Analogues of Sialic Acids as Potential Sialidase Inhibitors. Synthesis of C6 and C7 Analogues of N-Acetyl-6-amino-2,6-dideoxyneuraminic Acid

The piperidines 12 – 18 , piperidmose analogues of Neu5Ac ( 1 ) with a shortened side chain, were synthesized from N-acetyl-D -glucosamine via the azidoalkene 32 and tested as inhibitors of Vibrio cholerae sialidase. Deoxygenation at C(4) of the uronate 22 , obtained from the known D -GlcNAc derivative 20 , was effected by β-elimination (→ 23 ), exchange of the AcO at C(3) with a (t-Bu)Me₂SiO group and hydrogenation (→ 26 ; Scheme 1). Chain extension of 26 by reaction with Me₃SiCH₂MgCl gave the D -ido-dihydroxysilane 28 , which was transformed into the unsaturated L -xylo-mesylate 29 and further into the L -lyxo-alcohol 30 , the mesylate 31 , and the L -xylo-azide 32 . The derivatives 29 – 31 prefer a sickle zig-zag and 32 mainly an extended zig-zag conformation (Fig. 2). The piperidinecarboxylate 15 was obtained from 32 by ozonolysis (→ 33 ), intramolecular reductive animation (→ 34 ), and deprotection, while reductive animation of 34 with glycolaldehyde (→ 35 ) and deprotection gave 16 (Scheme 2). An intramolecular azide-olefin cycloaddition of 32 yielded exclusively the fused dihydrotriazole 36 , while the lactone 39 did not cyclize (Scheme 3). Treatment of 36 with AcOH (→ 37 ) followed by hydrolysis (→ 38 ) and deprotection led to the amino acid 18 . To prepare the (hydroxymethyl)piperidinecarboxylates 12 and 17 , 32 was first dihydroxylated (Scheme 4). The L -gluco-diol 40 was obtained as the major product, in agreement with Kishi's rule. Silylation of 40 (→ 42 ), oxidation with periodinane (→ 44 ), and reductive animation gave the L -gluco-piperidine 45 . It was, on the one hand, deprotected to the amino acid 12 and, on the other hand, N-phenylated (→ 46 ) and deprotected to 17 . While 45 and 12 adopt a ²C₅ conformation, the analogous N-Ph derivatives 46 and 17 adopt a ₅C² and a B_3,6 conformation, respectively, on account of the allylic 1,3-strain. The conformational effects of this 1,3-strain are also evident in the carbamate 47 , obtained from 45 (Scheme 5), and in the C(2)-epimerized bicyclic ether 48 , which was formed upon treatment of 47 with (diethylamino)sulfur trifluoride (DAST). Fluorination of 40 with DAST (→ 49 ) followed by treatment with AcOH led to the D -ido-fluorohydrin 50 . Oxidation of 50 (→ 51 ) followed by a Staudinger reaction and reduction with NaBH₃CN afforded the (fluoromethyl)piperidine 52 , while reductive amination of 51 with H₂/Pd led to the methylpiperidine 55 , which was similarly obtained from the keto tosylate 54 and from the dihydrotriazole 36 . Deprotection of 52 and 55 gave the amino acids 13 and 14 , respectively. The aniline 17 does not inhibit V. cholerae sialidase; the piperidines 12 – 16 and 18 are weak inhibitors, evidencing the importance of an intact 1,2,3-trihydroxypropyl side chain.     

Synthesis,X-ray crystal structure,and redox and electronic properties of iron(III)-polyimidazole complexes relevant to the metal sites of iron proteins

A new tripod N(3) ligand (L), containing three imidazole rings, was synthesized in good yield. At variance with usual aromatic ligands with N(2) or N(3) donor sets such as pyridine or pyrazole derivatives, L stabilizes the Fe(III) oxidation state. The corresponding iron(III) complexes [Fe(L)Cl(3)] (1) and [Fe(L)(2)](ClO(4))(3) (2) were prepared and characterized by X-ray structural analysis and spectroscopic methods. The coordination environment around all the Fe(III) centers has a distorted octahedral geometry. [Fe(L)Cl(3)] (1) belongs to the monoclinic system, space group P2(1)/n, a = 9.7406(5) A, b = 17.207(2) A, c = 14.615(2) A, beta = 104.448(9)(o) Z = 4, V = 2372.1(4) A(3); R = 0.044, R(w) = 0.055. [Fe(L)(2)](ClO(4))(3) (2) belongs to the monoclinic system, space group P2(1)/c, a = 16.1057(15) A, b = 11.1079(12) A, c = 26.283(2) A, beta = 102.062(10)(o), Z = 4, V = 4598.2(8) A(3); R = 0.0465, R(w) = 0.0902. The Fe-N((i)PrIm) bond lengths are systematically longer than the Fe-N(MeIm) ones. Compound 2 is a highly anisotropic low-spin Fe(III) complex displaying a rather unusual EPR spectrum with a sharp signal at g = 3.5 and a broad one at g approximately 1.6. The fitting of this EPR spectrum is discussed.     

Zum Kristallisations- und Schmelzverhalten von Auripigment und den strukturanalogen Verbindungen Claudetit und Arsenselenid

The rates of homogeneous nucleation of orpiment (As₂S₃), claudetite (As₂O₃) and arsenic selenide (As₂Se₃) in their undercooled melts are much lower than the calculated values. From the slow nucleation rates results the extremely low tendency towards crystallization of the pure undercooled melts. By seeding crystals or under conditions favouring a heterogeneous nucleation, crystalline As₂S₃ and As₂Se₃ were obtained from the melt. Orpiment shows a retarded melting behaviour and can be superheated beyond its melting point of 591 K.

     

Total synthesis of both enantiomers of the macrocyclic lactone citreofuran

Both enantiomers of the polyketide-derived lactone citreofuran (4) have been prepared. The enantiopure building blocks were obtained on a chemoenzymatic route ((S)-6) and by a chiral pool synthesis ((R)-6). The crucial step in the synthesis, a macrolactonization, was accomplished under Mitsunobu conditions.     

Synthesis, photophysical, photochemical and biological properties of caged GABA, 4-[[(2H-1-benzopyran-2-one-7-amino-4-methoxy) carbonyl] amino] butanoic acid

The photorelease of a caged neurotransmitter can be used to investigate the function of neuronal circuits in tissues. We have designed and synthesized a stable, caged gamma-aminobutyric acid (GABA) derivative, 4-[[(2H-1-benzopyran-2-one-7-amino-4-methoxy)carbonyl]amino] butanoic acid (BC204), that releases the neurotransmitter in physiological medium when irradiated with UV light at 300-400 nm in PBS at pH 7.4. The release of GABA occurs with the formation of the major photoproduct, 7-amino-4-(hydroxymethyl)-2H-1-benzopyran-2-one, via a solvolytic photodegradation mechanism of the coumarin moiety and was confirmed by electrospray mass spectrometry/mass spectrometry (ESI MS/MS). BC204 is chemically stable and shows no intrinsic activity after many hours under physiological dark conditions. These properties suggest that BC204 is an excellent form of caged GABA that is well suited for long-term biological studies.     

Chemical algebra. VI:G-weighted metrics of non-compact groups: Group of translations in the Euclidean space

The generalized definition equation of a G-weighted metricds ² from the datum of any groupG acting onto a vector space mapped by a continuous numerical function is applied whenE =R ⁿ andG = the group of translations inR ⁿ . Here,G does not act linearly inR ⁿ andR ⁿ is considered as an affine space. The solution readsds ² = –d ²(InI)/(Bp),I = (4ix ⁰) ^n/2 ·, wherex ⁰ = –i/(2p), is a solution of the Schrödinger-type equation +i/x ⁰ = 0, andB is a uniform term depending onx ⁰. Whenn = 3,p is interpreted as the reciprocal of a time variable. Attempts to identifyds ² with the spatial part of a space-time metric of general relativity failed except for the flat Robertson and Walker spaces. In the simplest case,B = 1/R ²(t) and . A uniform but non-constant imaginary potential energy of the space can be formally derived: V(x ⁰) = 3i/(2x ⁰). Despite a striking formal link with tools of physical mathematics, no physical validation of the propositions of chemical algebra is claimed.     

Hydrophobically modified polyelectrolytes used as reducing and stabilizing agent for the formation of gold nanoparticles

This paper is focused on the synthesis and characterization of hydrophobically modified polyelectrolytes and their use as reducing as well as stabilizing agents for the formation of gold nanoparticles. Commercially available poly(acrylic acid) has been hydrophobically modified with various degrees of grafting of butylamine introduced randomly along the chain. Different analytical methods are performed, i.e., IR and ¹H-NMR spectroscopy in combination with elemental analysis to determine the degree of grafting. The modified polymers can successfully be used for the controlled single-step synthesis and stabilization of gold nanoparticles. The process of nanoparticle formation is investigated by means of UV-vis spectroscopy. The size and shape of the particles obtained in the presence of unmodified or modified polyelectrolytes are characterized by dynamic light scattering, zeta potential measurements and transmission electron microscopy. The polyelectrolytes were involved in the crystallization process of the nanoparticles, and in the presence of hydrophobic microdomains at the particle surface, a better stabilization at higher temperature can be observed.     

Continuous production of inorganic magnetic nanocomposites for biomedical applications by laser pyrolysis

Magnetic composites of Fe-based nanoparticles encapsulated in carbon/silica (C/SiO₂@Fe) or carbon (C@Fe) matrices were prepared by laser-induced pyrolysis of aerosols. The powders were dispersed in aqueous solutions at pH 7 resulting in biocompatible colloidal dispersions with a high resistance to biodegradation. Structural and magnetic properties and the suitability of aqueous dispersions as contrast agent for MRI were analyzed. The results of these characterizations and the NMR relaxivity data are very encouraging for application of laser pyrolysis products in the field of living tissues.     

End‐functionalized polystyrene by ATRP: A facile approach to primary amino and carboxylic acid terminal groups

Monoamino‐terminated and monocarboxylic acid‐terminated polystyrenes containing active halogenated end groups were prepared by atom transfer radical polymerization (ATRP) using the so‐called initiator method and protective group chemistry. α‐Chloropropionates were synthesized and utilized as initiators containing the tert‐butoxycarbonyl (t‐BOC)‐protected amino and the tert‐butyl (t‐Bu)‐protected carboxylic acid function, respectively. Optimum polymerization conditions were attained using CuCl/N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine (PMDETA) as catalyst and 10 vol % n‐butanol as homogenizing agent at 110 °C. However, targeting larger quantities an alternative route was established employing 50 vol % N,N‐dimethylformamide (DMF). Subsequent hydrolysis of the ω‐tert‐butoxycarbonyl polystyrenes afforded well‐defined polymers with quantitative deprotection of the functional groups. Comparatively, thermolytic cleavage of the protective sites was studied. ¹H NMR verified the quantitative removal of the t‐BOC‐protecting groups. Furthermore, the resulting α‐amino‐ω‐chloro polystyrenes were reacted with Sanger reagent to confirm the existence of the thereby converted primary amino groups. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3845–3859, 2009