Ratio Of Cis Research Articles

Radiolytic preparation, chromatographic resolution and absorption spectra of cis- and trans-1-iodopropene

1-Iodopropene has been prepared in its cis and trans configurations by scavenging the 1-propenyl radical produced in the dissociative attachment of electrons to cis- and trans-1-bromopropene with iodine. Gel permeation methods have been used to separate these two isomers. The chromatographic data show that identical ratios of these isomers are produced from each of the 1-bromopropene isomers as the result of interchange of the cis and trans configurations of the vinyl like 1-propenyl radical on a time scale shorter than that for the scavenging process, i.e., <10 ns. Radiochemical studies show that at equilibrium the trans/ cis ratio for the two configurations of the radical is 1.76. This ratio indicates that the cis configuration of the radical is 0.35 kcal/mol higher in energy than the trans configuration.

DFT analysis of NMR scalar interactions across the glycosidic bond in DNA.

The relationship between the glycosidic torsion angle chi, the three-bond couplings (3)J(C2/4-H1') and (3)J(C6/8-H1'), and the one-bond coupling (1)J(C1'-H1') in deoxyribonucleosides and a number of uracil cyclo-nucleosides has been analyzed using density functional theory. The influence of the sugar pucker and the hydroxymethyl conformation has also been considered. The parameters of the Karplus relationships between the three-bond couplings and chi depend strongly on the aromatic base. (3)J(C2/4-H1') reveals different behavior for deoxyadenosine, deoxyguanosine, and deoxycytidine as compared to deoxythymidine and deoxyuridine. In the case of (3)J(C6/8-H1'), an opposite trans to cis ratio of couplings is obtained for pyrimidine nucleosides in contrast to purine nucleosides. The extremes of the Karplus curves are shifted by ca. 10 degrees with respect to syn and anti-periplanar orientations of the coupled nuclei. The change in the sugar pucker from S to N decreases (3)J(C2/4-H1') and (3)J(C6/8-H1'), while increasing (1)J(C1'-H1') for the syn rotamers, whereas all of the trends are reversed for the anti rotamers. The influence of the sugar pucker on (1)J(C1'-H1') is interpreted in terms of interactions between the n(O4'), sigma*(C1'-H1') orbitals. The (1)J(C1'-H1') are related to chi through a generalized Karplus relationship, which combines cos(chi) and cos(2)(chi) functions with mutually different phase shifts that implicitly accounts for a significant portion of the related sugar pucker effects. Most of theoretical (3)J(C2/4-H1') and (3)J(C6/8-H1') for uracil cyclo-nucleosides compare well with available experimental data. (3)J(C6/8-H1') couplings for all C2-bridged nucleosides are up to 3 Hz smaller than in the genuine nucleosides with the corresponding chi, revealing a nonlocal aspect of the spin-spin interactions across the glycosidic bond. Theoretical (1)J(C1'-H1') are underestimated with respect to the experiment by ca. 10% but reproduce the trends in (1)J(C1'-H1') vs chi.

Comparison of microbial communities in percolating water from plow layer and subsoil layer estimated by phospholipid fatty acid (PLFA) analysis

The amount and composition of phospholipid fatty acids (PLFAs) in the percolating water taken from different depths of soil (10 cm, PW10; 40 cm, PW40) and floodwater (FW) in a paddy field were compared during the period of rice cultivation. The amounts of PLFAs in PW10, PW40, and FW ranged from 22.6 to 46.2 μg L−1, from 22.3 to 54.5 μg L−1, and from 82.9 to 179.0 μg L−1, respectively. The PLFA profiles in PW10, PW40, and FW were similar to each other and 16 : 1ω7c, 18 : 1ω7, and 16 : 0 PLFAs were dominant components, irrespective of the sampling site and sampling time. High proportions of straight mono-unsaturated PLFAs ranging from 42.0 to 76.5% suggested that Gram-negative bacteria were the major members in the microbial communities of the water samples studied. A potential indicator of the environmental stresses imposed upon the microbiota that was represented by the trans vs. cis ratio of 16 : 17 PLFA was constantly low (< 0.05), indicating that the microbial communities at these sites were hardly stressed.

Light emitting poly( para-phenylenevinylene- alt-3- tert butyl- meta-phenylenevinylenes)

Poly( para-phenylenevinylene- alt-3- tert-butyl- meta-phenylenevinylene) was synthesized by Wittig condensation with varying ratio of cis and trans olefin bonds. The luminescent properties of the unisomerized and trans-isomerized polymers ( 1 and 2, respectively) were compared. Nearly identical emission spectra were observed for both polymers in dilute solutions, but the emission intensity of 1 is lower. This is attributed to the cis vinylic moieties in 1, which favor nonradiative excited state decay processes by limiting formation of a planar excited state. Double layer LEDs with the configuration ITO/PPV/( 1 or 2)/Ca–Al were fabricated. Substantially better efficiencies and luminances were observed for devices made with polymer 2 as the emissive layer, by comparison to 1. The results show that the elimination of cis olefin defects in these polymers is important for the optimization of LEDs.

Effect of 4-hydroxytamoxifen isomers on growth and ultrastructural aspects of normal human breast epithelial (HBE) cells in culture

In the search for a breast cancer prevention strategy which would avoid undesirable effects of orally administered tamoxifen, the percutaneous administration of the highly active metabolite 4OHTamoxifen (4OHTam) has been proposed. Percutaneous 4OHTam penetrates the skin to reach breast tissues. It, thus, avoids the hepatic first pass effect, and offers an optimal local/systemic effect. However, trans-4OHTamoxifen can spontaneously isomerize into the cis-isomer, which may have estrogen agonist action. The aim of this study was to examine the effect of cis-4OHTam on normal human breast epithelial (HBE) cells in culture. Spontaneous isomerization of trans- into cis-4OHTam occurred within 24–48 h, but stabilized rapidly at a trans/ cis ratio of 70/30, whether in stock solution, culture medium or cultured cells. The cis-4OHTam did not stimulate HBE cell growth according to histometric cell counts and scanning electron microscopy analysis, but inhibited E 2-induced cell growth, albeit two to three times less than trans-4OHTam. In conclusion, spontaneous isomerization of trans- to cis-4-OHTam is limited and 4OHTam retains a marked antiestrogenic effect. It may prove to be a useful alternative to tamoxifen in breast cancer prevention, especially if administered percutaneously.

Effect of dietary conjugated linoleic acids on the distribution of fatty acids in serum lipoprotein fractions and different tissues of growing pigs.

The effect of conjugated linoleic acid (CLA) on blood lipids [total cholesterol, high-density lipoprotein (HDL), low-density lipoprotein (LDL) and triacylglycerols (TAG)] and the fatty acid distribution of the lipoprotein fractions, backfat, muscle fat, and liver lipids were examined in an experiment with two groups of 40 pigs [Pietrain x (Landrace x Large White)] each. The 20 female and 20 male castrated pigs of each group were fed with isoenergetic and isonitrogenous diets supplemented with either 20 g/kg rapeseed oil (control) or 20 g/kg CLA-TAG. The CLA preparation contained 54.2% pure CLA consisting of approximately two-thirds cis,trans/trans,cis-isomers and one-third trans,trans-isomers. The fatty acids of lipoproteins, backfat, muscle lipids and liver lipids were analysed by gas chromatograph (GC). CLA supplementation did not significantly influence blood lipids and the LDL to HDL ratio. In the CLA-fed pigs the very low-density lipoprotein (VLDL) contained higher saturated fatty acid (SFA) concentrations at the cost of the monounsaturated fatty acids (MUFA). The percentage of polyunsaturated fatty acids (PUFA) remained unchanged. The highest CLA content was analysed in VLDL (4.00%) followed by LDL (2.78%) and HDL (1.45%). The ratio of cis,cis to trans,trans isomers increased from VLDL over LDL to HDL. The content of SFA, probably in backfat and muscle lipids, increased whereas the part of MUFA decreased as a result of reduced Delta9-desaturase activity. The percentage of PUFA (without CLA) was higher in backfat of the control group in accordance with the dietary PUFA supply. This shift in the fatty acid distribution was not observed in the liver lipids. In all the three tissues analysed, the CLA-fed pigs had a significantly increased CLA content: the highest increase was in the backfat (5.65%), followed by liver lipids (2.41%), and muscle lipids (1.47%). An isomer-specific accumulation was observed for cis,cis-CLA isomers in muscle, and for trans,trans-CLA isomers in backfat. We conclude that CLA supplementation results in a higher SFA content in backfat and muscle lipids but not in liver lipids. There is a discrimination of the trans-10, cis-12 isomer and the trans,trans isomers in the formation of the cell membranes.

Succession of microbiota estimated by phospholipid fatty acid analysis and changes in organic constituents during the composting process of rice straw

The succession of responsible microbiota during the composting process of rice straw (RS) was studied for 145 d in relation to the changes in the organic constituents of RS. During the composting process, the C / N ratio of RS decreased from 56 to 22. On a C basis, the relative contents of lipid, water-soluble organic matter (WSOM), hemicellulose, cellulose, and lignin fractions in RS changed from 5.6, 8.9, 32.9, 17.9, and 34.0%, to 7.3, 5.8, 30.7, 3.8, and 51.1%, respectively, indicating that the cellulose fraction was mainly decomposed in the composting process. Biomass C accounted for 18.3% (on day 75 when the total amount of phospholipid fatty acids (PLFAs) reached a peak) and 11.5% (at the end of composting) of the total C of RS under the composting process. As for PLFAs, the biomarkers of fungi and Gram-negative bacteria predominated in the RS material used. At the thermophilic stage (the first 2 weeks), biomarkers of Gram-positive bacteria and actinomycetes predominated. After the thermophilic stage, biomarkers of other Gram-positive bacteria became dominant. Finally, at the curing stage, the proportion of the biomarkers of Gram-negative bacteria and eukaryotes increased, indicating the co-contribution of Gram-positive and Gram-negative bacteria and fungi in the decomposition process at this stage. The trans / cis ratio of 16 : 1ω7 PLFA of RS under the composting process ranged from 0.18 to 0.30, indicating that the composting process of RS prepared a significantly lower environmental stress (p < 0.01) compared to the decomposition of RS in a submerged paddy soil.

Comparison of phospholipid fatty acid composition in percolating water, floodwater, and the plow layer soil during the rice cultivation period in a Japanese paddy field

The composition of phospholipid fatty acids (PLFAs) in the percolating water (PW), the floodwater (FW), and the plow layer soil (PLS) in a paddy field was compared during the period of rice cultivation. The amounts of PLFAs in PW, FW, and PLS ranged from 19.2 to 40.3 μg L−1, from 41.6 to 72.3 μg L−1, and from 56.4 to 75.3 μg g−1 dry-soil, respectively. There was no appreciable seasonal variation in the amount of PLFAs in PW and PLS. However, the amount of PLFAs in FW tended to decrease after the middle stage of rice cultivation. Dominant PLFAs in PW and FW were 16 : 1ω7c, 18 : 1ω7, and 16 : 0 PLFAs, and those in PLS were 16 : 1ω7c, 18 : 1ω7, 16 : 0, 18 : 1ω9, i15 : 0, ai15 : 0, and 10Me17 : 0 PLFAs. The proportion of straight, mono-nnsaturated PLFAs and branched-chain PLFAs ranged from 36.6 to 64.8% and from 9.8 to 23,8% in PW, from 49.2 to 63.7% and from 12.6 to 27.5% in FW, and from 23.4 to 28.1% and 41.7 to 47.7% in PLS, respectively. Principal component analysis of the PLFA composition enabled to clearly differentiate the community structure of the microbiota in PW and FW from that in PLS. A potential indicator of the environmental stress imposed upon the microbiota expressed by the trans vs. cis ratio of 16 : 1ω7 PLFA in the PLFA composition was consistently lower in PW and FW than in PLS, suggesting that the microbial community in PW was not appreciably stressed from the percolation through PLS.

Anionic ligand effect on the nature of epoxidizing intermediates in iron porphyrin complex-catalyzed epoxidation reactions.

We have studied an anionic ligand effect in iron porphyrin complex-catalyzed competitive epoxidations of cis- and trans-stilbenes by various terminal oxidants and found that the ratios of cis- to trans-stilbene oxide products formed in competitive epoxidations were markedly dependent on the ligating nature of the anionic ligands. The ratios of cis- to trans-stilbene oxides obtained in the reactions of Fe(TPP)X (TPP = meso-tetraphenylporphinato dianion and X(-) = anionic ligand) and iodosylbenzene (PhIO) were 14 and 0.9 when the X(-) of Fe(TPP)X was Cl(-) and CF(3)SO(3)(-), respectively. An anionic ligand effect was also observed in the reactions of an electron-deficient iron(III) porphyrin complex containing a number of different anionic ligands, Fe(TPFPP)X [TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion and X(-) = anionic ligand], and various terminal oxidants such as PhIO, m-chloroperoxybenzoic acid (m-CPBA), tetrabutylammonium oxone (TBAO), and H(2)O(2). While high ratios of cis- to trans-stilbene oxides were obtained in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl(-) and OAc(-), the ratios of cis- to trans-stilbene oxide were low in the reactions of iron porphyrin complexes containing nonligating or weakly ligating anionic ligands such as SbF(6)(-), CF(3)SO(3)(-), and ClO(4)(-). When the anionic ligand was NO(3)(-), the product ratios were found to depend on terminal oxidants and olefin concentrations. We suggest that the dependence of the product ratios on the anionic ligands of iron(III) porphyrin catalysts is due to the involvement of different reactive species in olefin epoxidation reactions. That is, high-valent iron(IV) oxo porphyrin cation radicals are generated as a reactive species in the reactions of iron porphyrin catalysts containing nonligating or weakly ligating anionic ligands such as SbF(6)(-), CF(3)SO(3)(-), and ClO(4)(-), whereas oxidant-iron(III) porphyrin complexes are the reactive intermediates in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl(-) and OAc(-).

Massive Steric Hindrance in Two‘Thiocarbonyl Ylides': Cycloadditions with Tetra-Acceptor-Substituted Ethylenesvia Zwitterionic Intermediates

The switch from a concerted to a two-step pathway of 1,3-dipolar cycloadditions was recently established for the reactions of sterically hindered ‘thiocarbonyl ylides' with acceptor ethylenes. This mechanism via zwitterionic intermediates is studied here for 1,3-dipoles 5A and 5B, which are derived from 2,2,5,5-tetramethylcyclopentanethione and 1,1,3,3-tetramethylindan-2-thione, respectively, and contain a highly screened reaction center. In the reactions of 8A and 8B (the precursors of 5A and 5B) with dimethyl 2,3-dicyanofumarate (15) and 2,3-dicyanomaleate (16), virtually identical ratios of cis- and trans-thiolanes were observed (17/18 93 : 7 for 5a and 94 : 6 for 5B). Thus, full equilibration of rotameric zwitterions precedes cyclization; an anteceding disturbing isomerization 15⇌16 had to be circumvented. The cis,trans assignment of the cycloadducts rests on three X-ray analyses. The kinetically favored cis-thiolanes 17 isomerize at >80° to 18 (trans), and irreversible cleavage leads to thione 7 and trans,cis isomeric dimethyl 1,2-dicyanocyclopropane-1,2-dicarboxylates (27 and 28, resp.). Furthermore, the zwitterionic intermediates equilibrate with the cyclic seven-membered ketene imine 21, which was intercepted under conditions where the solvent contained 2 vol-% of H2O or MeOH. Lactams 22 were obtained with H2O in high yields, and the primary products of capturing by MeOH were the cyclic ketene O,N-acetals 23, which subsequently tautomerized to the lactim methyl ethers 24. When 5B was reacted with ethenetetracarbonitrile in CDCl3/MeOH (98 : 2 vol-%), the analogous cyclic ketene imine 13B was trapped to the extent of 93%.

Uptake and effects of the cypermethrin-containing sea lice treatment Excis® in the marine mussel, Mytilus edulis

Wild and farmed mussels, Mytilus edulis, coexist with salmon farms in Scottish sea lochs. A synthetic pyrethroid, cypermethrin, is licensed for use on fish farms to control sea lice infestations as a formulation called Excis®. In this study, uptake of cypermethrin from Excis® exposure is investigated through the use of gas chromatography with mass-spectrometry. The effects of Excis® on mussels are also examined by measuring the neutral red retention time of lysosomes, aerial survival and shell closure. The isomeric ratios of cis:trans cypermethrin measured in mussels are around 80:20; a marked increase from 40:60 to which the mussels were exposed. This is most likely due to preferential metabolism of trans-isomers, as the same response is seen in vertebrates. There is a pronounced behavioural effect of shell closure, where mussels exposed to 1000 μg/l cypermethrin shut their shells within an hour of exposure. Arguments are presented for this effect being either a voluntary response on recognition of cypermethrin, or an effect arising from an involuntary action of cypermethrin on the adductor muscle. Even at 1000 μg/l cypermethrin, neutral red retention time and aerial survival are not affected. The data suggest that the responses of mussels shown here are unlikely to occur in the field, even at the concentrations of cypermethrin used in fish cages, for the treatment of sea lice.

Electronic effects in butadiene hydrogenation catalysed by the transition metals

Buta-1,3-diene hydrogenation to but-1-ene, trans-but-2-ene and cis-but-2-ene has been catalysed by evaporated films of the majority of the elements of Groups 3–11 of the Periodic Table. The effects on product composition of H-absorption into the metals of Groups 3–5 and of S-adsorption onto the metals of Groups 6–10 are also described. For the pure metals of each transition series, the but-1-ene yield increases with increasing Pauling electronegativity indicating that an electronic effect governs the overall extent of 1:2-addition. The effect of H-absorption into V, Zr, Nb, and Hf films and the general effect of S-adsorption onto films of the later transition elements is to increase the trans: cis ratio in the but-2-ene which indicates the stabilisation of π-allylic half-hydrogenated states at positively polarised metal atom sites, M δ +. The hydrided and sulphided systems differ in that 1:2-addition by simple hydrogenation of one double bond in the reactant is retained in the former but poisoned in the latter.

Changes in membrane fluidity and fatty acid composition of Pseudomonas putida CN-T19 in response to toluene.

A bacterial isolate, Pseudomonas putida CN-T19, could grow in a two-phase medium with toluene up to 50% (v/v). Changes in fatty acid composition and membrane fluidity of the isolate were investigated to understand how this microorganism responds toluene. The changes in the ratios of unsaturated to saturated fatty acids were insignificant between cells grown with and without toluene. The changes in the ratio of cis- to trans-fatty acids of C16:1 and C18:1 was, however, significantly lower in cells grown with toluene than cells grown without toluene, giving approximately 1.3 and 9.7, respectively. Toluene had a fluidizing effect on the membrane of cells grown without toluene, resulting in decrease in membrane polarization ratio. Less fluidizing effect of toluene on the membrane of cells grown with toluene was observed, giving 11% of polarization percentage, which was significantly lower than 53% in cells grown without toluene. These results suggest that cis/trans isomeration of C16:1 and C18:1 makes cell membranes more rigid to respond toluene, and is an adaptive strategy allowing P. putida CN-T19 to grow in the presence of organic solvent.

A multivariate study of α-tocopherol and hydroperoxide interaction during the oxidation of methyl linoleate

Interactions between α-tocopherol (α-TOH) and lipid hydroperoxides (LOOH) during the oxidation of methyl linoleate were studied in a model system using the response surface methodology statistical technique. When no α-TOH is present, LOOH had only slight pro-oxidative effects on the oxidation of methyl linoleate. High initial levels of α-TOH caused increased consumption and loss of efficiency for this antioxidant. The time for α-TOH consumption and the induction period for the oxidation of methyl linoleate were increased by increasing initial α-TOH concentration but were decreased by increasing initial LOOH as well as by interactions between LOOH and α-TOH. Interactions among LOOH molecules and between LOOH and α-TOH molecules had negative effects on the stability of α-TOH and the rate of oxidation of methyl linoleate as evidenced by the rate of α-TOH consumption, the rate of LOOH formation, the ratio of cis,trans/trans,trans LOOH and the rates of formation of hydroxy and ketodiene compounds during the induction period. Thus, the efficiency of α-TOH as an antioxidant is determined by, as yet unknown, interactions involving α-TOH and/or LOOH which are both present during the induction period.

Regioselective acetylation of 4- t-butylcyclohexanol in presence of β-cyclodextrin and its derivatives

Regioselective acetylation of 4- t-butylcyclohexanol (I) in the presence of β-CD, DMβ-CD, HPβ-CD and β-CD-polymer was carried out with 10–20 times excess acetic anhydride. At 1:10 ratio I and acetic anhydride, the control showed good amount of conversion (99.0%) to 4- t-butylcyclohexyl acetate (II) with a trans/ cis ratio of 3.48 with very little unreacted alcohols present ( trans/ cis ratio 2.5). With increase in concentration of β-CD, although the ester yield was less than the control, the trans/ cis ratio increased steadily from 3.46 for 1:0.1 (yield 57.7%) to 5.49 for 1:1 eq.(yield 74.4%) of I to β-CD. However, the trans ester yield was comparable to the control (76.9%) in the presence of 1 eq. of β-CD. The results indicated selectivity in esterification between the trans and cis alcohols due to inclusion inside β-CD cavity.

A Palladium-mediated DNA Base Pair of a β-C-Nucleoside Possessing a 2-Aminophenol as the Nucleobase

An approach we have used in this study for the incorporation of metal ions into DNA, is the direct modification of a DNA base itself, turning it into a metal-chelating nucleobase wherein two nucleobases are paired through metal coordination. Herein we report the X-ray crystal structure of a synthetic intermediate 6 for the aminophenol bearing nucleoside 3 and its metal coordination properties with Pd2+. The anomeric configuration of the nucleoside was unequivocally determined to be β-form by the X-ray analysis of 6; the structure has been resolved by direct methods (S1R97) and expanded using Fourier techniques (DIRDIF94) using 2628 independent reflections with I>2.00 [sgrave](I) and 425 parameters. Final R (Rw) was 0.037 (0.043): orthorhombic, space group P212121 (#19) with a=16.562(1) Å, b=16.933(1) Å, c=11.205(1) Å, and V=3142.2(4) Å3; Dc =1.369g/cm3 for Z=4, and molecular weight 647.65. This result is consistent with the tentative assignment by our previous 1H NOE differentiation experiments. Detailed 1H NMR studies showed that the nucleoside forms a stable 2:1 complex with Pd2+ with concomitant deprotonation of its phenolic proton. Although there are two possible structures (cis or trans) for the square-planar Pd2+ complex, the ratio of cis to trans was approximately 1:1. The electrospray ionization time-of-flight mass spectrum of the complex also provided clear evidence for the 2:1 complexation.

Olefin oxidation with dioxygen catalyzed by porphyrins and phthalocyanines intercalated in α-zirconium phosphate

Oxidation of cyclohexene and cis-stilbene with dioxygen in presence of metal phthalocyanines or metal tetraphenylporphyrins complexes intercalated in α-zirconium phosphate and isobutyraldehyde were studied. The degradation of free metal complexes in solution in the reaction media was verified. It was observed that the matrix protects the metal complexes from degradation and the activity of the catalytic system is preserved. Oxidation of cyclohexene with intercalated complexes gave epoxide as the predominant product, while allylic oxidation products were obtained in smaller proportion and the product distributions depended on the identity of the individual metal complexes. Since the addition of a free radical inhibitor stops the reaction, a free radical mechanism should be present. Oxidation of cis-stilbene with intercalated metal complexes gives different ratios of cis- to trans-stilbene oxide and of benzaldehyde which depend on the intercalated metal complex, suggesting that in addition to the free radicals there is another active oxidizing agent.

Methyl jasmonate is blowing in the wind, but can it act as a plant–plant airborne signal?

Abstract Interplant communication in nature is beginning to look like a reality with the field demonstration that tobacco plants downwind of damaged sagebrush suffer less herbivory, a response that appears to be mediated by an airborne signal. Sagebrush constitutively releases methyl jasmonate (MeJA), a compound that is highly active in inducing a number of physiological responses in plants. Damage increases the absolute quantity of the MeJA released as well as the proportion of MeJA in the isomeric cis form. Several studies have shown that volatile MeJA, when released in sufficient quantities, can simulate responses elicited by direct MeJA applications. Additionally, the thermodynamically unstable cis isomer, which is responsible for the characteristic jasmine odor, is thought to be the biologically active form of MeJA. To examine the hypothesis that the cis-MeJA release is responsible for the apparent inter-plant communication, we developed methods to: (1) entrain sagebrush constituents in water which preserved the isomeric shift in the MeJA released after damage; (2) chemically manipulate the MeJA trans : cis ratio; and (3) isolate nearly pure cis-MeJA by HPLC. These treatments were applied as aqueous sprays to a natural population of tobacco plants, however, an outbreak of specialist herbivores consumed all treated plants and chemical analysis on previously harvested treated leaf material was inconclusive. The hypothesis is currently being carefully investigated with laboratory experiments.

Scope and limitations of chiral bis(oxazoline) ligands in the copper-catalysed asymmetric cyclopropanation of trisubstituted alkenes

A series of derivatives of 3-methyl-2-buten-1-ol has been used to test the scope and limitations of the copper-catalysed asymmetric cyclopropanation of trisubstituted alkenes by ethyl diazoacetate in the presence of C 2-symmetric bis(oxazoline) ligands. In the best case, a trans/ cis ratio of 91:9, with 92% ee for the major isomer, was obtained from the reaction of the p-methoxybenzoate derivative. The highest ee was 95%, for the trans isomer of a 80:20 diastereomer mixture (acetate derivative).

Effect of solvent on the cis– trans conformational equilibrium of a proline imide bond of short model peptides in solution

Proton NMR spectra of proline-containing short peptides with N-terminal sequences of N-acetyl-prolyl- (Ac-Pro-) N- tert-butoxycarbonyl-phenylalanyl-prolyl- (Boc-Phe-Pro-) and N- tert-butoxycarbonyl-leucyl-prolyl- (Boc-Leu-Pro-) were measured in mixed solvents of hexadeuterodimethylsulfoxide and deuterochloroform (CDCl 3). Population ratios of cis and trans conformers with respect to the proline imide bond and chemical shifts of NH protons were obtained as a function of a CDCl 3 fraction of solvent. With increasing fraction of CDCl 3, the trans percentages of the Ac-Pro- imide bonds increased. On the other hand, those of Boc-Phe-Pro- decreased, and those of Boc-Leu-Pro- exhibited middle tendency. From the solvent-dependent variation of the chemical shifts of the NH protons, intramolecular hydrogen bonds that stabilize the trans form of Ac-Pro- and the cis form of Boc-Phe-Pro- were discussed. For the Ac-Pro- peptides, only the trans forms are found to the compatible with 7-, 10-, and 13-membered hydrogen-bonded rings that would be similar to the ordinary secondary structures, γ- and β-turns and α-helix, respectively. For the cis form of Boc-Phe-Pro-R (R=O-methyl or glycyl-O-ethyl), the hydrogen-bonded structure is found to be similar to the type-VIa β-turn. On the other hand, for Boc-Phe-Pro-Pro-Leu-Gly-NH 2, it has been suggested that two different hydrogen bonds, which are different from that of the type-VIa β-turn, support each other and cooperatively stabilize the cis form.