furans, by way of allenic ester intermediates. As gamma-acyloxy butynoates are readily obtained through condensation of ethyl propiolate with aldehydes followed by acylation, this method represents a powerful and mechanistically novel protocol for the convergent three-component construction of substituted furans.     

Synthesis of 1,3,4-oxadiazol-2-ones with aliphatic groups at N-3

A series of 3-substituted-5-methoxy-1,3,4-oxadiazol-2-ones were prepared from aldehydes, ketones, phenylacetic acids, and 1,2- and 1,3-diketones. Conditions for the formation of these oxadiazolones from the precursor N-carbamoyl chlorides depended on the structure, and varied from spontaneous ring closure to those requiring bases. Variation in the N-3 substituents sometimes produced mixtures of isomers which were separated and identified. These molecules were prepared in order to study the effect of the N-3 substituent variation on the biological properties of oxadiazolones.     

Analysis and interpretation of excess molar properties of amphiphile + water systems : Part 3. Excess molar volumes of isopropoxyethanol + water and isobutoxyethanol + water

Excess molar volumes at 25°C, are reported for the systems isoPrOEtOH + water and isoBuOEtOH + water across their entire composition ranges. These data have been compared to those for their normal alkyl analogs in order to assess the effects of both - and β-chain branching.

The results are discussed in terms of the relative abilities of the amphiphiles to form structured aggregates.