Unambiguous Assignment (unambiguous + assignment)

Distribution by Scientific Domains

Selected Abstracts

Electrospray ionization mass spectrometric study of end-groups in peroxydicarbonate-initiated radical polymerization

Michael Buback
Abstract Initiation by diethyl peroxydicarbonate (E-PDC), di- n -tetradecyl peroxydicarbonate (nTD-PDC), di- n -hexadecyl peroxydicarbonate (nHD-PDC), and di-2-ethylhexyl peroxydicarbonate (2EH-PDC) of free-radical polymerizations of methyl methacrylate in benzene solution was studied by end-group analysis via electrospray ionization mass spectrometry (ESI-MS). Unambiguous assignment of ESI-MS peaks allows for identification of the type of radical that starts chain growth. In case of initiation by dialkyl peroxydicarbonates with linear alkyl groups, almost exclusively alkoxy carbonyloxyl species, which are the primary fragments from initiator decomposition, occur as end-groups. With 2EH-PDC, however, both the primary 2-ethylhexoxy carbonyloxyl fragment and a second moiety, which is formed by decarboxylation of the 2-ethylhexoxy carbonyloxyl radical, are clearly observed as end-groups. The decarboxylation process is described by a concerted mechanism which involves a 1,5-hydrogen shift reaction. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6071,6081, 2008 [source]

Low-temperature 1H and 13C NMR spectra of N -substituted 1,2,3,4-tetrahydropyrazino[1,2- a]indoles

Alan R. Katritzky
Abstract The temperature-dependent 1H and 13C NMR spectra of 2-(2-butynyl)-10-methyl-1,2,3,4-tetrahydropyrazino[1,2- a]indole (4) (as a representative example of 1,9) in CFCl3 + CD2Cl2 solution are described and discussed. Below 183 K, the hexahydropyrazine ring inversions become slow on the NMR time-scale and 4 exists in principle as two conformational diastereomers. In fact, only one was observed with the N-2 substituent in an equatorial position as shown by a low-temperature NOESY experiment. The energy barrier for conformational interchange was calculated from NMR data to be 8.3 kcal mol,1 (1 kcal = 4.184 kJ), in agreement with quantum chemical calculations. Unambiguous assignments for all proton and carbon resonances of 1,9 were made using 1D (APT, DEPT, NOE difference) and 2D (COSY, NOESY, gHMQC, gHMBC) NMR techniques. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Absolute Conformations of the (,)-[9](2,5)Pyridinophane Molecule

Maxim Fedorovsky
Abstract [9](2,5)Pyridinophane was first synthesized, and its enantiomers were separated, more than 40 years ago, but the molecule's absolute conformations could not be determined up to now. We show here, by the comparison of measured and computed vibrational optical activity (VOA), that the CIP descriptor (P) applies to the (,)-enantiomer. This assignment is based on the VOA of bands from vibrations localized on the pyridine ring bent by the tense (CH2)9 chain extending from position 2 to 5. The VOA of vibrations localized on the chain is in agreement with this assignment. Its behavior differs from the VOA of the bent pyridine ring, and conclusions drawn from the chain's VOA alone would not be sufficient, because the close-to-enantiomorphic geometries of the chain present in some of the 14 conformers of (,)-[9](2,5)pyridinophane lead to VOA with an opposite sign. Understanding of how VOA is generated is crucial for the unambiguous assignment of the molecule's absolute conformations. [source]

The Absolute Configuration of (+)-Ethyl cis -1-Benzyl-3-hydroxypiperidine-4-carboxylate and (+)-4-Ethyl 1-Methyl cis -3-Hydroxypiperidine-1,4-dicarboxylate; a Revision

Abstract Discrepancies between chiroptical data from the literature and our determination of the structure of the title compounds (+)- 5 and (+)- 9a were resolved by an unambiguous assignment of their absolute configuration. Accordingly, the dextrorotatory cis -3-hydroxy esters have (3R,4R)- and the laevorotatory enantiomers (3S,4S)-configuration. The final evidences were demonstrated on both enantiomers (+)- and (,)- 5 by biological reduction of 4 by bakers' yeast and stereoselective [RuII(binap)]-catalyzed hydrogenations of 4 (Scheme,2), by the application of the NMR Mosher method on (+)- and (,)- 5 (Scheme,3), as well as by the transformation of (+)- 5 into a common derivative and chiroptical correlation (Scheme,4). [source]

NMR Characterization of Complex p- Oligophenyl Scaffolds by Means of Aliasing Techniques to Obtain Resolution-Enhanced Two-Dimensional Spectra

Damien Jeannerat
The usefulness of computer-assisted aliasing to secure maximal resolution of signal clusters in 1H- and 13C-NMR spectra (which is essential for structure determination by HMBC 2D NMR spectroscopy) in minimal acquisition time is exemplified by the complete characterization of the two complementary p -octiphenyls 1 and 2 with complex substitution patterns. The need for digital resolution near 1,Hz/pt to dissect the extensive signal clusters in the NMR spectra of these refined oligomers excluded structure determination under routine conditions. High resolution was secured by exploiting the low signal density in the 13C dimension of HMBC spectra by using computer-assisted aliasing to maximize signal density. Based on the observed shifts in DEPT and 1H-decoupled 13C-NMR spectra of 1 and 2, computer-assisted aliasing allowed to reduce the number of required time increments by a factor of 20 to 30 compared to full-width spectra with identical resolution. Without signal-to-noise constraints, this computer-assisted aliasing reduced the acquisition time for high-resolution NMR spectra needed for complete characterization of refined oligomers 1 and 2 by the same factor (e.g., from over a day to about an hour). With resolved signal clusters in fully aliased HSQC and HMBC spectra, unproblematic structure determination of 1 and 2 is demonstrated by unambiguous assignment of all C- and H-atoms. These findings demonstrate that computer-assisted aliasing of the underexploited 13C dimension makes extensive molecular complexity accessible by conventional multidimensional heteronuclear NMR experiments without extraordinary efforts. [source]

1H and 13C NMR assignments of all three isomeric o -fluoronaphthaldehydes and three o -fluorophenanthrene aldehydes

Carl A. Busacca
Abstract Three isomeric o -fluoronaphthaldehydes, 9-fluorophenanthrene, and three previously unreported o -fluorophenanthrene aldehydes were analyzed in detail by multiple NMR techniques to provide unambiguous assignment of structures and resonances. The six aldehydes serve as the key starting materials for novel chiral ligands used in highly enantioselective rhodium-catalyzed asymmetric hydrogenation reactions. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Investigation of the correlations between 19F and 1H NMR signals for various mono- and di-substituted octafluoro[2.2]paracyclophanes

Alex J. Roche
Abstract A selection of mono- and pseudo ortho di-substituted octafluoro[2.2]paracyclophane derivatives were analyzed using 19F- 1H HOESY, 1H COSY and 19F COSY techniques. This resulted in the unambiguous assignment of the 19F and 1H NMR resonances, and also revealed interesting solvent effects and noteworthy coupling patterns for various JHH, JHF, and JFF interactions, including observable through bond 7JFF and 8JFF couplings. For the four mono-substituted derivatives, the assignments were achieved through the combination of 19F- 1H HOESY, 1H COSY and 19F COSY techniques. The C2 symmetry of the six pseudo ortho di-substituted derivatives that were examined produced simplified spectra, and careful inspection of the characteristic 1H coupling patterns led to the assignment of 1H signals. Therefore only 19F- 1H HOESY experiments were required to complete the assignments for those molecules. Refinements and alternative strategies for previous protocols are presented for the molecules that were less responsive to nuclear Overhauser effect (nOe) experiments. Copyright © 2009 John Wiley & Sons, Ltd. [source]

The use of MPA amide for the assignment of absolute configuration of a sterically hindered cyclic secondary amine by ,mix and shake' NMR method

Jinhai Gao
Abstract We present here a new method using methoxyphenylacetic acid (MPA) as the chiral derivatizing agent (CDA) for the assignment of absolute configuration of cyclic secondary amines. The MPA amides were prepared using the purification-free ,mix and shake' method. A detailed conformational analysis for the two diastereomeric amides was conducted by 2D NMR experiments and molecular mechanics calculations. We have established that, in the most stable conformation of each syn rotamer of MPA amides, the H-, in the MPA moiety is oriented toward the bulky substituent group at the asymmetric carbon in the chiral amine, presumably to avoid steric and/or electrostatic interactions. The observed NMR data were correlated with the conformational model to allow unambiguous assignment of absolute configuration of secondary amines. The results demonstrate that the MPA can be used as a useful CDA in the case of sterically crowded cyclic secondary amines from which the MTPA amides are usually difficult to make. Copyright © 2008 John Wiley & Sons, Ltd. [source]

NMR characterization of new 10-membered-ring macrolactones and dihydrobenzophenazine-5-one, oxidized derivatives of benzo[a]phenazines

Marília O. F. Goulart
Abstract Peroxidation of the phenazine of ,-lapachone using m -ClC6H4CO3HCH2Cl2 furnished a macrolactone with a rigid 10-membered ring, and the corresponding N -oxide, along with a dihydrobenzophenazine-5-one. All of the new compounds were fully characterized by spectroscopic methods, with the unambiguous assignment of the hydrogens and carbon NMR signals for the N -oxide, with the aid of 2-D NMR, mainly COSY, HMQC, HSQC and HMBC. For the other two compounds some signals could not be assigned owing to their own intrinsic features. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Trabecular bone volume fraction mapping by low-resolution MRI

M.A. Fernández-Seara
Abstract Trabecular bone volume fraction (TBVF) is highly associated with the mechanical competence of trabecular bone. TBVF is ordinarily measured by histomorphometry from bone biopsies or, noninvasively, by means of high-resolution microcomputed tomography and, more recently, by micro-MRI. The latter methods require spatial resolution sufficient to resolve trabeculae, along with segmentation techniques that allow unambiguous assignment of the signal to bone or bone marrow. In this article it is shown that TBVF can be measured under low-resolution conditions by exploiting the attenuation of the MR signal resulting from fractional occupancy of the imaging voxel by bone and bone marrow, provided that a reference signal is available from a marrow volume devoid of trabeculation. The method requires accurate measurement of apparent proton density, which entails correction for various sources of error. Key among these are the spatial nonuniformity in the RF field amplitude and effects of the slice profile, which are determined by B1 field mapping and numerical integration of the Bloch equations, respectively. By contrast, errors from variations in bone marrow composition (hematopoietic vs. fatty) between trabecular and reference site are predicted to be small and usually negligible. The method was evaluated in phantoms and in vivo in the distal radius and found to be accurate to 1% in marrow volume fraction. Finally, in a group of 12 patients of varying skeletal status, TBVF in the calcaneus was found to strongly correlate with integral bone mineral density of the lumbar vertebrae (r2 = 0.83, p < 0.0001). The method may fail in large imaging objects such as the human trunk at high magnetic field where standing wave and RF penetration effects cause intensity variations that cannot be corrected. Magn Reson Med 46:103,113, 2001. © 2001 Wiley-Liss, Inc. [source]

Identification of isomeric dicaffeoylquinic acids from Eleutherococcus senticosus using HPLC-ESI/TOF/MS and 1H-NMR methods

Ari Tolonen
Abstract Liquid chromatography,electrospray time-of-flight mass spectrometry (HPLC-ESI/TOF/MS) and a novel NMR technique, developed to maximise the sensitivity obtained from the standard NMR spectrometer, have been applied to the identification of the phenolic constituents of Eleutherococcus senticosus. In addition, molecular modelling and dihedral bond angle calculations based on the vicinal 3JHH -coupling constants have been used in the unambiguous assignment of signals in the 1H-NMR spectra. 5,- O -Caffeoylquinic acid and three isomeric compounds, 1,,5,- O -dicaffeoylquinic acid, 3,,5,- O -dicaffeoylquinic acid and 4,,5,- O -dicaffeoylquinic acid, have been isolated and identified from a sample. The isolation and structure determination of the latter two compounds are reported for the first time from this plant. Copyright © 2002 John Wiley & Sons, Ltd. [source]

mtDNA from hair and nail clarifies the genetic relationship of the 15th century Qilakitsoq Inuit mummies

M. Thomas P. Gilbert
Abstract The 15th century Inuit mummies excavated at Qilakitsoq in Greenland in 1978 were exceptionally well preserved and represent the largest find of naturally mummified specimens from the Arctic. The estimated ages of the individuals, their distribution between two adjacent graves, the results of tissue typing, and incomplete STR results led researchers to conclude that the eight mummies formed two distinct family groups: A grandmother (I/5), two daughters (I/3, I/4), and their two children (I/1, I/2) in one grave, and two sisters (II/6, II/8) and a daughter (II/7) of one of them in the other. Using mtDNA from hair and nail, we have reanalyzed the mummies. The results allowed the unambiguous assignment of each of the mummies to one of three mtDNA haplogroups: A2b (I/5); A2a (I/2, I/3, II/6, II/8); A2a-311 (I/1, I/4, II/7), excluded some of the previous relations, and pointed to new ones. I/5 is not the grandmother/mother of the individuals in Grave I, and she is not maternally related to any of the seven other mummies; I/3 and I/4 are not sisters and II/7 is neither the daughter of II/6 nor of II/8. However, I/1 may be the child of either I/4 or II/7 and these two may be sisters. I/2 may be the son of I/3, who may be the daughter of either II/6 or II/8, and these two may be sisters. The observation of haplogroups A2a and A2b amongst the 550-year-old Inuit puts a lower limit on the age of the two lineages in Greenland. Am J Phys Anthropol, 2007. © 2007 Wiley-Liss, Inc. [source]

Complete and unambiguous assignments of 1H and 13C chemical shifts of new arylamino derivatives of ortho -naphthofuranquinones

Eufrânio N. da Silva Júnior
Abstract Six new nor-,-lapachones have been synthesized from reaction of 3-bromo-nor-,-lapachone with arylamines. These derivatives have potent anticancer properties against several cell lines. Here, we report complete unambiguous assignments of 1H and 13C chemical shifts of the new compounds. The assignments were made using a combination of one- and two-dimensional NMR techniques (1H, 13C, 1H1H COSY, 1H13C HSQC, and 1H13C HMBC). Copyright © 2008 John Wiley & Sons, Ltd. [source]

Structural and spectral assignment of a new diterpenoid isolated from Ballota undulata and a complete 1H and 13C NMR data assignment for three other structurally related compounds

Ahmed A. Hussein
Abstract The structure of 3,-hydroxyballotinone, a new labdane diterpenoid isolated from Ballota undulata, has been established by NMR spectroscopic studies. In addition, complete and unambiguous assignments of the 1H and 13C NMR spectra of three other already known labdanes (ballotinone, ballonigrin and ballonigrinone) isolated from the same source have been achieved. The assignments are based on 2D shift-correlated 1H1H COSY, 1H13C gHSQC [1J(C,H)] and 1H13C gHMBC [nJ(C,H) (n = 2 and 3)], and NOE experiments. Copyright © 2007 John Wiley & Sons, Ltd. [source]

1H and 13C NMR assignments for two anthraquinones and two xanthones from the mangrove fungus (ZSUH-36)

Changlun Shao
Abstract We report the unambiguous assignments of the 1H and 13C NMR spectra of one new natural product, namely, 6,8-di-O-methyl versiconol (1) together with one known anthraquinone aversin (2) and two xanthones 5-methoxysterigmatocystin (3) and sterigmatocystin (4). These compounds were all isolated from the mangrove endophytic fungus ZSUH-36 from the South China Sea. 1D and 2D NMR experiments including COSY, HMQC and HMBC were used to elucidate the structures. Variations in the 1H NMR spectrum of 6,8-di-O-methyl versiconol (1) were also observed in the temperature range 25,75 °C. In addition, the plausible biogenetic path from 1 to 2 is discussed. Copyright © 2007 John Wiley & Sons, Ltd. [source]