Concise synthesis of AHMHA unit in perthamide C. Structural and stereochemical revision of perthamide C

Diastereomers of 3-amino-2-hydroxy-6-methylheptanoic acid (AHMHA), a new amino acid unit in perthamides C and D, have been synthesized from commercially available 4-methylpentanol in a concise manner and 50% average overall yield. Comparison of the ¹H and ¹³C NMR data, optical rotation data and Marfey’s analysis of the resulting isomers with the natural fragment unambiguously allowed the configurational assignment of the natural residue as (2R,3R). A structural revision of perthamides C and D is also reported.     

A H, C and N NMR spectroscopic and GIAO DFT study of ethyl 5-oxo-2-phenyl-4-(2-phenylhydrazono)-4,5-dihydro-1H-pyrrole-3-carboxylate

¹⁵N-Labelled ethyl 5-oxo-2-phenyl-4-(2-phenylhydrazono)-4,5-dihydro-1H-pyrrole-3-carboxylate was synthesized by azo-coupling of diazotized aniline (using Na¹⁵NO₂ , 99% ¹⁵N) with ethyl 4,5-dihydro-5-oxo-2-phenyl-(1H)-pyrrole-3-carboxylate. The product was formed as a tautomeric hydrazone mixture as confirmed by ¹³C and ¹⁵N chemical shifts, and was obtained as a mixture of E and Z isomers according to ⁿJ(¹⁵N, ¹³C). A comparison of the ¹H NMR data with GIAO DFT calculations enabled determination of the configuration of the carboxy ester group in both isomers.     

Synthesis and NMR spectra of the syn and anti isomers of substituted cyclobutanes—evidence for steric and spatial hyperconjugative interactions

The syn and anti isomers of cis,cis-tricyclo[5.3.0.0^2,6]dec-3-ene derivatives have been synthesized and their ¹H and ¹³C NMR spectra unequivocally analyzed. Both their structures and their ¹H and ¹³C NMR chemical shifts were calculated by DFT, the latter two calculations employing the GIAO perturbation method. Additionally, calculated NMR shielding values were partitioned into Lewis and non-Lewis contributions from the bonds and lone pairs involved in the molecules by accompanying NBO and NCS analyses. The differences between the syn and anti isomers were evaluated with respect to steric and spatial hyperconjugation interactions.     

quantitation of p- and m-methylstyrenes by liquid chromatography and 13C nuclear magnetic resonance spectrometry

The molar ratio of p- and m-methylstyrene in mixtures is determined by liquid chromatography and ¹³C NMR spectrometry. In the former instance, separation and quantitations are achieved using an adamantyl-modified column with a mobile phase of ethanol/water (30 + 70, v/v) containing 0.01 M β-cyclodextrin. In the latter instance, a ¹H-decoupled ¹³C NMR technique is described for the determination of the positional isomers. The results obtained by both methods are self-consistent, with a standard deviation for the molar ratio of the para and meta isomers of less than ±0.01.     

Determination of astaxanthin and astaxanthin esters in the microalgae <Emphasis Type="Italic">Haematococcus pluvialis</Emphasis> by LC-(APCI)MS and characterization of predominant carotenoid isomers by NMR spectroscopy

The oily product ZANTHIN® consists of natural astaxanthin, which is manufactured from the microalgae Haematococcus pluvialis by supercritical CO₂ extraction. An HPLC method was developed to separate all of the components of the complex astaxanthin extract using a C₃₀ column. The separation resulted in different isomers of astaxanthin accompanied by two other carotenoids. The main component consisted of astaxanthin singly esterified with several different fatty acids. C18:3, C18:2, C18:1 and C16:0 were identified as the most commonly occurring fatty acids. Doubly esterified astaxanthin was also found, although in lower concentrations compared to singly esterified astaxanthin. After performing a detailed fatty acid analysis by GC-MS, the peaks from the extract were assigned via HPLC-MS. A trans to cis transmutation of the all-trans compound was performed by thermal treatment in order to obtain an enrichment of cis isomers as the basis for unambiguous identification via NMR experiments. The all-trans as well as the 9- and 13-cis isomers of astaxanthin were characterized in detail by UV/Vis, ¹H, and ¹H,¹H COSY NMR spectroscopy.     

Investigation of the exo and endo isomers of dioxolane-type benzylidene derivatives of carbohydrates by 13C NMR spectroscopy

The absolute configurations of nine 2,3-O-benzylidene-α-L-rhamno- and α-D-mannopyranoside diasteteomeric pairs were determined and the ¹³C NMR spectra of further thirteen α-L-rhamno- and α-D-mannopyranosides, having various substituents, were completely assigned.Four ¹³C shifts were found suitable for the determination of the absolute configuration of the dioxolane skeleton. (1) The chemical shift of the acetal carbon in the endo isomers is between 103.9 and 104.7 ppm whereas for the exo isomers this region extends from 102.8 to 103.4 ppm; (2) The formation of the dioxolane ring causes a deshielding effect for the bridgehead carbons, in the exo isomers this effect is more pronounced for C-3 whereas in the endo isomers for C-2. For C-4, shielding effect was found in the exo isomers and deshielding effect in the endo ones; (3) The chemical shift of the quaternary carbon of the phenyl group is greater in the exo isomers than in the endo ones; (4) The difference between the shift of the acetal carbon and that of the quaternary carbon of the phenyl group in the exo isomers is greater than 35.4 ppm, in the endo isomers is less than 33.7 ppm.     

Fourier transform NMR investigations of organotin compounds : VIII. Propenyl- and isopropenyl-tin compounds; detection and identification of isomers using 119Sn and 13c NMR

Starting from the equilibrium mixture of cis- and trans-1-bromo-1-propene, isomeric mixtures of compounds Me_n Sn(CH=GHMe)_4-n (n = 0–3) have been prepared and studied. While proton NMR only allows distinction between the methyltin signals of the various isomers (except where n = 3), the ¹³C spectra show separate signals for almost all isomeric carbons even when n = 0. In the ¹¹⁹Sn spectra the signals due to the various isomers are separated by ca. 20 ppm for a given value of n; the peak areas can be used to estimate the proportions of cis- and trans-propenyl residues present in the mixtures. Addition of 2-bromo-propene to the starting 1-bromo-1-propenes leads to the formation of further isomers, which can in all cases be observed and identified in the ¹¹⁹Sn spectra; ¹¹⁹Sn shifts can be calculated using the shifts for the Me₃SnC₃H₅ isomers as increments.     

On the structure of n-sulphonylformamidines

The structure of N²-sulphonylformamidines has been investigated by means of IR, ¹H and ¹³CNMR spectroscopy. The compounds are shown to exist as two rotamers in solution. A previous report on the isolation of Z and E isomers (regarding the CN²-bond) for one N²-methane-sulphonylformamidine is shown to be dubious, the spectroscopical data indicating the two isomers to be rotamers at the C-N¹ -bond.     

(Trifluoromethoxy)Phenylboronic Acids: Structures,Properties, and Antibacterial Activity

Three isomers of (trifluoromethoxy)phenylboronic acids were studied in the context of their physicochemical, structural, antimicrobial and spectroscopic properties. They were characterized by ¹H, ¹³C, ¹¹B and ¹⁹F NMR spectroscopy. The acidity of all the isomers was evaluated by both spectrophotometric and potentiometric titrations. The introduction of the -OCF₃ group influences the acidity, depending, however, on the position of a substituent, with the ortho isomer being the least acidic. Molecular and crystal structures of ortho and para isomers were determined by the single crystal XRD method. Hydrogen bonded dimers are the basic structural motives of the investigated molecules in the solid state. In the case of the ortho isomer, intramolecular hydrogen bond with the -OCF₃ group is additionally formed, weaker, however, than that in the analogous -OCH₃ derivative, which has been determined by both X-Ray measurements as well as theoretical DFT calculations. Docking studies showed possible interactions of the investigated compounds with LeuRS of Escherichia coli. Finally, the antibacterial potency of studied boronic acids in vitro were evaluated against Escherichia coli and Bacillus cereus.     

Restricted rotation of the olefin and amine ligands in cis-dichloro(p-toluidine)(ethyl vinyl ether)platinum(II)

cis-Dichloro(p-toluidine)(olefin)platinum(II) complexes, in which the olefin is either ethyl vinyl ether or ethylene, have been investigated by ¹H NMR spectroscopy over a range of temperatures (+50 to ?60° C) in order to study the rotation of the olefin ligand.The results give no evidence of rotation of the amine or vinyl ether ligands even at the highest temperature investigated, +50°C; only one of the two possible rotational isomers is present, and this is attributed to the presence of intramolecular hydrogen bonding between the oxygen atom of the vinyl ether and the -NH- group of the amine.In contrast, the amine and vinyl ligands in the ethylene complex rotate freely at room temperature, coalescence being observed above ?25° C for the ethylene protons.     

Natural abundance 17O NMR study of 2- and 4-substituted benzoyl chlorides

Natural abundance ¹⁷O NMR chemical shift data for 17 ortho and para benzoyl chlorides recorded in acetonitrile at 75°C are reported. ¹⁷O NMR data for the para substituted benzoyl chlorides are correlated with ¹⁷O NMR data for similarly substituted acetophenones and methyl benzoates. The ¹⁷O NMR signals for ortho isomers are downfield (ca 30 ppm) from their para isomers; the downfield shifts are consistent with torsion angle change. The ¹⁷O NMR data for the para isomers gave good correlations with σ⁺ constants and with dual substituent parameters (DSP).     

The stereochemistry of 2,4-disubstituted γ-butyrolactones and 2,4,6-trisubstituted-5,6-dihydro-4H-1, 3-oxazines

2,4-Disubstituted butyrolactones and 2,4,6-trisubstituted-5,6 dihydro-4H-1,3-oxazines show similar features in their ¹H and ¹³C- NMR spectra. Two geminal ring hydrogens of cis isomers give rise to a complex ABXY spectra when the substituent is alkyl or aryl. In spectra of trans isomers these patterns are degenerated. When R is OMe(in 4) or OCOMe (in 6) the difference in chemical shifts of geminal protons and vicinal coupling constants cannot be used for diagnosis. In ¹³C spectra ring carbons C-2 and C-3 in lactones and C-4 and C-5 in oxazine of trans isomers show a small but consistent shift to higher fields.     

η5-cyclopentadienyl-η3-1-vinylallylnickel-komplexe

The η⁵-cyclopentadienylpentadienylnickel complexes 5 and 6 contain η³-1-vinylallyl groups. There is no evidence of coordination of the vinylic CC bond to nickel in either the syn or the anti isomers 6a and 6b.¹H NMR saturation transfer experiments indicate that 6b exists in an equilibrium between two valence isomers.     

Synthesis and characterization of all four isomers of methyl 2,4-decadienoate for an investigation of the pheromone components of Pityogenes chaicographus

Methyl (2E,4Z)-2,4-decadienoate (1), an important pheromone component of Pityogenes chalcographus, and its three geometrical isomers (2–4) have been synthesized and obtained in 99 % isomeric purity. Urea inclusion complexes were used in the final purification procedures. Spectroscopic data (MS, ¹H and ¹³C NMR) of all four isomers are discussed. A photoisomerization study of the decadienoates is presented.     

Synthesis and structures of sila-macrobicyclic compounds: syn and anti isomers of hexasilabicyclo[12.8.8]triacontane

Syn and anti isomers of a novel saturated sila-macrobicyclic compound, hexasilabicyclo[12.8.8]triacontane, were synthesized via the functionalization of 1,1,10,10-tetraphenylhexasilacyclooctadecane derivative, which was obtained by the diimide hydrogenation of the corresponding hexasilacyclooctadecahexayne derivative. The structures of these macrocyclic compounds were determined by ¹H, ¹³C, and²⁹Si NMR and X-ray crystallography.     

Geometrical isomerism of the o-substituted oximes of some keto-steroids

O-Benzyl- and/or O-iso-propyl-oximes of a number of 3-, 7- or 20-oxo-steroids and of some 3,20-di-oxosteroids have been prepared. The products were characterized by their C,H,N-content and physical properties and studied by UV, ¹H- and ¹³C-NMR spectroscopy. The ketoximes of 3-oxo-Δ⁴-steroids were invariably found to consist of a mixture of two geometrical isomers, denominated syn and anti. In some cases both isomers could be obtained in a pure state by column-chromatography. Oximes of the 7-oxo-Δ⁵-steroids were found to consist only of the syn isomer, whereas the 20-oximino compounds showed no isomerism.     

1H and 13C NMR study of the conformations of the atropisomers of some 1-(11-naphthyl)-2,4-dioxo-(or 2-thio-4-oxo)-hexahydro-pyriminides

The ¹H and ¹³C NMR spectra of the atropisomers of 6-(or 5-)-methyl-1-(1¹-naphthyl)-2, 4-dioxo-(or 2-thio-4-oxo)-hexahydro-pyrimidines, 1 to 4, are assigned on the basis of NOE and double resonance experiments. The syn-anti configurations of the isomers of 6-methyl-2,4-dioxo-pyrimidine follow from the close similarity of the NMR parameters to those observed with the isomers of the 6-methyl-2-thio derivative of known configuration. That of the 5-methyl derivatives was assigned from NOE and magnetic anisotropy effects of the naphthalene ring and of the (thio) amide group. Allylic strain between the naphthyl and 5-methyl groups makes the axial confomation prefered int he anti isomers. In the syn isomers, however, the equational and axial conformations are approximately equally populated, apparently due to interference between the remote benzene ring and the axial methyl group. The equatorial conformation is prepared with the 5-methyl derivatives.     

Synthesis and Isomer Composition of 2-Polyfluoroalkoxy-1,3,2-dioxaphospholanes and -phosphinanes

Polyfluoroalkanols readily reacted with 2-chloro-1,3,2-dioxaphospholanes and 2-chloro-1,3,2-dioxaphosphinanes in hexane in the presence of triethylamine (–10 to 25°C, 5 h) to give 2-polyfluoroalkoxy-1,3,2- dioxaphospholanes and 2-polyfluoroalkoxy-1,3,2-dioxaphosphinanes in 48–72% yield. The products were found to exist as mixtures of cis and trans isomers with the trans isomer predominating for the phospholanes and cis isomer predominating for the phosphinanes according to the ¹H, ¹³C, ¹⁹F, and ³¹P NMR data.     

Kinetically and thermodynamically controlled interaction of organo-silicon (-boron) halides with lithium salts of acylphosphines. Influence of solvents and electrophilic catalysts

Kinetically controlled interaction of (R¹PCOR²)Li with R₃SiCl in nonsolvating media gives P-silylated acylphosphines, which rearrange to O-E isomers. Direct formation of O-Z isomers was observed in solvating media with R₂BX and R₃SiX. Equilibria of E/Z isomers exist for B- or Si-derivatives of isobutyrylphosphines. Both isomers rearrange irreversibly to vinylphosphines.     

Influence of conjugation of donor and acceptor on the properties of hydrogen bonds of Cis and trans isomers

Cis and trans, and syn and anti. isomers are studied by IR and NMR in acetonitrile-d₃ solutions. Cis and syn isomers form completely intramolecular N⁺H…N⇌N…H⁺N bonds. They show, however, nearly no proton polarizability since the donor and acceptor are electronically conjugated. The trans and anti isomers form intermolecular N⁺H…N⇌N…H⁺N bonds showing large proton polarizability.