NMR study on 9‐S and 9‐R oxirane derivatives of ascomycin

Structure elucidation of 9‐S and 9‐R oxirane derivatives of ascomycin, a 23‐membered immunomodulating macrolactam, was performed using NMR spectroscopy. The total ¹H and ¹³C signal assignments required the gradient‐selected versions of COSY (gs‐COSY), heteronuclear multiple quantum‐correlation spectroscopy (gs‐HSQC), heteronuclear multiple‐bond correlation spectroscopy (gs‐HMBC), and nuclear Overhauser methods. The data sets then were used to examine the dependence of ketone–hemiketal and cis–trans amide equilibria on the substitution pattern and the absolute configuration of the chiral oxirane. Copyright © 2002 John Wiley & Sons, Ltd.     

cis‐trans Peptide‐Bond Isomerization in α‐Methylproline Derivatives

α‐Methyl‐L ‐proline is an α‐substituted analog of proline that has been previously employed to constrain prolyl peptide bonds in a trans conformation. Here, we revisit the cis‐trans prolyl peptide bond equilibrium in derivatives of α‐methyl‐L ‐proline, such as N‐Boc‐protected α‐methyl‐L ‐proline and the hexapeptide H‐Ala‐Tyr‐αMePro‐Tyr‐Asp‐Val‐OH. In Boc‐α‐methyl‐L ‐proline, we found that both cis and trans conformers were populated, whereas, in the short peptide, only the trans conformer was detected. The energy barrier for the cis‐trans isomerization in Boc‐α‐methyl‐L ‐proline was determined by line‐shape analysis of NMR spectra obtained at different temperatures and found to be 1.24 kcal/mol (at 298 K) higher than the corresponding value for Boc‐L ‐proline. These findings further illuminate the conformationally constraining properties of α‐methyl‐L ‐proline.     

A New Benzotriazole‐Mediated Stereoflexible Gateway to Hetero‐2,5‐diketopiperazines

Open chain Cbz‐L ‐aa¹‐L ‐Pro‐Bt (Bt=benzotriazole) sequences were converted into either the corresponding trans‐ or cis‐fused 2,5‐diketopiperazines (DKPs) depending on the reaction conditions. Thermodynamic tandem cyclization/epimerization afforded selectively the corresponding trans‐DKPs (69–75 %). Complementarily, tandem deprotection/cyclization led to the cis‐DKPs (65–72 %). A representative set of proline‐containing cis‐ and trans‐DKPs has been prepared. A mechanistic investigation, based on chiral HPLC, kinetics, and computational studies enabled a rationalization of the results.     

Conformational Analysis of a 20‐Membered Cyclic Peptide Disulfide from Conus virgo with a WPW Segment: Evidence for an Aromatic–Proline Sandwich

A novel peptide containing a single disulfide bond, CIWPWC (Vi804), has been isolated and characterised from the venom of the marine cone snail, Conus virgo. A precursor polypeptide sequence derived from complementary DNA, corresponding to the M‐superfamily conotoxins, has been identified. The identity of the synthetic and natural peptide sequence has been established. A detailed analysis of the conformation in solution is reported for Vi804 and a synthetic analogue, CI^DWPWC (^DW3‐Vi804), in order to establish the structure of the novel WPW motif, which occurs in the context of a 20‐membered macrocyclic disulfide. Vi804 exists exclusively in the cis W3?P4 conformer in water and methanol, whereas ^DW3‐Vi804 occurs exclusively as the trans conformer. NMR spectra revealed a W3?P4 type VI β turn in Vi804 and a type II′ β turn in the analogue peptide, ^DW3‐Vi804. The extremely high‐field chemical shifts of the proline ring protons, together with specific nuclear Overhauser effects, are used to establish a conformation in which the proline ring is sandwiched between the flanking Trp residues, which emphasises a stabilising role for the aromatic–proline interactions, mediated predominantly by dispersion forces.     

cis‐ and trans‐Dichloro(3,6‐dihydro‐1,2‐oxazine‐N)(dimethyl sulfoxide‐S)‐platinum(II)

Both the cis, (I), and trans, (II), isomers of the title complex, [PtCl₂(C₄H₇NO)(C₂H₆OS)], possess relatively undistorted square‐planar geometries about the Pt atoms. For (I), cisL—Pt—L angles are in the range 88.8 (2)–91.08 (8)°, while trans angles are 178.61 (8) and 179.4 (2)°. For (II), cisL—Pt—L 86.1 (3)–93.7 (1)°, and transL—Pt—L 175.5 (1) and 179.1 (3)°. The di­methyl sulfoxide (dmso) ligand adopts a normal pyramidal geometry in both complexes. In (I), the S=O bond essentially eclipses the adjacent Pt—N bond, while the oxazine ligand in (I) is twisted so as to avoid steric interactions with the adjacent chloride ligand. By contrast, the dmso ligand in (II) is rotated such that the S=O bond is approximately perpendicular to the square plane, while the oxazine ligand is once again twisted out of the plane by a similar amount as in (I). These are the first structural examples of square‐planar platinum(II) complexes containing a 1,2‐oxazine ligand.     

A linear free-energy correlation in the low-energy tandem mass spectra of protonated tripeptides Gly–Gly–Xxx

The backbone cleavages of protonated tripeptide ions of the series Gly—Gly—Xxx, where Xxx ? Gly, Ala, Val, d-Leu, l-Leu, Ile, Phe, Tyr, Trp, Pro, Met and Glu, were studied in a hybrid tandem mass spectrometer. C-Terminal y-type ions and N-terminal a- and b-type ions were noted. A linear relationship between log (y₁/b₂) and the proton affinity of the C-terminal amino acid substituents was found: as the proton affinity of the C-terminal residue increases, the fraction of y₁ ion formation increases. When the C-terminal substituent was more basic than Trp, the b₂ ion was not observed. It is likely that the site of protonation changes from peptide bond to side-chain for just these residues, Lys, His and Arg.     

Synthesis and 1H and 13C NMR structural analysis of cis‐ and trans‐2‐imino‐1,3‐ and ‐3,1‐perhydrobenzoxazines and their 3‐ and 1‐N‐methyl derivatives

cis‐ and trans‐2‐imino‐1,3‐ and ‐3,1‐perhydrobenzoxazines and the N‐methyl derivatives of the latter were synthesized from the corresponding cyclic 1,3‐amino alcohol with cyanogen bromide. The configurations of the studied compounds were confirmed by ¹H and ¹³C NMR spectra. All trans‐fused compounds exist in biased chair–chair conformations as expected, whereas the cis‐fused 1,3‐benzoxazines attain exclusively the O‐in conformations. The cis‐fused 3,1‐benzoxazines, especially the 1‐methyl‐substituted derivatives, tend to favor the N‐out form, obviously owing to the favorable axial orientation of this N‐methyl. Copyright © 2003 John Wiley & Sons, Ltd.     

Ring opening of pyridines: the pseudo‐cis and pseudo‐trans isomers of tetra‐n‐butylammonium 4‐nitro‐5‐oxo‐2‐pentenenitrilate

The reaction of 2‐chloro‐5‐nitropyridine with two equivalents of base produces the title carbanion as an intermediate in a ring‐opening/ring‐closing reaction. The crystal structures of the tetra‐n‐butylammonium salts of the intermediates, C₁₆H₃₆N⁺·C₅H₃N₂O₃⁻, revealed that pseudo‐cis and pseudo‐trans isomers are possible. One crystal structure displayed a mixture of the two isomers with approximately 90% pseudo‐cis geometry and confirms the structure predicted by the S_N(ANRORC) mechanism. The pseudo‐cis intermediate undergoes a slow isomerization over a period of months to the pseudo‐trans isomer, which does not have the appropriate geometry for the subsequent ring‐closing reaction. The structure of the pure pseudo‐trans isomer is also reported. In both isomers, the negative charge is highly delocalized, but relatively small differences in C—C bond distances indicate a system of conjugated double bonds with the nitro group bearing the negative charge. The packing of the two unit cells is very similar and largely determined by the interactions between the planar carbanion and the bulky tetrahedral cation.     

The Implications of (2S,4S)‐Hydroxyproline 4‐O‐Glycosylation for Prolyl Amide Isomerization

The conformations of peptides and proteins are often influenced by glycans O‐linked to serine (Ser) or threonine (Thr). (2S,4R)‐4‐Hydroxyproline (Hyp), together with L ‐proline (Pro), are interesting targets for O‐glycosylation because they have a unique influence on peptide and protein conformation. In previous work we found that glycosylation of Hyp does not affect the N‐terminal amide trans/cis ratios (K_trans/cis) or the rates of amide isomerization in model amides. The stereoisomer of Hyp—(2S,4S)‐4‐hydroxyproline (hyp)—is rarely found in nature, and has a different influence both on the conformation of the pyrrolidine ring and on K_trans/cis. Glycans attached to hyp would be expected to be projected from the opposite face of the prolyl side chain relative to Hyp; the impact this would have on K_trans/cis was unknown. Measurements of ³J coupling constants indicate that the glycan has little impact on the C^γ‐endo conformation produced by hyp. As a result, it was found that the D ‐galactose residue extending from a C^γ‐endo pucker affects both K_trans/cis and the rate of isomerization, which is not found to occur when it is projected from a C^γ‐exo pucker; this reflects the different environments delineated by the proline side chain. The enthalpic contributions to the stabilization of the trans amide isomer may be due to disruption of intramolecular interactions present in hyp; the change in enthalpy is balanced by a decrease in entropy incurred upon glycosylation. Because the different stereoisomers—Hyp and hyp—project the O‐linked carbohydrates in opposite spatial orientations, these glycosylated amino acids may be useful for understanding of how the projection of a glycan from the peptide or protein backbone exerts its influence.     

Unraveling the Mechanism and Regioselectivity of the B12‐Dependent Reductive Dehalogenase PceA

PceA is a cobalamin‐dependent reductive dehalogenase that catalyzes the dechlorination of perchloroethylene to trichloroethylene and then to cis‐dichloroethylene as the sole final product. The reaction mechanism and the regioselectivity of this enzyme are investigated by using density functional calculations. Four different substrates, namely, perchloroethylene, trichloroethylene, cis‐dichloroethylene, and chlorotheylene, have been considered and were found to follow the same reaction mechanism pattern. The reaction starts with the reduction of Co^II to Co^I through a proton‐coupled electron transfer process, with the proton delivered to a Tyr246 anion. This is followed by concerted C?Cl bond heterolytic cleavage and proton transfer from Tyr246 to the substrate carbon atom, generating a Co^III?Cl intermediate. Subsequently, a one‐electron transfer leads to the formation of the Co^II?Cl product, from which the chloride and the dehalogenated product can be released from the active site. The substrate reactivity follows the trend perchloroethylene>trichloroethylene?cis‐dichloroethylene?chlorotheylene. The barriers for the latter two substrates are significantly higher compared with those for perchloroethylene and trichloroethylene, implying that PceA does not catalyze their degradation. In addition, the formation of cis‐dichloroethylene has a lower barrier by 3.8 kcal mol^?1 than the formation of trans‐dichloroethylene and 1,1‐dichloroethylene, reproducing the regioselectivity. These results agree quite well with the experimental findings, which show cis‐dichloroethylene as the sole product in the PceA‐catalyzed dechlorination of perchloethylene and trichloroethylene.     

cis‐Bis(3,6‐di­hydro‐2H‐1,2‐oxazine‐N)­di­iodo­platinum(II) and cis‐bis(3,4,5,6‐tetra­hydro‐2H‐1,2‐oxazine‐N)­di­iodo­platinum(II)

The title complexes, [Pt(C₄H₇NO)₂I₂], (I), and [Pt(C₄H₉NO)₂I₂], (II), possess similar square‐planar coordination geometries with modest distortions from ideality. For (I), the cis‐L—Pt—L angles are in the range 87.0 (4)–94.2 (3)°, while the trans angles are 174.4 (3) and 176.4 (3)°. For (II), cis‐L—Pt—L are 86.1 (8)–94.2 (6)° and trans‐L—Pt—L are 174.4 (6) and 177.4 (5)°. One 3,6‐di­hydro‐2H‐1,2‐oxazine ligand in (I) is rotated so that the N—O bond is out of the square plane by approximately 70°, while the N—C bond is only ca 20° out of the plane. The other oxazine ligand is rotated so that the N—C bond is about 80° out of the plane, while the N—O bond is out of the plane by approximately 24°. In (II), the 3,4,5,6‐tetra­hydro‐2H‐1,2‐oxazine ligands are also positioned with one having the N—O bond further out of the plane and the other having the N—C bond positioned in that fashion. Both ligands, however, are rotated approximately 90° compared with their positions in (I). In both complexes, this results in an unsymmetrical distortion of the I—Pt—N bond angles in which one is expanded and the other contracted. These features are compared to those of reported cis‐di­amine­di­iodo­platinum(II) complexes.     

Synthesis and structure of cycloalkane‐ and norbornane‐condensed 6‐aryl‐1,2,4,5‐tetrahydropyridazinones

The C=N double bond of certain cis‐ or trans‐cycloalkane and diexo‐ or diendo‐norbornane‐condensed pyridazinones was reduced with NaBH₃CN. The cis‐ or trans nature of the starting cycloalkane derivatives was always retained in the saturated products, with a high degree of diastereoselectivity: the hydrogen on the new stereocenter and the annelational hydrogen next to the carbonyl always exhibited the same steric orientation. The stereostructures were determined by means of nmr measurements and confirmed by molecular modelling.     

Ruthenium Lewis Acid‐Catalyzed Asymmetric Diels–Alder Reactions: Reverse‐Face Selectivity for α,β‐Unsaturated Aldehydes and Ketones

Acrolein, methacrolein, methyl vinyl ketone, ethyl vinyl ketone, 3‐methyl‐3‐en‐2‐one, and divinyl ketone were coordinated to a cationic cyclopentadienyl ruthenium(II) Lewis acid incorporating the electron‐poor bidentate BIPHOP–F ligand. Analysis by NOESY and ROESY NMR techniques allowed the determination of conformations of enals and enones present in solution in CD₂Cl₂. The results were compared to solid‐state structures and to the facial selectivities of catalytic asymmetric Diels–Alder reactions with cyclopentadiene. X‐Ray structures of four Ru‐enal and Ru‐enone complexes show the α,β‐unsaturated C=O compounds to adopt an anti‐s‐trans conformation. In solution, enals assume both anti‐s‐trans and anti‐s‐cis conformations. An additional conformation, syn‐s‐trans, is present in enone complexes. Enantioface selectivity in the cycloaddition reactions differs for enals and enones. Reaction products indicate enals to react exclusively in the anti‐s‐trans conformation, whereas with enones, the major product results from the syn‐s‐trans conformation. The alkene in s‐cis conformations, while present in solution, is shielded and cannot undergo cycloaddition. A syn‐s‐trans conformation is found in the solid state of the bulky 6,6‐dimethyl cyclohexanone‐Ru(II) complex. The X‐ray structure of divinyl ketone is unique in that the Ru(II) center binds the enone via a η² bond to one of the alkene moieties. In solution, coordination to Ru–C=O oxygen is adopted. A comparison of facial preference is also made to the corresponding indenyl Lewis acids.     

Synthesis,hole mobility,and photovoltaic properties of two alternating poly[3‐(hex‐1‐enyl)thiophene‐co‐thiophene]s

Two alternating poly[3‐(hex‐1‐enyl)thiophene‐co‐thiophene]s, Pa (with 77% trans‐isomer and 23% cis‐isomer) and Pb (with 100% trans‐isomer), were synthesized by the coupling of 2,5‐dibromo‐3‐hex‐1‐enyl‐thiophene to 2,5‐bis(tributylstannyl)thiophene via a Stille reaction and compared with poly(3‐hexylthiophene‐co‐thiophene) ( P1 ) to study the effect of changing the carbon(α)–carbon(β) single bond into a carbon–carbon double bond on the properties of the polymers. From P1 to Pb and to Pa , the ultraviolet–visible absorption peaks of the polymers were slightly redshifted, and their electrochemical bandgaps decreased by 0.05–0.1 eV. X‐ray diffraction analysis indicated that Pa had a better lamellar structure than Pb . The hole mobilities of the three polymers, determined with the space‐charge‐limited current model, were 5.23 × 10^?6 ( P1 ), 2.34 × 10^?4 ( Pb ), and 7.02 × 10^?4 cm²/V s ( Pa ). The power conversion efficiencies (PCEs) of polymer solar cells based on the three polymers were 0.87 ( P1 ), 1.16 ( Pb ), and 1.70% ( Pa ). The increase in the hole mobility and PCE revealed the important effect of changing the carbon(α)–carbon(β) single bond into a carbon–carbon double bond on the properties of polythiophene derivatives containing 3‐alkylthiophene. The strategy used in this work enlarges the thinking to obtain novel, efficient donor polymers for optoelectronic applications. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 629–638, 2007     

trans‐Di­chloro­tetrakis(4‐methylpyrimidine‐N1)­ruthenium(II)

The title compound, trans‐[Ru^IICl₂(N¹‐mepym)₄] (mepym is 4‐methylpyrimidine, C₅H₆N₂), obtained from the reaction of trans,cis,cis‐[Ru^IICl₂(N¹‐mepym)₂(SbPh₃)₂] (Ph is phenyl) with excess mepym in ethanol, has fourfold crystallographic symmetry and has the four pyrimidine bases coordinated through N¹ and arranged in a propeller‐like orientation. The Ru—N and Ru—Cl bond distances are 2.082 (2) and 2.400 (4) Å, respectively. The methyl group, and the N³ and Cl atoms are involved in intermolecular C—H?N and C—­H?Cl hydrogen‐bond interactions.     

Probing the γ‐Turn in a Short Proline Dipeptide Chain

The small peptide derived from proline, N‐acetyl‐prolinamide (Ac‐Pro‐NH₂), has been investigated using a combination of Fourier transform microwave spectroscopy with laser ablation. Spectral signatures belonging to only one conformer have been detected in the supersonic expansion. Rotational constants and nuclear quadrupole coupling constants of the two ¹⁴N nuclei have been used in the characterization of a γ‐turn structure in the gas phase, which is stabilized by a CO???HN intramolecular hydrogen bond closing a seven‐membered ring. A methyl group internal rotation barrier of 354 cm^?1 has been determined from the analysis of the A–E splittings.     

trans‐Selective Insertional Dihydroboration of a cis‐Diborene: Synthesis of Linear sp3‐sp2‐sp3‐Triboranes and Subsequent Cationization

The reaction of aryl‐ and amino(dihydro)boranes with dibora[2]ferrocenophane 1 leads to the formation 1,3‐trans‐dihydrotriboranes by formal hydrogenation and insertion of a borylene unit into the B=B bond. The aryltriborane derivatives undergo reversible photoisomerization to the cis‐1,2‐μ‐H‐3‐hydrotriboranes, while hydride abstraction affords cationic triboranes, which represent the first doubly base‐stabilized B₃H₄⁺ analogues.     

cis-trans-and Intramolecular enol-enolic equilibrium of β-ketoaldehydes

Tautomerism of aromatic β-ketoaldehydes p-XPhCOCH₂CHO ( 1 , X = NMe₂, OMe, Me, H, Br, NO₂), aliphatic β-ketoaldehydes and benzoylacetaldehyde RCOCH₂CHO ( 2 , R = Me, i-Bu, t-Bu, Ph), RCOCH(Me)CHO ( 3 , R = Me, Et, i-Pr) and methyl 2-formylpropionate MeOCOCH(Me)CHO ( 4 ) has been studied by the ¹H NMR technique. In basic solvents both cis- and trans-enol forms of these compounds co-exist. trans-Enolisation, which occurs exclusively at the formyl group, is most favoured in compound ( 4 ) and least favoured in compounds ( 1 ) and ( 2 ). The increasing electron-attracting property of the substituent X in the aromatic β-ketoaldehydes ( 1 ), as well as increasing solvent basicity in the series propanediol-1, 2-carbonate, acetone < dimethylformamide < dimethylacetamide < pyridine, also shifts the equilibrium towards the trans-enol form. The trans-enol form is absent in aprotic solvents of low basicity such as CCl₄, C₂HCl₃ and toluene. The thermodynamic parameters of the cis-trans-enol (C ? T) and cis-enol-enolic (C ? C') equilibria have been estimated from the temperature dependences. The transition from the cis-to the trans-enol form is accompanied by an entropy decrease of about 10 cal mol^?1 degree^?1. Nevertheless the trans-enol form is stabilised due to its lower enthalpy. The cis-trans-enol equilibrium is determined by the relative strength of the intramolecular hydrogen bond in the cis-enol form and the intermolecular hydrogen bonds with basic solvent molecules of the trans-enol form. The enthalpy difference of the two cis-enolic forms does not exceed 1.0 kcal/mol, in rough agreement with the data calculated by the CNDO/2 approximation. Polar solvents favour the hydroxymethyleneketone form (C) for both groups of compounds 2 and 3 . The content of the hydroxymethyleneketone form is about the same within series 2 where R = Me, i-Bu, Ph and is a little higher for the t-Bu derivative. A decrease of temperature only slightly shifts the equilibrium of compounds 1 and 2 to the hydroxymethyleneketone form, while in the case of 2-methyl-β-ketoaldehydes (3) this effect is markedly pronounced.     

Stereocomplementary Synthesis of cis‐ and trans‐2‐(p‐Bromophenyl)‐5‐methylthiazolidin‐4‐ones: Useful Umpolung‐type Suzuki–Miyaura Cross‐coupling Partner and Donor

Novel cis‐ and trans‐2‐(p‐bromophenyl)‐5‐methylthiazolidin‐4‐ones, S,N‐containing heterocyclic compounds, were provided in a cis‐stereocomplementary and trans‐stereocomplementary synthetic manner. cis‐Selective cyclo‐condensation proceeded between 2‐sulfanylpropanoic acid (thiolactic acid) and an imine derived from 4‐bromobenzaldehyde and methylamine, whereas Ti(OiPr)₄ and Ti(OiBu)₄‐promoted trans‐selective cyclo‐condensation proceeded between benzyl 2‐sulfanylpropanoate and the imine. The obtained cis‐ and trans ‐ 2‐(p‐bromophenyl)‐5‐methylthiazolidin‐4‐ones were successfully converted to 2‐(3‐furyl)phenyl derivatives and bis(pinacolato)diborane derivatives utilizing Suzuki–Miyaura and Miyaura–Ishiyama cross‐coupling reactions, respectively, in an umpolung manner.     

Enantioselective Divergent Synthesis of (−)‐cis‐α‐ and (−)‐cis‐γ‐Irone by Using Wilkinson’s Catalyst

A simple, efficient synthesis is reported for (?)‐cis‐α‐ and (?)‐cis‐γ‐irone, two precious constituents of iris oils, in ≥99 % diastereomeric and enantioselective ratios. The two routes diverge from a common intermediate prepared from (?)‐epoxygeraniol. Of general interest in this approach is the installation of the enone moiety of irones through a NHC?Au^I‐catalyzed Meyer–Schuster‐like rearrangement of a propargylic benzoate and the use of Wilkinson’s catalyst for the stereoselective hydrogenation of a prostereogenic exocyclic double bond to secure the critical cis stereochemistry of the alkyl groups at C2 and C6 of the irones. The stereochemical aspects of this reaction are rationally supported by DFT calculation of the conformers of the substrates undergoing the hydrogenation and by a modeling study of the geometry of the rhodium η² complexes involved in the diastereodifferentiation of the double bond faces. Thus, computational investigation of the η² intermediates formed in the catalytic cycle of prostereogenic alkene hydrogenation by using Wilkinson’s catalyst could be highly predictive of the stereochemistry of the products.