Carbons, quaternary/methine

The structure was determined by NMR spectral analysis including a variety of two-dimensional NMR techniques. The 500-MHz XH NMR spectrum of 77 taken in CDCI3 (Figure 26) revealed the presence of 5 aromatic protons, 15 olefinic protons, a methoxy (63.65), an allylic methyl (62.14) and a tertiary methyl group (61.33). The 13C NMR spectrum showed signals due to all 34 carbons, which were assigned to 7 quaternary carbons, 23 methines, 1 methylene and 3 methyls by DEPT experiments. The 13C and XH NMR spectral data are summarized in Table 27. 

Examination of the C-NMR spectra of roseadine (23) (Table XI) through comparison with vindoline (3) and leurosine (11) permitted the assignment of all carbons of the dihydroindole unit. The carbons of the indole nucleus were assigned by comparison with vinblastine (1), and the presence of three deshielded carbons, a methine carbon at 8 142.9 and two quaternary carbons at 8 133.2 and 169.2, were observed. The latter was assigned to the methoxycarbonyl carbon, which is shielded somewhat from its characteristic chemical shift of 8 174 1 ppm in the vinblastine series by attachment of an olefinic unit. The other two deshielded carbons at 8 133.2 and 142.9 could be assigned as C-18 and C-17, respec-... 

DEPT Spectra Identifying Quaternary, Methine, Methylene and Methyl Carbons... 

Mohan et al. [8] used an NMR spectroscopic method to characterize impurity D in samples of clopidogrel bisulfate. The method entailed XH NMR (at 400.13 MHz) and 13C NMR (at 100.62 MHz), with a sample concentration of 1 mg/ml in DMSO-t/f, (this solvent also served as an internal chemical shift standard). It was found that the 1H NMR spectrum of impurity D exhibited one hydrogen band than did that of clopidogrel, while the 13C NMR and DEPT135 NMR spectra indicated the presence of one methyl carbon, two methylene carbons, eight methine carbons, and five quaternary carbons. This corresponded to a similar structure as for clopidogrel, but with one less methylene carbon and one more methine carbon. 

A methyl group (Cby is a primary carbon centre (attached to one carbon), a methylene group (CH2) is a secondary carbon centre (attached to the other carbons), a methine group (CH) is a tertiary carbon centre (attached to three other carbons) and a carbon centre with four alkyl substituents (C) is a quaternary carbon centre (attached to four other carbons) ... 

The J-coupling patterns in spectra can help to elucidate chemical structure, but they also comphcate the spectrum. Spectral-editing experiments allow such interactions to be observed in a controlled way. One such technique, DEPT (ifistortionless enhancement by polarization transfer), edits the spectrum based on scalar coupUng the pulse sequence is depicted in Fig. 15. DEPT is actually a set of three experiments, with tip angles, /, of 45°, 90°, and 135°. In NMR, appropriate addition and subtraction of the resulting subspectra separate the contributions from methyl, methylene, and methine carbons quaternaries are not observed. 

VI) Finally, any alkyl group attached to only one other carbon is called primary if attached to two other carbons, then it is secondary to three other carbons, tertiary and attached to four other carbons, quaternary. While these designations are not part of formal lUPAC names, they are part of the systematic description of compounds. Thus, in the earlier example of the isomers of C4FI10, butane and 2-methylpropane, the former may be said to have two primary carbons (in the methyl groups) and two secondary carbons (in the methylene groups), while the latter has three primary carbons (in the methyl groups) and one tertiary carbon (in the methine). 

Several other techniques now allow for the easy determination of the number of hydrogens attached to a carbon. One of the most convenient techniques goes by the acronym DEPT (distortionless enhancement with polarization transfer). In this technique, several separate NMR spectra are determined for a molecule under conditions that allow for the appearance of only methine (CH), methylene (CH2), or methyl (CH3) carbons. Quaternary carbons can be determined by difference Any signal in the full spectrum that does not appear in the separate spectra for CH, CH2, and CH3 carbons must belong to a quaternary carbon. 

The methine chain is obtained by reacting ethyl o-formate (method A ) or ethylisoformanilide (method B) with a bis quaternary salt of bis-(2-thiazolyllbutane. Concerning dyes with fused thiazolo rings pyrrolo[2. lb]thiazoIe. thiazolo[2.3a]indole. thiazolo[2.3c]1.4-benzox-azine. the a carbon directly linked to the carbon 2 of the thiazoJe ring is also responsible for the classical syntheses giving trimethine or penta-methine dyes. 

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

With T set at V2J, the quaternary carbons generally appear with greater intensity than the other carbons, which will be of near-zero intensities, thereby allowing them to be distinguished, particularly from the CH2 carbons, as compared to the normal APT spectrum, in which both CH2 and quaternary carbons appear with positive amplitudes. A difference APT spectrum, in which an APT spectrum recorded with t set at %/is subtracted from another APT spectrum recorded with t set at /sj, can provide useful information. The methyl carbons will then appear with reduced intensities in the difference spectrum as compared to the methine carbons, allowing us to distinguish between them. 

The GASPE spectrum of vasicinone is shown. The peak at 8 126.5 is a cluster of three peaks at 8 126.3 and 126.7 representing methine carbons. Similarly, the signal at 8 160 on the positive phase of the spectrum represents two close singlets at 8 160.4 and 160.5. Predict the chemical shift values of various protonated and quaternary carbons in the structure. 

The broad-band decoupled C-NMR spectrum of ethyl acrylate shows five carbon resonances the DEPT (6 = 135°) spectrum displays only four signals i.e., only the protonated carbons appear, since the quaternary carbonyl carbon signal does not appear in the DEPT spectrum. The CH and CH3 carbons appear with positive amplitudes, and the CHj carbons appear with negative amplitudes. The DEPT (6 = 90°) spectrum displays only the methine carbons. It is therefore possible to distinguish between CH3 carbons from CH carbons. Since the broadband decoupled C spectrum contains all carbons (including quaternary carbons), whereas the DEPT spectra do not show the quaternary carbons, it is possible to differentiate between quaternary carbons from CH, CHj, and CH3 carbons by examining the additional peaks in the broad-band spectrum versus DEPT spectra. The chemical shifts assigned to the various carbons are presented around the structure. 

Organic compounds contain four types of carbon atom methyl, methylene, methine and quaternary. And so if we simply record the spectrum as we would a proton spectrum, the result will be a series of quartets, triplets, doublets and singlets, each associated with a carbon-proton one-bond coupling constant of between 125 and 250 Hz. If we are dealing with a complex molecule, these multiplets will overlap and give us spectra which are almost impossible to analyse. In addition, coupling interactions over two or more bonds complicate the picture still further. 

DEPT Distortionless enhancement by polarization transfer. A useful one-dimensional technique which differentiates methyl and methine carbons from methylene and quaternary carbons. 

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