Daniel Bayeul

Position of side-chain branching and handedness of turns and helices of homopeptides from chiral Cα-methylated amino acids. Crystal-state structural analysis of (αMe)Leu trimer and tetramer

Terminally blocked homotri- and homotetra-peptides from (αMe)Leu, a chiral Cα-methylated, γ-branched α-amino acid, have been prepared by solution methods and fully characterized. The molecular and crystal structures of pBrBz-[D-(αMe)Leu]3-OH monohydrate and pBrBz-[D-(αMe)-Leu]4-OBut(where pBrBz indicates p-bromobenzoyl) were determined by X-ray diffraction. The tripeptide carboxylic acid adopts a type-III β-turn conformation followed by an uncommon oxyanalogue of a type-III β-turn, the latter being stabilized by a 1â†�4 CO ⋯ H–O intramolecular H-bond. The three independent molecules in the asymmetric unit of the tetrapeptide ester are folded in a regular right-handed 310-helix. All (αMe)Leu residues exhibit φ, Ψ torsion angles in the helical region of the conformational map. These results indicate that: (i) the (αMe)Leu residue is an effective β-turn and helix promoter and (ii) the relationship between (αMe)Leu chirality and turn and helix handedness is the same as that shown by the γ-branched (αMe)Phe residue, but it is opposite to that characteristic of isovaline (Iva), with a linear side chain, the β-branched (αMe)Val residue and protein amino acids (including Leu).

Synthesis, crystal structures and reactivity of ruthenium-(II) and -(III) complexes containing β-ketophosphine or phosphino enolate ligands

Reaction of 1, 2 or 3 equivalents of (diphenylphosphino)acetophenone, Ph2PCH2C(O)Ph (L), with [RuCl2(PPh3)3] in toluene afforded selectively trans,mer-[[graphic omitted])Ph}(PPh3)2]1, trans,cis,cis-[[graphic omitted])Ph}2]2 or trans,mer-[[graphic omitted])Ph}{Ph2PCH2C(O)Ph}2]3, respectively. Complex 1 hydrogenates and isomerises hex-1-ene at atmospheric hydrogen pressure, whereas 2 is inactive. Complex 3 exhibits a dynamic behaviour on the 1H NMR time-scale which corresponds to exchange between chelating and terminally bound phosphines. The activation energy of this process was calculated to be 63.7 ± 0.7 kJ mol–1. Reaction of 3 with TIPF6 in CH2Cl2 gave mer-[[graphic omitted])Ph}2{Ph2PCH2C(O)Ph}]PF69 which reacted with a second equivalent of TIPF6 in MeCN to give the dicationic complex trans,cis,cis-[[graphic omitted];)Ph}2(NCMe)2][PF6]210. The fac and mer isomers of the tris(enolato) RuII complex [Na][[graphic omitted])Ph}3]11a and 11b were obtained by reaction of 9 with NaH in tetrahydrofuran (thf). Treatment of 3 with NaOMe in toluene afforded selectively 11b, which has been crystallised by addition of 1,4,7,10,13-pentaoxacyclopentadecane (15-crown-5) to give [Na(15-crown-5)·H2O][mer-[graphic omitted])Ph}3]·thf 12. Protonation of 11b with HBF4·Et2O gave the neutral intermediate mer-[[graphic omitted])Ph}]13, characterised spectroscopically, and the final product mer-[[graphic omitted])Ph}3][BF4]214. Treatment of [RuCl3(AsPh3)2(MeOH)] with 2 equivalents of L yielded the ruthenium(III) complex mer,trans-[[graphic omitted])Ph}{Ph2PCH2C(O)Ph}]15, which was easily reduced to the ruthenium(II) complex 2. The solid state structures of complexes 2, 10 and 12 have been determined by single crystal X-ray analysis. The co-ordination of the metal in 2 and 10 is slightly distorted octahedral with the two phosphorous atoms (and oxygen atoms) in a cis position and the chloride and acetonitrile ligands, respectively, in a trans position. In the chiral anion of complex 12 the distorted octahedral structure has a meridional arrangement of the P atoms and electronic delocalisation occurs within the chelating phosphino enolate ligands.

Synthesis, structure and electrochemistry of Ph2PCH2PPh2-bridged, heterometallic complexes containing a (η-C5H4Me)Mn(CO)2 fragment

Heterometallic complexes and clusters were prepared by using the new metallophosphine [Mn(η-C5H4Me)(CO)2(dppm-P)]1 obtained from [Mn(η-C5H4Me)(CO)3] and Ph2PCH2PPh2(dppm). The dark green complexes [(OC)2(η-C5H4Me)M[graphic omitted]dCl2] and [{(η-C5H4Me)[graphic omitted]h(µ-Cl)}2], which contain metal–metal bonds, have been obtained by the reactions of 1 with [PdCl2(NCPh)2] and [{Rh(cod)(µ-Cl)}2](cod = cycloocta-1,5-diene), respectively. Starting from [Pt(cod)2], the trimetallic complex [Pt{(µ-dppm)Mn(η-C5H4Me)(CO)2}2] was formed and a reversible Mn–Pt bond formation has been observed by variable-temperature 31P-{1H} NMR spectroscopy. Reactions of 1 with [PtCl2(NCPh)2], [{Re(CO)3(thf)(µ-Br)}2](thf = tetrahydrofuran), [{RuCl(CO)3(µ-Cl)}2] and [{Ir(cod)(µ-Cl)}2] led in high yields to complexes of the type Mn–dppm–M–dppm–Mn (M = Pt, Re, Ru or Ir) having no metal–metal interaction. With [Mn(η-C5H4Me)(CO)2(thf)], [{Pd(η3C-3H4Me)(µ-Cl)}2] or [AuBr(tht)](tht = tetrahydrothiophene) bimetallic complexes of the type Mn-dppm-M′(M′= Mn, Pd or Au) were obtained again with no metal–metal interaction. Another route to this type of bimetallic complexes consists of the reaction of [(OC)2(η-C5H4Me)M[graphic omitted]dCl2] with two-electron donor ligands such as isocyanides RNC (R = 2,6-xylyl or But), which yielded [(OC)2(η-C5H4Me)Mn(µ-dppm)Pd(CNR)Cl2]. Reaction of the isocyanide complexes with azetidine did not lead to the expected carbene complexes, instead the isocyanide ligand was substituted by azetidine, yielding [(OC)2(η-C5H4Me)Mn(µ-dppm)Pd(NHC3H6)Cl2]. The structures of the xylyl isocyanide and azetidine complexes have been determined by X ray diffraction. Reaction of [(OC)2(η-C5H4Me)Mn(µ-dppm)AuBr] with the metalate K[Fe{Si(OMe)3}(CO)3(PPh3)] gave in high yield the heterotrimetallic chain complex [(OC)2(η-C5H4Me) Mn(µ-dppm)AuFe{Si(OMe)3}(CO)3(PPh3)]. Reaction of 1 with trans-[Pt{W(η-C5H4Me)(CO)3}2(NCPh)2] afforded the cluster [Pt2W2(η-C5H4Me)2(µ3-CO)2(µ-CO)4{(µ-dppm)Mn(η-C5H4Me)(CO)2}2]. An electrochemical study of some of the complexes has provided evidence for possible electronic communication between the metal centres.

The crystal state conformation of Aib-rich segments of peptaibol antibiotics

Ac‐(Aib‐Ala)3‐OH (a protected segment of the peptaibols gliodeliquescin and paracelsin), Z‐Leu‐Aib‐Val‐Aib‐Gly‐OtBu (a segment of [Leu]7‐gliodeliquescin), Z‐Val‐Aib‐Aib‐Gln‐OtBu (a common segment of alamethicin, paracelsin, and hypelcin), and Ac‐Aib‐Pro‐(Aib‐Ala)2‐OMe and Z‐Aib‐Pro‐(Aib‐Ala)2‐OMe, which represent differently Nα‐protected 1–6 segments of alamethicin and hypelcin, have been synthesized by solution methods. The crystal‐state conformations of these five Aib‐containing peptides have been determined by X‐ray diffraction analysis. We have confirmed that the 310‐helical structure is preferentially adopted by Aib‐rich short peptides. An experimentally unambiguous proof for the 310→α‐helix conversion has been provided by the two differently N‐blocked ‐Aib‐Pro‐(Aib‐Ala)2‐OMe hexapeptides. The β‐bend ribbon conformation, commonly observed in the (Aib‐Pro)n sequential oligopeptides, is not found in the ‐Aib‐Pro‐Aib‐Ala‐Aib‐Ala‐ sequence. As expected on the basis of the l‐configuration of the Cα‐monoalkylated residues, a right‐handed helix screw sense was found in all peptides investigated.

Further studies on dppm-stabilized mixed-metal clusters: X-ray structure of PdPtCoCl(CO)3(μ-dppm)21

The metalloligated mixed-metal cluster [PdPtCo2(CO)7(μ-dppm)2] (2) (dppm = μ-Ph2PCH2PPh2) possesses numerous potential reaction centers (e.g., metal(s), metal-metal bonds, CO, and dppm ligands) and this has previously led to an investigation of the site selectivity of reactions with nucleophiles. The exocyclic CO(CO)4 fragment was substituted with a chloride ligand and the resulting chiral, triangular cluster PdPtCoCl(CO)3(μ-dppm)2 (4) has been structurally characterized. The Pd-Co and Pt-Co edges of this almost equilateral triangle are bridged by a dppm ligand, and two of the three carbonyls borne by the Co atom are semi-triply bridging above and below the plane of the metals. The “Co(CO)3P” fragment behaves as an anionic 4-electron donor organometallic bridging group toward thed9-d9 Pd(I)-Pt(I) unit. Crystal data for4, monoclinic space groupP21/n with Z=4 in a unit cell of dimensionsa=12.291(3),b=19.321(4),c=23.680(5) Å,β=100.05(2)°. The structure has been solved from diffractometer data by Patterson, Fourier methods and refined by full-matrix least squares on the basis of 3512 observed reflections (l>3σ) toR(F) andRw(F) values of 0.059 and 0.061.

Elimination Reactions of 2-Deoxy 2-C-Methyl Sugars : Application to the Synthesis of Methyl 2,3,4-Trideoxy-2,4-Di-C-Methyl-6-O-Triphenylmethyl α-D-Lyxo Hexopyranoside and its X Ray Crystal Structure

Abstract A facile and regiospecific dehydration of 2-deoxy 2-C-methyl sugars using the triphenylphosphine-diethylazodicarboxylate reagent is reported. This reaction allowed us to prepare the title compound which is a crucial intermediate in the total synthesis of natural products. Its absolute configuration is firmly established by X Ray analysis.

An entry to enantiomerically pure cis decalinic structures from carbohydrates

Tri-O-acetylglucal is the starting material of a new route to chiral highly oxygenated cis decalinic structures by tandem IMDA-Ferrier carbocyclization reactions, single-crystal X-ray analysis of one of these aldol structures is performed.