Protons axial

The multiplets and coupling constants of the axial) protons at = 3.15, 3.50 and 4.08 moreover confirm the equatorial positions of all three OH groups, as can be seen in formula B. Here the couplings from 10.0 to 11.5 Hz, respectively, identify vicinal protons in diaxial configurations, whilst values of 4.5 and 5.0 Hz, respectively, are typical for axial-equatorial relationships. As the multiplets show, the protons at 5 = 3.50 and 4.08 couple with two axial and one equatorial proton (triplet of doublets) respectively, whereas the proton at = 3.15 couples with one axial and one equatorial proton (doublet of doublets). 

The rates of removal of axial and equatorial protons from 4-t-butylcyclohexane in NaOD/dioxan have been measured by an NMR technique. The rate of removal of an axial proton is 5.5 times faster than for an equatorial proton. What explanation can you offer for this difference ... 

There is no generally reliable rule of thumb to predict whether an axial or equatorial proton will exchange faster. For instance, in the examples discussed above, the slowest rate of exchange is found for the S -axial proton in 5a-androstan-7-one (1) [see(2)-(4)] and the 2fi-equatorial proton in 5a-androstan-l 1-one (5) [see (6)-(9)]. Furthermore, the results of base-catalyzed exchange cannot necessarily be predicted from the corresponding... 

One property of NMR spectroscopy is that it is too slow a technique to see the individual conformations of cyclohexane. What NMR sees is the average environment of the protons. Because chau -chair interconversion in cyclohexane converts each axial proton to an equatorial one and vice versa, the average environments of all the protons are the sfflne. A single peak is observed that has a chemical shift midway between the true chemical shifts of the axial and the equatorial protons. 

The tetrasubstituted isomer of the morpholine enamine of 2-methyl-cyclohexanone (20) because cf the diminished electronic overlap should be expected to exhibit lower degree of enamine-type reactivity toward electrophilic agents than the trisubstituted isomer. This was demonstrated to be the case when the treatment of the enamine with dilute acetic acid at room temperature resulted in the completely selective hydrolysis of the trisubstituted isomer within 5 min. The tetrasubstituted isomer was rather slow to react and was 96% hydrolyzed after 22 hr (77). The slowness might also be due to the intermediacy of quaternary iminium ion 23, which suffers from a severe. 4< strain 7,7a) between the equatorial C-2 methyl group and the methylene group adjacent to the nitrogen atom, 23 being formed by the stereoelectronically controlled axial protonation of 20. 

The formation of the less favored trisubstituted isomer (37) occurs by the usual intramolecular axial proton transfer from the 6 position, whereas that of the tetrasubstituted isomer (38) involves the intramolecular proton transfer of the stereoelectronically less favored equatorial proton either via a four-membered transition state (39) or a six-membered transition state (40). 

The NMR spectrum of d-inositol has a signal at 3.83 p.p.m. for four axial protons, and a signal at 4.10 p.p.m. for two equatorial protons. The NMR spectrum of kasugamycin contains similar signals of d-inositol. 

The following experiments exclude the structure having an axial proton at C-2 (8c). The two patterns of methylene protons are different, when C-2 proton at 3.15 p.p.m. and C-4 proton at 2.92 p.p.m. are irradiated separately (A and B in Figure 5). This evidence excludes the structure... 

The power of the new spectrometer to reveal configurations of difficult cyclitols or sugars was first tested with mt/o-inositol (2), using deuterium oxide as solvent. At 60 or 100 MHz. the one equatorial and five axial protons appear to have different chemical shifts as shown by Lemieux in 1956 with a 40 MHz. instrument (14,15). However, since the five-proton axial signal could not be resolved, one could probably not have assigned the configuration 2 (which was already known from laborious chemical correlations extending over many years.)... 

The proton spectra analysis of thietane, thietane oxide and thietane dioxide at 100 and 300 MHz in the temperature range — 140 to 190 °C confirmed the puckered structure for the oxide (5a) with the sulfinyl oxygen in the equatorial orientation, as inferred from chemical-shift considerations180. It appears that the repulsive-type 1,3-interactions between the oxygen and the 3-substituents184 are operating between oxygen and the axial proton on C-3 in the unsubstituted thietane oxide (5a). For the thietane dioxide (5b ... 

Elimination on (26) must remove an axial proton so can give only (25), oxidation gives (24) and axial... 

Figure 18. A 0.2-s delayed COSY spectrum of the aliphatic region of 10 (2mg, CDCLj). Long-range "W-type" coupling of 19 and 21 axial protons to 30-CHj and coupling across the gem dimethyls from I9eq to 21 eq establish the position of oxidation at C-22. The spectrum was obtained under conditions similar to those in Figure 1, except that 32 transients were acquired for each of 128 x 512 data point spectra (17).

With octalin (55), rotation to the trans chair form (59) followed by protonation from the outside face of the double bond at the less substituted carbon atom leads to the tertiary cation (60) with net equatorial introduction of the proton. Rotation to a trans boat form would lead to net axial protonation<84) ... 

It is also worth noting that nine times out of ten, equatorial protons absorb at somewhat lower field than the corresponding axial protons. This can be reversed in certain cases where the specific anisotropies of the substituents predominate over the anisotropies of the rings themselves but this is relatively rare. The difference is typically 0.5-1.0ppm, but may be more. 

Catalytic reduction of bridgehead enol lactone over Pd/C indicates that, indeed, the syn addition from the exo face of the bridgehead double bound establishes the relative configuration of all substituents [264], Equilibration studies performed in EtONa/EtOH also established that the ratio of the epimers corresponds to an equilibrium mixture. Under mild basic conditions (NajCOj/ EtOH), the product isomerization occurs to a very small extent. The product distribution is best understood by rapid conformational relaxation to one of the two low-eneigy half-chair conformations. The stereochemistry is established at the subsequent protonation step. This takes place with a strong preference for axial protonation from the /I face at carbon 2 to produce the most stable chair conformation (Scheme 14.12). 

In saturated cyclic hydrocarbons, diamagnetic currents are induced due to circulating bond electrons. Although, the effect is weak, but this is sufficient to distinguish between axial and equatorial protons is cyclohexane. The axial protons are shielded and they absorb 0.5 ppm upfield as compared to the equatorial. 

Carbon-13. Steric compression by axial P substituents produces a so-called y-effect by their interaction with the two axial protons on the same side of the ring, and there is a resultant upfield shift of the carbon atoms bearing the axial protons. This effect has been noted for a number of dioxaphosphorinans (22)42>43 as well as for phos-phorinans.22 A small but significant shift of the C-4 resonance occurs when the... 

As for any desymmetrization of meso compounds, enantioselectivity comes from the ability of a homochiral base to distinguish between two enantiotopic protons, in this particular case, to discriminate between the two pseudo-axial protons of the rapidly equilibrating enantiomeric half-chair conformations 51 and 52 (Scheme 25). 

The same features exist at a reduced level for POCCH couplings in phosphorinanes and these are about 2-6 Hz for an equatorial proton at C-5, while it is less than 1 Hz for an axial proton. Similarly, coupling of phosphorus to an equatorial methyl at C-4 is 2 to 3 Hz while that to axial methyl is less than 1 Hz (1968>83) similar trends have been found in the corresponding phosphonates.<1969,52)... 

The latter are, indeed, of considerable interest. They have a long history in conformational chemistry [258,259] and deserve attention for the major role they play in the discussion and prediction of stmcmral feamres. Typically, we refer here to gauche interactions exemplified by one of the methyl protons of the axial methylcyclohexane (for instance) interacting with the axial protons at C-3 and C-5 of the ring, or to the three gauche interactions occurring in cw-decalin. ... 

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