Synthesis of Cyclopentapeptides with Three to Five Aib Units

Four new Aib‐containing cyclopentapeptides have been synthesized by cyclization of the corresponding linear pentapeptides using the diethyl phosphorocyanidate (DEPC)/EtN(ⁱPr)₂ method. The linear precursors were prepared via the ‘azirine/oxazolone method’, i.e., the Aib units were introduced by the reaction of amino acids or peptide acids with a 2,2‐dimethyl‐2H‐azirin‐3‐amine, followed by selective hydrolysis of the terminal amide function. Most remarkably, cyclo[(Aib)₅] exists in CDCl₃ solution in a symmetrical conformation, i.e., no intramolecular H‐bonds are detectable.     

Structural Elucidation of Trace Amounts of Volatile Compounds Using the Crystalline Sponge Method

Volatile organic compounds are widely present as scents and odors in our daily lives and are readily found in a variety of natural extracts. Because these compounds are highly volatile and usually available only in minute quantities, little attention has been paid to X‐ray diffraction as a technique for their structure determination. Here, we show that the structures of volatile organic compounds are easily elucidated using minute quantities of the compounds and the crystalline sponge method. The compound vapors can be directly absorbed into the sponge crystals, or alternatively, preparative gas chromatography can be used to separate the desired compound from a natural mixture.     

Oxazolidinone cross-alkylation during Evans’ asymmetric alkylation reaction

Starting from Evans’ imidazolidin-2-one (1) two compounds were obtained by trans-N-acylation: the expected one 3 with S,R configuration and a second compound 5, that is, related to 3 by the loss of a CH(CH₃)CO fragment. The stereochemistry of 3 was established by NMR spectroscopy, mainly NOE experiments, as expected, the new center has an S configuration, the compound being thus S,R. The structure of compound 5 was determined by X-ray crystallography. A mechanism of formation of 5 was proposed.     

Synthesis and spectral characterization of mercury(II) complexes with the bidentate Schiff base ligand N,N′-bis(2,3-dimethoxybenzylidene)-1,2-diaminoethane: The crystal structures of [Hg((23-MeO-ba)2en)I2] and [Hg((23-MeO-ba)2en)Br2]

Three mercury(II) complexes, [Hg((23-MeO-ba)₂en)X₂] (X = I (1), Br (2) and Cl(3)), and the ligand (23-MeO-ba)₂en ((23-MeO-ba)₂en = N,N′-bis(2,3-dimethoxybenzylidene)-1,2-diaminoethane) have been synthesized and characterized by elemental analyses, FT-IR and ¹H NMR spectroscopy. The crystal and molecular structures of 1 and 2 were determined by X-ray crystallography from single-crystal data. The metal-to-ligand ratio was found to be 1:1. The mercury(II) center in 1 and 2 has a distorted tetrahedral geometry with HgN₂I₂ and HgN₂Br₂ chromophores, respectively. The Schiff base ligand (23-MeO-ba)₂en acts as a chelating ligand, coordinating via the two nitrogen atoms to the mercury(II) center, and it adopts an E,E conformation. The coordination sphere of the mercury(II) center in 1 and 2 is completed by the two I and Br atoms, respectively. In complex 1 an inter-molecular non-classical hydrogen bond of the type C-H?O was found, while in complex 2 inter- and intra-molecular non-classical hydrogen bonds of the type C-H?X (X = O and Br) were found. The ¹H NMR spectra of the complexes exhibit downfield as well as upfield shifts of the free ligand resonances, reflecting changes in the ligand’s geometry during its coordination.     

Syntheses, structural, electrochemical and optical studies of heterobinuclear ruthenium-osmium alkynyl complexes

The complexes trans-[Os(CCC₆H₄-4-CCR)Cl(dppe)₂] (R = SiPrⁱ₃1, H 2), trans,trans-[(dppe)₂ClOs(CCC₆H₄-4-CC)RuX(dppe)₂] (X = Cl 3, CCC₆H₄-4-CCSiPrⁱ₃4), trans-[Os(CCC₆H₄-4-CCC₆H₄-4-CCR)Cl(dppe)₂] (R = SiPrⁱ₃5, H 6), and trans,trans-[(dppe)₂ClOs(CCC₆H₄-4-CCC₆H₄-4-CC)RuCl(dppe)₂] (7) have been synthesized, and the identities of 1, 2, and 6 confirmed by single-crystal X-ray diffraction studies. Cyclic voltammetry shows that the mononuclear complexes 1, 2, 5, and 6 are oxidized at potentials within a narrow range (0.45-0.49 V), in processes centered on the osmium ethynyl neighbourhood and for simplicity assigned as Os^II/III, while the heterobinuclear complexes 3, 4, and 7 exhibit lower oxidation potentials for Os^II/III and a second oxidation process assigned in a similar fashion to Ru^II/III; the difference in potential between the Os- and Ru-localized processes decreases as the π-bridge is lengthened. UV-vis-NIR spectroelectrochemical studies on 1 and 5 reveal the appearance on oxidation of a low-energy band ascribed to chloro to metal-ethynyl charge transfer. Osmium-centered oxidation at the heterobinuclear complexes 4 and 7 results in appearance of a low-energy band, which blue-shifts and increases in intensity on further oxidation to 4²⁺ and 7²⁺.     

Synthesis,Characterization and X-ray Crystal Structure of N,N＇-Bis（4-nitrophenylcarba-mothioyl）-isophthalamide·DMF

A new crystal of N,N'-bis(4-nitrophenylcarbamothioyl)isophthalamide DMF solva-te has been prepared at room temperature and characterized by elemental analysis and IR,MS,1H NMR spectra and X-ray single-crystal determination.The complex crystallizes in monoclinic,space group P21/c with a = 11.2093(12),b = 22.081(2),c = 13.9640(15) ,β= 112.128(2)°,V = 3201.8(6) 3,C28H30N8O8S2,Mr = 670.72,Dc = 1.391 Mg/m3,Z = 4,μ= 0.228 mm-1,F(000) = 1400,the final R = 0.0483 and wR = 0.1411(I > 2σ(I)).The molecular packing in the crystal is the result of N-H…O hydrogen bonding.     

Crystallographic analysis of metal-ion binding to human ubiquitin

The metal-binding ability of human ubiquitin (hUb) towards a selection of biologically relevant metal ions and complexes has been probed. Different techniques have been used to obtain crystals suitable for crystallographic analysis. In the first type of experiments, crystals of hUb have been soaked in solutions containing copper(II) acetate and two metallodrugs, Zeise salt (K[PtCl(3)(η(2)-C(2)H(4))]·H(2)O) and cisplatin (cis-[PtCl(2)(NH(3))(2)]). The Zeise salt is used in a test for hepatitis, whereas cisplatin is one of the most powerful anticancer drugs in clinical use. The Zeise salt readily reacts with hUb crystals to afford an adduct with three platinum residues per protein molecule, Pt(3)-hUb. In contrast, copper(II) acetate and cisplatin were found to be unreactive for contact times up to one hour and to cause degradation of the hUb crystals for longer times. In the second type of experiments, hUb was cocrystallized with a solution of copper(II) or zinc(II) acetate or cisplatin. Zinc(II) acetate gives, at low metal-to-protein molar ratios (8:1), crystals containing one metal ion per three molecules of protein, Zn-hUb(3) (already reported in previous work), whereas at high metal-to-protein ratios (70:1) gives crystals containing three Zn(II) ions per protein molecule, Zn(3)-hUb. In contrast, once again, copper(II) acetate and cisplatin, even at low metal-to-protein ratios, do not give crystalline material. In the soaking experiment, the Zeise anion leads to simultaneous platination of His68, Met1, and Lys6. Present and previous results of cocrystallization experiments performed with Zn(II) and other Group 12 metal ions allow a comprehensive understanding of the metal-ion binding properties of hUb with His68 as the main anchoring site, followed by Met1 and carboxylic groups of Glu16, Glu18, Glu64, Asp21, and Asp32, to be reached. In the case of platinum, Lys6 can also be a binding site. The amount of bound metal ion, with respect to that of the protein, appears to be a relevant parameter influencing crystal packing.     

ent‐Kaurane Diterpenes and Further Constituents from Wedelia trilobata

Two new ent‐kaurane diterpenes, wedelidins A ( 8 ) and B ( 9 ), together with eighteen other constituents, including the sesquiterpenoids 1 and 2 , ent‐kaurane diterpenes 3 – 7 , triterpenoids 10 and 11 , steroids 12 – 14 , and flavonoids 15 – 17 as well as benzene derivatives 18 – 20 , were isolated from the aerial parts of Wedelia trilobata. The structures of wedelidins A ( 8 ) and B ( 9 ) were elucidated by extensive spectroscopic analyses (including UV, IR, NMR, and MS). Furthermore, the structures of compounds 2 and 3 were confirmed by X‐ray single‐crystal diffraction analyses.