Quaternary carbon, meaning

In the retrosynthetic direction this means that wherever we have a quaternary carbon atom at the a-position to a keto group -either because it already existed in the target molecule or because it has been introduced by a FGI or a FGA- the molecule can be "disconnected" as shown in the following sequence ... 

In another early application to natural product synthesis, Fleming and coworkers utilized this approach in the efficient formation of the gelsemine model (47) from 45 according to Scheme 8. The cyclization step to form the spiro-oxindole (46) proceeded in 85% yield and provided a means of generating the spiro-fused quaternary carbon without the need for carbenium ion or carbanion chemistry. 

As mentioned earlier, the irradiation of directly bonded hydrogen atoms increases the intensity of C signals through the NOE, which means that CH, CH2 and CH3 groups usually have a relatively high intensity in a NMR spectrum, while we can often recognize quaternary carbon atoms because of their low intensity. This difference in intensity is due to the different relaxation rates, and is the reason we do not integrate C NMR spectra. 

A constant evolution period t, is the new feature of. /-modulated spin-echo, DEPT, and INADEQUATE sequences. During this time period, a 7-modulation or a polarization transfer may evolve. Such pulse sequences provide FID signals S(t2) which are still functions of one variable time t2. The Fourier transforms, however, are NMR spectra with specific information, depending on the constant evolution period ti. One simple example is the generation of the quaternary carbon-13 subspectrum by means of a. /-modulated spin-echo experiment with an evolution time of tj2 = x = as in... 

It was envisioned that the addition of an indole derived from a tryptamine to the activated iminium ion, arising from imidazolidinone catalyst 3 and an a,p-unsaturated aldehyde, would generate a C(3)-quaternary carbon-substituted indo-lium ion. As a central feature this intermediate cannot undergo re-aromatization by means of proton loss, in contrast to the analogous 3-H indole addition pathway. As a result, 5-exo-heterocyclization of the pendant ethylamine would provide the corresponding pyrroloindoline compounds. In terms of molecular complexity, this cascade sequence should allow the rapid and enantioenriched formation of stereochemically defined pyrroloindoline architecture from tryptamines and simple a,/i-unsaturated aldehydes. 

The palladium-catalyzed arylation and alkenylation of olefins, which were first discovered in the 1970 s by Heck (7,2) and Mizoroki (3) and have been often called the "Heck reaction", are versatile synthetic means for making a carbon-carbon bond. These reactions have been extensively used for organic synthesis during the past two decades (4-7). However, no reports on the "asymmetric Heck reaction" have been appeared until very recently. Shibasaki reported an asymmetric intramolecular cyclization of alkenyl iodides to give c/j-decalin derivatives of 80-91% ee (8-10). Overman reported an intramolecular cyclization of alkenyl triflate, giving a chiral quaternary carbon center of 45% ee (77). We report herein the first example of intermolecular asymmetric Heck-type arylation of cyclic olefins catalyzed by (7 )-BINAP-coordinated palladium complexes (Scheme 1) (12,13). 

In addition to these solvent peaks, we can count 26 peaks in the spectrum. Because there are 27 carbons in the cholesterol molecule (three are lost in the biosythesis from a triterpene precursor), there must be one peak that accounts for two carbons. The tallest peak (labeled 1, m ) in fact corresponds to two different carbons with nearly identical chemical shifts. The most downfield peaks ( aa and z ) are in the olefin/aromatic region of the 13 C spectrum (120-140 ppm), so they must correspond to C-5 andC-6. Peak aa is less intense ( shorter ) than all of the other peaks because of slow relaxation it must be a quaternary carbon. We will see that the proximity of protons is the primary means of relaxation of 13 C nuclei, so carbons lacking a proton relax much more slowly and give less intense peaks, especially if... 

Novel methods for functionalizing piperidines at the 3- and 4-positions were also introduced. Mete and co-worker synthesized 3-diazo-piperidin-2-one and characterized its reactivity in transition-metal catalyzed reactions, particularly H-X insertion reactions and cyclopropanation reactions <02T3137>. Christoffers and co-workers developed an asymmetric Michael addition reaction with a chirally modified 4-piperidone-enamine. They were able to create a quaternary carbon center in >95% de and elaborate the compound on through classical means to the functionalized piperidine 107 (Scheme 21) <02EJ01505>. 

The 13C spectrum of an organic molecule arises from 13C nuclei coupled through one bond to zero, one, two, or three protons. It can be very helpful to ascertain for each line in a proton-decoupled 13C spectrum whether it arises from a CH3, CH2, CH, or quaternary carbon. In our discussion of INEPT in Section 9.7, we noted that each of the first three moieties has different precession behavior and refocuses at a specific time, hence can serve as a means of distinguishing among these groups. We describe here three methods that are particularly useful for such... 

Migration cannot occur past a quaternary carbon, however, since the required elimination is blocked. At equilibrium, the major trialkylborane is the least-substituted terminal isomer that is accessible, because it is the one that minimizes unfavorable steric interactions. The availability of the isomerization provides a means for thermodynamic control of the hydroboration reaction. 

These examples should give you an idea of the capabihties of the DEPT technique. It is an excellent means of distinguishing among methyl, methylene, methine, and quaternary carbons in a NMR spectrum. 

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