Quaternary Carbons C

An unprotonated carbon simply shows one magnetization vector, which precesses at the Larmor frequency (i.e., it always points along the F axis). A normal peak is recorded at time t = 1/7. 

602 Nuclear Magnetic Resonance Spectroscopy Part Five Advanced NMR Techniques 

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For quaternary carbons (C) For methine carbons (CH) For methylene carbons (CH2) For methyl carbons (CHj) ... 

At this point, we have assigned all of the protons but still cannot differentiate between the methyl groups at H-8 and H-9. Since we know all the H assignments, it is a trivial task to transfer assignments to the 13C signals through the HMQC spectrum. The quaternary carbons C-3 and C-7 have no attached protons and cannot be correlated in the HMQC spectrum. An HMBC spectrum could be used to corre-... 

Several structural features of (-)-rhazinilam 3 raise interesting synthetic challenges the axially chiral phenyl-pyrrole A-C biaryl bond, the fused pyrrole-piperidine C-D rings, the stereogenic quaternary carbon (C-20) ortho to the phenyl-pyrrole axis, the nine-membered lactam firing. Three racemic (Smith, Sames, Magnus) and one asymmetric (Sames) total syntheses have been published to date, which all proceed via construction of the pyrrole ring and diastereoselective control of the axial chirality by the central chirality at C-20. 

Furthermore, the fragmentation of the six-membered ring of XXVI that obviously must be taking place can be construed on several accounts (not indicated). In order to achieve this, it is crucial to take into account the transformation of the quaternary carbon C into a tertiary center. It is also significant that a new C-C bond is being constructed from C. ... 

Both possible disubstituted compounds 52 and 53 were obtained from alkylation (halides and sulfate) of 51. In determining the structures, the N(1)-CH2 correlated exclusively with the quaternary carbon C-8a, while the N(3)-CH2 or O-CH2 correlated with carbon C-4. In the latter case, a deshielding in both proton and carbon signals was observed that additionally confirmed the O-substitution (Equation 2) <2003BMC2395, 2000JMC3218>. 

HMBC ( H-Detected Multiple-bond Heteronuclear Multiple Quantum Coherence Spectrum) NMR of 26a allowed one to show that the isopropyl group (H , H ) is connected to the olefinic carbon (C ), whereas methyl (H ) and methylene (H-, H- ) groups are bonded to another olefinic carbon (C ) and fullerene carbon (C ) through quaternary carbon (C ) shown in 26a and 26b (Scheme 8). 

Although this compound has a total of six carbons, there are only four peaks in the NMR spectrum. The atoms that are equivalent appear at the same chemical shift. The single methyl carbon a appears at highest field (9 ppm), while the three equivalent methyl carbons b appear at 29 ppm. The quaternary carbon c gives rise to the small peak at 30 ppm, and the methylene carbon d appears at 37 ppm. The relative sizes of the peaks are related, in part, to the number of each type of carbon atom present in the molecule. For example, notice in Figure 4.10 that the peak at 29 ppm (b) is much larger than the others. This peak is generated by three carbons. The quaternary carbon at... 

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