Rapid,Traceless, AgI‐Promoted Macrocyclization of Peptides Possessing an N‐Terminal Thioamide

Peptide macrocyclization is often a slow process, plagued by epimerization and cyclodimerization. Herein, we describe a new method for peptide macrocyclization employing the Ag^I‐promoted transformation of peptide thioamides. The Ag^I has a dual function: chemoselectively activating the thioamide and tethering the N‐terminal thioamide to the C‐terminal carboxylate. Extrusion of Ag₂S generates an isoimide intermediate, which undergoes acyl transfer to generate the native cyclic peptide, resulting in a rapid, traceless macrocylization process. Cyclic peptides are furnished in high yields within 1 hour, free of epimerization and cyclodimerization.     

Samarium(II)-mediated reactions of gamma,delta-unsaturated ketones. Cyclization and fragmentation processes

[reaction: see text] On treatment with samarium(II) iodide, gamma,delta-unsaturated ketones undergo very different processes depending upon the nature of the reaction conditions. Employing samarium(II) iodide and MeOH, functionalized syn-cyclopentanol products are obtained stereoselectively. Mechanistic studies suggest that this cyclization occurs via a sequential reduction/intramolecular aldol reaction. With samarium(II) iodide and HMPA, products of a 4-exo-trig cyclization and of an interesting fragmentation reaction are observed.     

Intercombination lines of Mg I-, Al I- and Si I-like ions in the beam foil spectra of Ti,Fe, Ni and Cu

In delayed spectra of foil-excited beams of Ti, Fe, Ni and Cu ions lines have been observed which are identified with intercombination transitions 3s ^{2 1} S ₀ — 3s 3p ³ P ₁ ⁰ , 3s ² 3p ² P ⁰ — 3s 3p ^{2 4} P and 3s ² 3p ^{2 3} P — 3s 3p ^{3 5} S ₂ ⁰ in magnesiumlike, aluminiumlike and siliconlike spectra, resp. Wavelengths and decay properties have been determined. The results are compared to recent theoretical predictions.     

Factors affecting the efficiency and stereoselectivity of α-amino acid synthesis by the Petasis reaction

The use of chiral secondary amines containing only one branched substituent has been shown to give optimal yields and stereoselectivities in the preparation of α-amino acids using the Petasis reaction. While the use of chiral primary amines generally gives products in low to moderate diastereoselectivity, chiral secondary amines generally give products in >95:5 diastereoselectivity. Additionally, the use of amines with two chiral (and by definition, branched) N-alkyl substituents results in significantly reduced yields with respect to to secondary amines with one or no branched N-alkyl substituents.     

Role of 2‐oxo and 2‐thioxo modifications on the proton affinity of histidine and fragmentation reactions of protonated histidine

A combination of electrospray ionisation (ESI), multistage and high‐resolution mass spectrometry experiments was used to compare the gas‐phase chemistry of the amino acids histidine (1), 2‐oxo‐histidine (2), and 2‐thioxo‐histidine (3). Collision‐induced dissociation (CID) of all three different proton‐bound heterodimers of these amino acids led to the relative gas‐phase proton affinity order of: histidine >2‐thioxo‐histidine >2‐oxo‐histidine. Density functional theory (DFT) calculations confirm this order, with the lower proton affinities of the oxidised histidine derivatives arising from their ability to adopt the more stable keto/thioketo tautomeric forms. All protonated amino acids predominately fragment via the combined loss of H₂O and CO to yield a₁ ions. Protonated 2 and 3 also undergo other small molecule losses including NH₃ and the imine HN=CHCO₂H. The observed differences in the fragmentation pathways are rationalised through DFT calculations, which reveal that while modification of histidine via the introduction of the oxygen atom in 2 or the sulfur atom in 3 does not affect the barriers against the loss of H₂O+CO, barriers against the losses of NH₃ and HN=CHCO₂H are lowered relative to protonated histidine. Copyright © 2010 John Wiley & Sons, Ltd.     

Experience in making rheological measurements at high pressures

Summary We review the difficulties encountered in the design and operation of apparatus for rheological studies on liquids at pressures up to 900 MN m^–2 and temperatures from –30 to 120°C. Such rheological information is required in connection with elastohydrodynamic lubrication in which high pressures and shear rates are encountered.With 3 figures