Diastereotopic Protons Research Articles

Boophiline, an antimicrobial sterol amide from the cattle tick boophilus microplus

AbstractBoophiline (1), a new sterol amide was isolated from the cattle tick Boophilus microplus (Ixodidae). The structure was assigned as N‐[3‐(sulfooxy)‐25ξcholest‐5‐en‐26‐oyl]‐L‐isoleucine by detailed 2D NMR investigations in conjunction with FAB mass spectrometry and acidic hydrolyses. Complete assignment of the diastereotopic methylene protons of the ring system could be deduced from the NMR data. In agar dilution assays, 1 exhibited antifungal properties against Cladosporium cucumerinum and antibacterial activity against Bacillus subtilis and Escherichia coli.

13C−1H and 13C−13C Spin-Coupling Constants in Methyl β-d-Ribofuranoside and Methyl 2-Deoxy-β-d-erythro- pentofuranoside: Correlations with Molecular Structure and Conformation

Methyl β-d-ribofuranoside 5 and methyl 2-deoxy-β-d-erythro-pentofuranoside (methyl 2-deoxy-β-d-ribofuranoside) 6 were synthesized with single sites of 13C-enrichment at each carbon, and a complete set of 13C−1H and 13C−13C spin-coupling constants (37 couplings in 5, 41 couplings in 6) in these molecules were obtained by 1D and 2D NMR spectroscopy. 2JCH coupling signs were determined from the observation of relative cross-peak displacements in 2D TOCSY data. The 13C−1H couplings (one-, two-, and three-bond) were interpreted in structural and conformational terms with assistance from conformational models of 5 and 6 based on conventional two-state pseudorotational analysis of 3JHH values (PSEUROT) and on theoretical predictions of conformational energies and JCH values obtained from ab initio molecular orbital calculations on the ten envelope and planar conformers of β-d-ribofuranose 4 and 2-deoxy-β-d-erythro-pentofuranose (2-deoxy-β-d-ribofuranose) 3. A comparison of theoretical JCH values in 3 and 4 allowed an assessment of the effect of C2 structure (C2 oxy vs C2 deoxy) on coupling magnitudes and signs. Results show that the behavior of related JCH values in 5 and 6 may differ, especially when C2 is involved as a coupled nucleus. In addition, several 1JCH, 2JCH, and 3JCH values in β-d-ribo and 2-deoxy-β-d-ribo rings were found to be sensitive to furanose ring and hydroxymethyl group conformation; in the deoxyribo ring, the presence of diastereotopic protons at C2 produces paired 13C−1H coupling pathways involving H2R and H2S that are complementary, and differences in the resulting paired JCH values are, in some instances, sensitive to ring conformation. JCC values in β-d-ribo and 2-deoxy-β-d-ribo rings also reflect differences in ring structure and geometry, although fewer (e.g., 2JC3,C5, 3JC1,C5, 3JC2,C5) appear useful as conformational probes. The correlations drawn between ring structure/conformation and JCH/JCC magnitude and sign in 5 and 6 will be useful in anticipated applications of these couplings to assess furanose ring conformation/dynamics in DNA and RNA oligomers and in other biomolecules containing β-d-ribo and 2-deoxy-β-d-ribo rings.

Addition of Organolithium Nucleophiles to the Diiron Allenyl Complex [Fe2(CO)6(μ-PPh2){μ-η1:η2α,β-(H)CαCβCγH2}]: Synthesis and Characterization of Organodiiron-Coordinated β,γ-Unsaturated Ketones

The binuclear allenyl complex [Fe2(CO)6(μ-PPh2){μ-η1:η2α,β-(H)CαCβCγH2}] (1) has been prepared, and its reactivity with organolithium nucleophiles is described. Prop-2-yne bromide reacts with [Fe2(CO)7(μ-PPh2)]-Na+, via an SN2 mechanism, to give [Fe2(CO)6(μ-PPh2){μ-η1:η2α,β-(H)CαCβCγH2}], the first example of a phosphido-bridged allenyl complex. The molecular structure of [Fe2(CO)6(μ-PPh2){μ-η1:η2-(H)CαCβCγH2}] (1) was determined by single-crystal X-ray diffraction and shows that the allenyl ligand is coordinated through Cα−Cβ. Variable-temperature 1H and 13C NMR studies reveal a high-energy exchange process that equilibrates the diastereotopic allenyl protons, presumably via a zwitterionic intermediate, as well as two independent trigonal rotations that act to exchange the carbonyl ligands on each unique Fe(CO)3 group. Complex 1 reacts with organolithium reagents (RLi; R = Me, nBu, Ph, C4H3S), via allenyl−carbonyl−nucleophile coupling, to afford the binuclear β,γ-unsaturated ketones [Fe2(CO)5{P(OMe)3}(μ-PPh2)(μ-η1:η2-{RC(O)CH2}CCH2)] (R = Me, 3a; nBu, 3b; Ph, 3c; C4H3S, 3d), and a single-crystal X-ray structure determination of 3a was undertaken to confirm the connectivity of the hydrocarbyl ligand. The most likely mechanism for the formation of 3a−d involves nucleophilic attack of R- at CO to give an acylate intermediate followed by migration of RCO to Cα of the allenyl and protonation of the resulting enolate to give the unstable alkenyl complexes [Fe2(CO)5(μ-PPh2)(μ-η1(C):η1(C):η2(C)-{RC(O)CH2}CCH2)] (R = Me, 2a; Bu, 2b; Ph, 2c; C4H3S, 2d). Finally, substitution of the metal-coordinated ester carbonyl in 2a−d with trimethylphosphite affords 3a−d as stable crystalline products.

Solution conformation of α, β or γ-methylglutamyl-containing derivatives as probes of vitamin K-dependent carboxylase using molecular modelling and nuclear magnetic resonance

In the present study, the conformational behaviour of methyl substituted N-BOC glutamic acid methyl esters (2M, 3T, 3E, 4T, 4E) has been completely characterized through combined NMR and molecular modeling studies. Heteroand homonuclear coupling constants were measured in order to assign the remaining diastereotopic methylene protons at C(3) and/or C(4), and used for comparison with theoretical data. In parallel, the complete conformational analysis of these analogues has been achieved using molecular mechanics and molecular dynamics (MD) methods. The conformation of the glutamyl residue is established by the excellent agreement between the experimental and calculated side chain scalar coupling constants. The theoretical NMR data were calculated taking into account all the accessible conformations and using the averaging methods appropriate for internal motions. There is a significant influence of the methyl group on the conformational behaviour and on the biological relevance of these structures. Steric effect or electrostatic interaction may also have a considerable influence in stabilizing a conformational population in D 2O solution. The conformational preferences of those different analogues in aqueous and methanol solution are discussed in the light of biological results obtained on the vitamin K-dependent carboxylase system.

Conformational analysis of glutamic acid analogues as probes of glutamate receptors using molecular modelling and NMR methods. Comparison with specific agonists

The activity of five glutamic acid analogues substituted in position 3 or 4 by a methyl ( 3T, 3E, 4T, and 4E) or a methylene group ( 4M) has been examined at one cloned Glu receptor subtype, MGluR 1. These analogues interact with glutamate receptors of the central nervous system, especially the ligand 4T [(2 S,4 S)-4-methylglutamic acid] at the metabotropic glutamate receptor mGluR 1. It was observed that only the 4T isomer is as potent an agonist as glutamic acid, whereas other isomers are less active. Furthermore, 4E [(2 S,4 R)-4-methylglutamic acid] exhibited an exceptional selectivity for the KA ionotropic receptor subtype while 4M [(2 S)-4-methyleneglutamic acid] was active at the NMDA receptors. These molecules represent suitable tools among a population of similar glutamate analogues for a classical structure-function relationship study. We have undertaken a conformational analysis by 1H and 13C NMR spectroscopy and molecular modelling of these molecules. Hetero- and homonuclear coupling constants were measured in order to assign the diastereotopic methylene protons at C(3) or C(4), and used for comparison in molecular dynamics (MD) simulations. The hydrogen-bonding possibility, steric effects or electrostatic interactions may be a considerable influence in stabilizing a conformational population in D 2O solution. The conformations may be grouped by the two backbone torsion angles, χ 1 [α-CO − 2-C(2)-C(3)-C(4)] and χ 2 [ +NC(2)-C(3)-C(4)-γCO − 2] and by the two characteristic distances between the potentially active functional groups, αN ???—γCO − 2 ( d 2) and αCO − 2—γCO − 2 ( d 2). The conformational preferences in solution of 4T, 4E and ( 3T, 3E, 4M) are discussed in the light of the physical features known for a specific metabotropic agonist (ACPD) and specific ionotropic agonists (KA) and (NMDA), respectively.

Determination of Homonuclear Scalar Coupling Constants by Combining Selective Two-DimensionalNMR Spectroscopy with Convolution and Deconvolution: Applications to Paclitaxel (Taxol)

Homonuclear proton-proton coupling consts. can be detd. accurately by recording selective two-dimensional correlation spectra and by analyzing the resulting multiplets by two-dimensional convolution and deconvolution methods. These methods were evaluated by attempting to det. the complete set of homonuclear couplings in paclitaxel (taxol). Convolution methods have been applied successfully to ten pairs of complementary multiplets recorded by selective correlation spectroscopy (soft-COSY) and pure in-phase correlation spectroscopy (PICSY). These pairs of multiplets gave reliable measurements of ten active couplings. In some cases, satisfactory PICSY spectra could not be obtained, so that the complementary information of soft-COSY and PICSY could not be exploited. Furthermore, couplings involving two strongly-coupled diastereotopic protons (14Ha and 14Hb) could not be detd. in satisfactory manner. Further problems arose due to partial overlap of the chem. shifts of two other protons (10H and 13H). Deconvolution has been applied successfully to all 14 soft-COSY multiplets, yielding 14 ests. of active couplings and 38 ests. of passive splittings. This work provides between one and seven ests. for 21 coupling consts. The accuracy of these different ests. must be weighed with the complexity of the methods used. This work may help to assess the perspectives of completely automated computer-supported anal. of two-dimensional spectra. [on SciFinder (R)]

Metal(II) N2S2 Schiff-base complexes incorporating pyrazole or isoxazole (M = Ni, Cu or Zn). Spin states, racemization kinetics and electrochemistry

A. la Cour, M. Findeisen, A. Hazell, R. Hazell and G. Zdobinsky, J. Chem. Soc., Dalton Trans., 1997, 121 DOI: 10.1039/A602281B

Organometallic Macrocycle Chemistry. 5.1 σ-Vinyl and σ-Aryl Complexes of Ruthenium(II) Ligated by 1,4,7-Trithiacyclononane: X-ray Crystal Structure of [Ru(CHCH2)(CO)(PPh3)([9]aneS3)]PF6·2CH2Cl2

The reaction of coordinatively unsaturated σ-vinyl complexes [Ru(CRCHR‘)Cl(CO)(PPh3)2] with 1,4,7-trithiacyclononane ([9]aneS3) provides chiral salts of the “half-sandwich” complexes [Ru(CRCHR‘)(CO)(PPh3)([9]aneS3)]+. The synthesis of the σ-aryl complex [Ru(C6H4Me-4)(CO)(PPh3)([9]aneS3)]+ is also described. A representative NMR study of one example allowed the assignment of individual chemical shifts for the 12 diastereotopic proton environments of the macrocycle. The crystal structure of [Ru(CHCH2)(CO)(PPh3)([9]aneS3)]PF6.2CH2Cl2 is also reported.

Re-face stereospecificity at C4 of NAD(P) for alcohol dehydrogenase from Methanogenium organophilum and for ( R)-2-hydroxyglutarate dehydrogenase from Acidaminococcus fermentans as determined by 1H-NMR spectroscopy

The two diastereotopic protons at C4 of NAD(P)H are seen separately in 1H-NMR spectra. This fact was used to determine the stereospecificity at C4 of NAD(P) for the NADP-dependent alcohol dehydrogenase from Methanogenium organophilum and for the NAD-dependent ( R)-2-hydroxyglutarate dehydrogenase from Acidaminococcus fermentans. The reduction of NADP + with [ 2H 6]ethanol was found to yield (4 R)-[4- 2H 1]NADPH and the oxidation of (4 R)-[4- 2H 1]NADH with 2-oxoglutarate to yield unlabelled [4- 1H]NAD +. These results indicate that both enzymes are Re-face stereospecific at C4 of the pyridine nucleotides.

C5′ Methylene Proton Signal Assignment of DNA/RNA Oligomers Labeled with C5′-Monodeuterated Nucleosides by1H-31P HSQC Spectroscopy

A new strategy for assigning the methylene protons attached to the C5′ of nucleic acids is described for a DNA dodecamer duplex, d(CGCGAATTCGCG)2. It employs 2D 1H–31P HSQC spectroscopy for DNA composed of deoxyribonucleotides, having one deuteron at their C5′ positions. Due to the lack of scalar as well as dipolar coupling between the C5′ methylene proton pair, the cross peaks for the residual H5′/H5″ protons and the 31P nuclei (at the 5′-terminal side) appear as much narrower signals, as compared to those of the intact proton pairs in the unlabeled dodecamer. The overall resolution and sensitivity of the 1H–31P HSQC cross peaks for the residual signals of the deuterated C5′-methylenes have thus been drastically improved over those for the unlabeled oligomer. All of the H5′/H5″ signals of the dodecamer are clearly observed, except for the unphosphorylated 5′-terminus, and each has been assigned. Most of the diastereotopic proton pairs at C5′ showed different relative peak intensities, which may be related to the fact that the present C5′ monodeuteration procedures proceed slightly stereoselectively, namely 60–70% 2H at H5′ (pro-S) and 40–30% 2H at H5″ (pro-R), respectively. The possibility of correlating these peak intensity difference with the stereospecific assignment is discussed.

Treatment of NOE constraints involving equivalent or nonstereoassigned protons in calculations of biomacromolecular structures

Two modifications to the commonly used protocols for calculating NMR structures are developed, relating to the treatment of NOE constraints involving groups of equivalent protons or nonstereoassigned diastereotopic protons. Firstly, a modified method is investigated for correcting for multiplicity, which is applicable whenever all NOE intensities are calibrated as a single set and categorised in broad intensity ranges. Secondly, a new set of values for 'pseudoatom corrections' is proposed for use with calculations employing 'centre-averaging'. The effect of these protocols on structure calculations is demonstrated using two proteins, one of which is well defined by the NOE data, the other less so. It is shown that failure to correct for multiplicity when using 'r(-6) averaging' results in overly precise structures, higher NOE energies and deviations from geometric ideality, while failure to correct for multiplicity when using 'r(-6) summation' can cause an avoidable degradation of precision if the NOE data are sparse. Conversely, when multiplicities are treated correctly, r(-6) averaging, r(-6) summation and centre averaging all give closely comparable results when the structure is well defined by the data. When the NOE data contain less information, r(-6) averaging or r(-6) summation offer a significant advantage over centre averaging, both in terms of precision and in terms of the proportion of calculations that converge on a consisten result.

Experimental and Theoretical Studies of the Internal Stereodifferentiation Occurring During the Lithiation of β‐Stereogenic Alkyl Carbamates. Kinetic Resolutions by (‐)‐Sparteine‐Mediated Deprotonation

AbstractThe deprotonation of β‐stereogenic alkyl carbamates 8 with sec‐butyllithium/TMEDA proceeds kinetically controlled with differentiation between the diastereotopic protons at the α‐methylene group. The lithium intermediates 9A and 9B are trapped by electrophiles to give carboxylic acid esters 10A/B and stannanes 11A/B, 13A/B. The diastereomeric ratio of A to B is independent of the electrophile. The highest efficiency is achieved with β‐aryl‐β‐alkyl‐substituted starting compounds 8d–f. The trends in the experimentally observed diastereoselectivities are reflected by the results of semiempirical PM3 calculations on the transition states of the deprotonation reaction. — As a result of double stereodifferentiation, application of the chiral base sec‐butyllithium/(‐)‐sparteine gives rise to efficient kinetic resolution of racemic alkyl carbamates.

Quantitative Determination of Cross-Relaxation Rates in NMR Using Selective Pulses to Inhibit Spin Diffusion

Overhauser effects and hence internuclear distances can be measured accurately with selective experiments designed to suppress spin diffusion. It is essential to consider effects of both transverse and longitudinal relaxationduringselective radiofrequency pulses. Fitting procedures allow one to refine selected cross-relaxation rate constants, and hence determine internuclear distances with improved accuracy. For the sake of illustration, selected cross-relaxation rates are determined that correspond to short- and long-range distances involving two diastereotopic sugar protons in a double-stranded B-DNA dodecamer. Such distances are difficult to distinguish by traditional Overhauser methods because of spin-diffusion effects.

Determination of γ and Stereospecific Assignment of H5‘ Protons by Measurement of2Jand3JCoupling Constants in Uniformly13C Labeled RNA

The conformational analysis of the backbone angle γ (O5‘−C5‘−C4‘−C3‘) and the stereospecific assignment of the H5‘(pro-S), H5‘(pro-R) diastereotopic protons in a uniformly 13C,15N labeled RNA oligonucleotide was performed using new Exclusive COSY (E.COSY) type multidimensional heteronuclear NMR experiments designed to measure 3J(H4‘,H5‘) homonuclear and 2J(C4‘,H5‘) heteronuclear coupling constants. The experiments were demonstrated on a uniformly 13C,15N labeled 19-mer RNA hairpin (5‘-rGCACCGUUGGUAGCGGUGC-3‘) derived from the RNA I transcript involved in Col E1 replication control. From the small 3J(H4‘,H5‘) couplings constants observed for the RNA hairpin, it was concluded that all γ angles assume a gauche+ rotamer conformation (γ = 60°). From the signs of the 2J(C4‘,H5‘) coupling constants, the H5‘ protons were stereospecifically assigned. In the helical stem region of the hairpin, the H5‘(pro-S) protons were found to resonate downfield (∼0.4 ppm) of the H5‘(pro-R) protons. In the loop region of the hairpin, the chemical shift differences between the H5‘(pro-S), H5‘(pro-R) resonances were found to be smaller, and in most cases, the H5‘(pro-S) protons were found to resonate upfield of the H5‘(pro-R) protons. The different chemical shift patterns observed for the H5‘(pro-S) and H5‘(pro-R) protons in the two secondary structure elements are discussed.

Conformational analysis by NMR spectroscopy and molecular simulation in water of methylated glutamic acids, agonists at glutamate receptors

We have undertaken a conformational analysis by 1H and 13C NMR spectroscopy and molecular modelling of five glutamic acid analogues substituted in position-3 or -4 by a methyl (3T, 3E, 4T, 4E) or a methylene group (4M). These analogues interact with glutamate receptors of the central nervous system, especially the ligand 4T, at one cloned metabotropic receptor (mGluR1a), and the ligand 4E, at the ionotropic receptor (KA). A combination of one- and two-dimensional NMR techniques was used to completely assign the 1H and 13C NMR chemical shifts of the different isomers according to the geometrical isomerism of the methyl. Hetero- and homo-nuclear coupling constants were measured in order to assign the diastereotopic methylene protons at C(3) or C(4), and used for comparison in molecular dynamics (MD) simulations. The system with two carbon–carbon single bonds allows the possibility of trans and gauche forms ‘A, B, C’ and ‘a, b, c’ corresponding to the χ1 and χ2 backbone torsion angles, respectively. The hydrogen-bonding possibility, steric effect or electrostatic interaction may be a considerable influence in stabilizing the different conformations in D2O solution. The conformations may be grouped by the two characteristic distances between the potentially active functional groups, αN+⋯γCO2–(d1) and αCO2–⋯γCO2–(d2) and by the two backbone torsion angles, χ1[α-CO2––C(2)–C(3)–C(4)] and χ2[+NC(2)–C(3)–C(4)–γCO2–].

Conformational Analysis of (2-Substituted-alkyl)phosphoryl Compounds. 1. NMR Spectroscopic Studies of Dialkyl (2-Hydroxyalkyl)phosphonates and Their Carboxylic Esters

Twelve (2-substituted-alkyl)phosphonates (2, 3) were synthesized and conformational analyses carried out by NMR spectroscopy. The conformational populations in six solvents with different polarities and in the presence of metal ions (Li+, Na+, Zn2+), were studied. Steric effects were studied by variation of the sizes of alkyl groups R1 and R2 and intramolecular interactions studied by derivatization of the hydroxy group X. In (2-hydroxyalkyl)phosphonates the most stable conformer ga, in which the phosphoryl group is gauche to the hydroxy group and anti to the alkyl group (R1), is favored particularly in less polar solvents such as C6D6 and CDCl3. In polar solvents the population of the less stable conformer ag, in which the phosphoryl group is gauche to the alkyl group (R1) and anti to the hydroxy group, is increased. Smaller alkyl groups R1 (Me, Prn) allow more conformational freedom. Conversion of the hydroxy group to its carboxylic ester reduces the population of conformer ga so that, for R1 = Prn, conformers ga and ag are approximately equally populated. The presence of metal salts in the acetone-d6 solutions causes an increase in the population of conformer ga in both (2-hydroxyalkyl)phosphonates and their carboxylic ester derivatives. The chemical shift difference between the diastereotopic α-methylene protons has been shown to be proportional to the population of conformer ga.

Heteroatom-directed lithiations of chiral alkyl carbamates: A powerful tool for enantioselective synthesis

Abstract

{H(3-(t)Bupz)B(3-(t)Bupz)(2)-eta(2)}AlEt(2) and {H(3-(t)Bupz)B(3-(t)Bupz)(5-(t)Bupz)-eta(2)}AlEt(2). Structure, Dynamic Solution Behavior, and the 1,2-Borotropic Shift.

HB(3-(t)Bupz)(3)Tl and AlEt(3) in benzene yield {H(3-(t)Bupz)B(3-(t)Bupz)(2)-eta(2)}AlEt(2), 1, as a hydrocarbon-soluble crystalline solid. Compound 1 is also obtained in a related reaction involving ClAlEt(2) via a preferential metathesis of the Al-Cl bond. Crystal data for 1 at -101 degrees C: a = 11.770(3) Å, b = 11.054(3) Å, c = 21.973(6) Å, beta = 95.57(1) degrees, Z = 4, space group P2(1)/a. In 1 the Al center is four-coordinate with Al-C = 1.97(1) Å and Al-N = 1.99(1) Å and with C-Al-C = 127 degrees and N-Al-N = 101 degrees being the largest and smallest angles, respectively. The average N-B-N angle is 109(1) degrees. In toluene-d(8) and tetrahydrofuran-d(8), 1 shows two types of 3-(t)Bupz groups in the integral ratio 2:1 and two distinct ethyl ligands. At low temperature there is a broadening of the 3-(t)Bupz singlet that is assigned to the eta(2)-(t)Bupz ligands. Up to +60 degrees C, compound 1 is nonfluxional on the NMR time scale but does isomerize to {H(3-(t)Bupz)B(3-(t)Bupz)(5-(t)Bupz)-eta(2)}AlEt(2), 2. Crystal data for 2 at -172 degrees C: a = 29.235(5) Å, b = 11.298(1) Å, c = 22.033(3) Å, beta = 129.66(1) degrees, Z = 8, space group = C2/c. In 2 there is a pseudotetrahedral Al center with Al-C = 1.97(1) Å (average) and Al-N = 1.95(1) Å (average) and with C-Al-C = 119 degrees and N-Al-N = 98 degrees as the largest and smallest angles, respectively. The average N-B-N angle is 108(1) degrees. In 2 the eta(2)-tris(alkylpyrazolyl)borate ligand isomerizes by a 1,2-borotropic shift to give one 5-(t)Bupz fragment that is part of the eta(2)-N,N' aluminum-bonded ligand. Variable-temperature (1)H NMR spectra of 2 in toluene-d(8) and THF-d(8) reveal temperature-dependent exchange involving the 3-(t)Bupz moieties, with more rapid site exchange in toluene-d(8) than in THF-d(8). At low temperature there are two ethyl signals, one of which indicates diastereotopic methylene protons, as well as three (t)Bu signals in the ratio 1:1:1. The dynamic behavior of 2 is consistent with an eta(2) right harpoon over left harpoon eta(3) exchange process as opposed to an eta(2) right harpoon over left harpoon eta(1) exchange wherein the Al center is transiently three-coordinate. The isomerization of 1 to 2 has been studied in benzene-d(6) (DeltaH() = 21.0(2) kcal/mol, DeltaS() = -15(1) eu) and THF-d(8) (DeltaH() = 18.3(4) kcal/mol, DeltaS() = -15(1) eu) and compared to a related isomerization involving {H(2)B(3-(t)Bupz)(2)-eta(2)}AlMe(2) reported by Parkin and Looney [Polyhedron 1990, 9, 265] in benzene-d(6) (DeltaH() = 34.5(8) kcal/mol, DeltaS() = 6(2) eu). It is proposed that the rate-determining 1,2-borotropic shift in the 1 --> 2 reaction occurs in a noncoordinating (t)Bupz group and that this is followed by a rapid associative interchange of pz groups wherein the sterically less demanding 5-(t)Bupz moiety remains bound to the metal.

NMR Studies of Agricultural Compounds. Applications of Achiral and Chiral Lanthanide Shift Reagents to the Herbicide, Fenoxaprop-Ethyl

Abstract The 200 MHz H NMR spectra of the herbicide, fenoxaprop-ethyl, 1, have been studied in CDCl3 solution at ambient temperatures as the racemic ester free base with the added chiral lanthanide shift reagent (LSR), tris [3 - (heptafluoropropylhydroxymethylene) - (+) - camphorato]europium(III), Eu(HFC)3, 2, with some additional runs using the achiral LSR, tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato) europium(III), Eu(FOD)3, 3, for supplemental spectral simplification. With 2, definite enantiomeric shift differences (ΔΔδ) were observed only for the CH3CH2O resonance. Although increased spectral complexity for the CH2O signal was seen with added 2, this could have resulted from anisochrony of the diastereotopic protons, Ha and Hb, of this methylene group rather than true ΔΔδ, since the achiral 3 led to near-baseline separation between the CHaHbO signals. Lanthanide-induced shift (LIS) magnitudes were interpreted as consistent with predominant LSR binding at the ester carbonyl.

Secondary Kinetic Isotope Effects of Diastereotopic Protons in Pericyclic Reactions: A New Mechanistic Probe

Secondary Kinetic Isotope Effects of Diastereotopic Protons in Pericyclic Reactions: A New Mechanistic Probe