Synthesis of guanidinium-derived receptor libraries and screening for selective peptide receptors in water

A library of "tweezer" receptors, incorporating a guanidinium "head group" and two peptide derived side arms has been prepared on the solid-phase using an orthogonally protected guanidinium scaffold 12. The library was screened with various tripeptide derivatives in an aqueous solvent system. A tweezer receptor 25 for the side chain protected tripeptide 19 was identified from the screening experiments. Receptor 25 was resynthesised and solution binding studies were carried out, which revealed that 25 binds to tripeptide 19 with K(a)=8.2 x 10(4) +/- 2.5 x 10(4) (15 % DMSO/H(2)O, pH 8.75) and with appreciable selectivity over the tripeptide enantiomer 22 and the side chain deprotected tripeptide 20.     

Structural investigations of metalcarbonyl complexes with acyclic diamino carbenes

Here, we report the synthesis of acyclic and cyclic aminocarbene substituted carbonyl complexes of group 6 metals. The reaction of bis(diisopropyl)aminocarbene with photolytically produced (ether)M(CO)₅ leads to unstable η¹-carbene complexes, which convert to η²-carbene complexes; crystallographic data are presented for the tungsten complex, where one amino group of the carbene coordinates to the metal. The Mo and W η²-complexes can be converted to an imine-complex under elimination of propene; characterized by chemical, spectroscopic and crystallographic data. DFT-calculations of a new bis(dialkyl)aminocarbene and the corresponding methylidene salt were performed and compared with the corresponding crystallographic data of the latter one. In a side-reaction during the reaction of (diethyl ether)Cr(CO)₅ with a free six-membered cyclic carbene a three-nuclear dianionic metal carbonyl nido complex (C₁₄H₁₀Cr₃O₁₂)²⁻ is formed, which could be characterized by spectroscopic and crystallographic data.     

Practical asymmetric synthesis of a selective endothelin A receptor (ETA) antagonist

[structure: see text]. A practical, chromotography-free asymmetric synthesis was developed for the large scale preparation of an endothelin receptor antagonist 2. This synthesis includes a new efficient process for the preparation of 6-bromo-2,3-dihydrobenzofuran, a stereoselective conjugate addition of an aryllithium followed by stereospecific addition of the Grignard reagent of the top aryl bromide, and an aminophosphate-mediated sterospecific intramolecular enolate alkylation, which led to the formation of the five-membered ring bearing three contiguous asymmetric centers.     

Preparation and Transmetallation of a Triphenylstannyl β-D-Glucopyranoside: A highly stereoselective route to β-D-C-glycosides via glycosyl dianions

The triphenylstannyl β-D -glucopyranoside 4 was synthesized in one step from the 1,2-anhydro-α-D -glucopyranose 3 with (triphenylstannyl)lithium (Scheme 1). Transmetallation of 4 with excess BuLi, followed by quenching the dianion 7 with CD₃OD gave (1S)-1,5-anhydro-3,4,6-tri-O-benzyl-[1-²H]-D - glucitol ( 8 ) in 81% yield (Scheme 2). Trapping of 7 with benzaldehyde, isobutyraldehyde, or acroleine gave the expected β-D -configurated products 11, 12 , and 13 in good yields. Preparation of C-acyl glycosides from acid chlorides, such as acetyl or benzoyl chloride was not practicable, but addition of benzonitrile to 7 yielded 84% of the benzoylated product 14 . Treatment of 7 with MeI led to 15 (30%) along with 40% of 18 , C-alkylation being accompanied by halogen-metal exchange. Prior addition of lithium 2-thienylcyanocuprate increased the yield of 15 to 50% and using dimethyl sulfate instead of MeI led to 77% of 15 . No α-D -anomers could be detected, except with allyl bromide as the electrophile, which yielded in a 1:1 mixture of the anomers 16 and 17 .     

Suicide Inactivation of Dioldehydrase by 2‐Chloroacetaldehyde: Formation of the ‘cis‐Ethanesemidione’ Radical,and the Role of a Monovalent Cation

Dioldehydrase is an adenosylcobalamin‐dependent enzyme that catalyzes the dehydration of (R)‐ or (S)‐propane‐1,2‐diol to propanal. The reaction proceeds by a radical mechanism initiated by the homolytic scission of the covalent Co? C(5′) bond in the coenzyme to form cob(II)alamin and the 5‐deoxyadenosyl radical as transient intermediates. Dioldehydrase is subject to ‘suicide inactivation’ by substrate/product analogs. Inactivation by 2‐chloroacetaldehyde converts the inactivator into the ‘cis‐ethanesemidione’ radical. A mechanism for this process includes reaction of chloroacetaldehyde in the reverse of the normal catalytic process to a rearranged radical that eliminates HCl. K⁺ and other monovalent cations of similar size, including Tl⁺, are required for dioldehydrase activity and for suicide inactivation by glycolaldehyde or 2‐chloroacetaldehyde. A K⁺ ion is bound to propane‐1,2‐diol in dioldehydrase. Both EPR and pulsed‐EPR experiments show that the magnetic nuclei of thallous ions (²⁰³Tl⁺, ²⁰⁵Tl⁺) do not interact with the unpaired electron in the cis‐ethanesemidione radical at the active site of dioldehydrase. Pulsed‐EPR experiments implicate a ¹⁴NH group, possibly of His¹⁴³, interacting with the radical at the active site.     

Rheological studies of the interactions in cellulose/ethylene diamine/salt systems

Degree of polymerization (DP) of cellulose was measured to confirm that aging time and salt concentration did not cause cellulose degradation. Dynamic rheological studies of cellulose solutions were carried out to probe the evolving interactions between cellulose and ethylene diamine (EDA)/salt solvent system. Potassium thiocyanate (KSCN) was used as the salt in these studies. Steady shear studies indicated that all solutions exhibited shear‐thinning behavior. The empirical Cox‐Merz rule did not hold true for the cellulose system with weak gel microstructure. The shear viscosities at the shear rates explored decreased with aging time. The zero‐shear viscosity, however, increased with increasing salt concentration. Oscillatory shear studies were investigated and the time temperature superposition (TTS) method was used to extend the experimental frequency range of the instrument. The results showed that the average relaxation time of the cellulose system decreased as the sample aged and increased with increasing salt concentration, indicative of dynamic interactions between cellulose and the solvent system in solution. The conformations of cellulose chains were constantly changing over time. The system gelled when the salt concentration was increased to a critical point. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2326–2334, 2008     

Inelastic neutron scattering study of the dynamics of the AlNiCo decagonal phase

The dynamics of the decagonal AlNiCo phase has been investigated on a single-grain quasicrystalline sample using inelastic neutron scattering. The decagonal structure can be viewed as a periodic stacking of quasiperiodic planes. The anisotropy between the modes propagating in the periodic and quasiperiodic directions is found to be much weaker than theoretically predicted. A strong resonance splitting is observed at an energy transfer of 15 meV for transverse modes polarized in the quasiperiodic plane. Received: 18 November 1998 / Accepted: 27 November 1998