H and C NMR Spectroscopy

FIGURE 25.5 Distribution of furanose, pyranose, and open-chain forms of o-ribose in aqueous solution as measured by H and C NMR spectroscopy. 

Energy Barriers to the Tautomeric Rearrangements 40a 40b as Determined by H AND C NMR Spectroscopy... 

As relatively few standard compounds are available from commercial or other sources, identification of flavonol glycosides has to be achieved by alternative means, for example UV-, H- and C-NMR spectroscopy. Therefore hydrolysing all glycosides to aglycones followed by HPLC determination offers a practical method for the quantitative determination of flavonoids in tea (Hertog et al, 1993a Wang and Helliwell, 2001). 

The structure of this compoimd 63a was determined by H and C NMR spectroscopy. The H NMR spectrum shows singlets at 1.45 ppm for the methyl protons, and at 4.17 and 6.30 ppm for the hydroxylic and ethylenic protons. The addition regioselectivity in the formation of 63a was established by H- C HMBC 2D-NMR, which shows the C5-C4-C3-Me linkages. The ethylenic proton correlates only with three carbon atoms Ce, C5, and C3. The methyl protons correlate with C3 and with the ethylenic carbon C4, consistent with the neighboring C3-C4 connection. The NOESY spectrum shows... 

The relatively basic (hydroxyalkyl)phosphines act toward LMCs as reductants and, compatible with this, also as strong nucleophiles. We have studied such reactions in aqueous and D2O solutions by P-, H-, and C-NMR spectroscopies (including 2D correlation methods), product isolation and, when possible, X-ray analysis of isolated compounds or their derivatives. Thus, aromatic aldehyde moieties present in lignin (e.g., 3) are reduced to the corresponding alcohols (see 4) with co-production of the phosphine oxide in D2O, -CH(D)OD is formed selectively (36). The mechanism proceeds via a phosphonium species formed by initial nucleophilic attack of the P-atom at the carbonyl C-atom, i.e., via ArCH(OH)P%, where Ar is the aromatic residue and R is the hydroxyalkyl substituent (36). When the aldehyde contains a 4-OH substituent, the alcohol product... 

Sesbanimide is a novel cytotoxic compound isolated from Sesbania puni-cea seeds [84JCS(P1)1311]. According to high-field H- and C-NMR spectroscopy, in solution a solvent-dependent equilibrium exists between the... 

N02)2C6H3] is stable in solution the other hydroxypyrazolines 72B revert to 73 in several solvents, which prevents investigation of the equilibrium 72A 72B. By means of H- and - C-NMR spectroscopy, a three-compo-... 

SB. Actually, these tautomeric equilibria are more complex because each of these three forms can exist as two geometric isomers ( and Z for 75A and 75A) or two conformers (rotamers E and Z for 75B). When + H, two diastereomers cis and trans and R ) are possible for the cyclic tautomer 75B. These equilibria have been thoroughly investigated (88TH2) by means of electronic, IR, H-, and C-NMR spectroscopy. The structure of the solid 5-hydroxypyrazoline 74B- (R = Me R = H R = Ph R" = i-Pr) was confirmed by X-ray diffraction (87MI5). An approximate generalization of the influence of the structural factor on these equilibria is shown in Table X. 

The chemical shift of a nucleus reflects the molecular structure and it can therefore be used to obtain structural information. Further, as hydrogen and carbon (and therefore iR and nuclei) are universal constituents of organic compounds the amount of structural information available from H and C NMR spectroscopy greatly exceeds in value the information available from other forms of molecular spectroscopy. 

Se(0Et)2 reacts with silica(soo) to give a single-site species that has been fully characterized by IR, solid H and C NMR spectroscopies, isotopic labeling. 

Quenching polystyryllithium and polybutadienyllithiums with A-benzylidenemethyl-amine (PhCH=NMe) in benzene solution leads to amine-functionalized polymers (equation 103), that can be characterized by SEC, TLC, acid-base titration and H and C NMR spectroscopies. The end groups are monomeric. Gradual addition of Et20 reduces the yields. This is attributed to increased reactivity of the alkyllithium residues in the polymer, bringing about a concurrent metallation reaction of the imine reagent (e.g. equation 101) . 

H and C NMR spectroscopies were systematically used to elucidate the structure of C-glycosylflavonoids the assignment of signals was based upon various experiments, HMBC, HMQC, COSY, etc., as shown in the following examples ... 

Ciyoscopic determination of the molecular weight of f-Bu2Mg in benzene solution pointed to a dimer (86), the structure of which was conhrmed by H and C NMR spectroscopy and ultimately determined by XRD crystallographic analysis . ... 

Reaction of two equivalents of 2-[(dimethylamino)methyl]ferrocenyUithium with ZnCl2 affords the corresponding diorganozinc compound (68) in which two 2-[(dimethylamino) methyl]ferrocenyl ligands are C,Af-chelate bonded to zinc. Due to the chirality of the bidentate ligands 68 exists in two diastereoisomeric forms. Indeed, in solution a meso one and an S,S)/ R,R) enantiomeric parr in a 1 4 ratio were observed by H and C NMR spectroscopy. However, it is only the S,S)/ R,R) enantiomeric pair that preferentially crystallizes from solution. An X-ray crystallographic study of such crystals afforded the structure of (R,R)/(S,S)-6S in the solid state (Figure 35). 

Tables of chemical shifts can be used lo determine the molecular environment of all nuclei, but are especially useful in H and C NMR spectroscopy.

Amino acid specific Maillard products were isolated from the extracts by preparative GC or HPLC and identified by MS-, IR-, 1 H- and c-NMR-spectroscopy. Proline derived components are important constituents in bread, malt and beer. More than 120 proline specific Maillard products were characterized. Cysteine and methionine derived components were predominant in roasted coffee and meat flavors. Thirty cysteine- and twenty methionine-specific Maillard products were identified for the first time. 

The ( )-structure of the natural antioxidant marginalin 50 has been established by comparison with analogous synthetic isoaurones as well as by H and C NMR spectroscopy <2006T9855>. 

Nitrenium cation is an extremely unstable species (89-91). That is why nitrenium complexes are usually prepared by the interaction of organic compounds with some sources of nitrenium cation (91, 92). Thus, the interaction of O-mesityl-sulfonylhydroxylamine with 1,10-phenanthroline results in the formation of l-amino-l,iO-phenanthrolinium mesitylenesulfonate (93-99). The structure of this salt has been confirmed by H and C NMR spectroscopy (93,... 

Upon reaction of O-mesitylsulfonylhydroxylamine with X-1,10-phenanthrolines (56a-d) in CH2CI2 the respective isomeric cations (55a-d) and (57a,e-g) were formed (Scheme 24) 99), and their structures were confirmed by H and C NMR spectroscopy. 

Norbornene-based and oxa-norbornene-based monomers bearing dendritic side chains, XXX and XXXI (Fig. 19), were synthesized and polymerized via ROMP with initiator 6 [83]. Based on size exclusion chromatography data, the polymerization shows hving-like character up to DP=70. H- and C-NMR-spectroscopy revealed 35% cis and 65% tram sequences. These polymers displayed enantiotropic nematic and smectic mesophases, except for DP=5. In contrast to other classes of SCLCPs, the dependence of the DP on the transition temperatirre of the polymer was very weak. Glass transition and isotropization temperatures became independent of molecular weight above a degree of polymerization of about 10. 

Penta-, hexa- and hepta-methyl substituted bicyclo[3.2.0]heptadienyl cations have been investigated by Hogeveen and van Kruchten (1979). Some ions contained besides the methyl groups D, CD3, cyclopropyl and/or t-butyl groups. The rearrangements of the ions were studied under conditions of long life by H- and C-nmr spectroscopy. They observed five-carbon... 

Several silacyclopropenes 62a-c were synthesized by photochemical isomerization of ethynyldisilanes (Scheme 28), while l,l-dimesityl-2,3-diphenylsilacyclopropene 62d was obtained by the reaction of the photoche-mically generated dimesitylsilylene with diphenylacetylene (Equation 23). All silacyclopropenes are air stable and were characterized by H- and C-NMR spectroscopy <2003OM2436>. 

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