Coupling 3/, axial/equatorial couplings

Proton c can be defined by the fact that it is not equatorial and it is highly coupled. The multiplet at 3.82 ppm satisfies these requirements. It is in the right ball park for chemical shift and is highly complex in that this proton is already the X part of an ABX system coupled to both protons alpha to the chlorine (the AB part). It is then further coupled with a 10 Hz, axial-axial coupling (reciprocated in the dd at 2.07 ppm) and with a 2 Hz axial-equatorial coupling which is reciprocated in the ddd at 2.90 ppm. Note that c and d are not fully resolved from each other. Such overlap inevitably complicates the issue. 

The /3-configuration of the hydroxyl group was inferred from the small coupling constant (J 2.5 cps) corresponding to axial-equatorial coupling. 

In the first molecule, proton H has two neighbours, one axial (H ) and one equatorial (H ) so it will appear as a double doublet with characteristic large axial/axial and small axial/equatorial couplings. By contrast the two... 

Spectral data of 4 were very close to 3, and indicated it was a stereoisomer of 5. The stereochemistry at C-6 was determined indirectly as in the case of the compound 3. Assuming axial orientation of H-4, J= 9.1 Hz could be assigned to axial-axial coupling between H-4 and H-Sax. Starting from this, J = 4.2 Hz then must be due to axial-equatorial coupling H-Sax and H-6. Therefore, the configuration of the hydroxyl group at C-6 was concluded to be S. 

The case of cyclohexyl iodide provides an example of the use of NMR spectroscopy to determine the conformational equilibrium constant and the value of -AG°. At -80 C, the NMR spectrum shows two distinct peaks in the area of the CHI signal, as shown in Fig. 3.6. The multiplet at higher field is a triplet of triplets with coupling constants of 3.5 and 12 Hz. This pattern is characteristic of a hydrogen in an axial position with two axial-axial couplings and two axial-equatorial couplings. The broader peak at lower field is characteristic of a proton at an equatorial... 

In the trans isomer, the H is axial and therefore will experience two large axial-axial couplings and one smaller axial-equatorial coupling (Section 7.2C). This proton appears at 4.70 ppm (ddd, J = 10.2, 10.2, 4.4 Hz). In the cis isomer, the H is equatorial and therefore experiences three small couplings 2 axial-equatorial and 1 equatorial-equatorial. This proton appears at 5.25 ppm and is technically a ddd, but the small J values result in significant overlap of the lines as just discussed. Therefore, the cis diastereomer is the major component and the trans diastereomer is the minor component. 

Neighbouring diaxial protons of cyclohexane can be clearly identified by their large coupling constants 11-13 Hz, Table 2.10) which contrast with those of protons in diequatorial or axial-equatorial configurations ( Jee 2-4 Hz). Similar relationships hold for pyranosides as oxy-... 

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). 

Stereoselective epoxidation can be realized through either substrate-controlled (e.g. 35 —> 36) or reagent-controlled approaches. A classic example is the epoxidation of 4-t-butylcyclohexanone. When sulfonium ylide 2 was utilized, the more reactive ylide irreversibly attacked the carbonyl from the axial direction to offer predominantly epoxide 37. When the less reactive sulfoxonium ylide 1 was used, the nucleophilic addition to the carbonyl was reversible, giving rise to the thermodynamically more stable, equatorially coupled betaine, which subsequently eliminated to deliver epoxide 38. Thus, stereoselective epoxidation was achieved from different mechanistic pathways taken by different sulfur ylides. In another case, reaction of aldehyde 38 with sulfonium ylide 2 only gave moderate stereoselectivity (41 40 = 1.5/1), whereas employment of sulfoxonium ylide 1 led to a ratio of 41 40 = 13/1. The best stereoselectivity was accomplished using aminosulfoxonium ylide 25, leading to a ratio of 41 40 = 30/1. For ketone 42, a complete reversal of stereochemistry was observed when it was treated with sulfoxonium ylide 1 and sulfonium ylide 2, respectively. ... 

When methylene protons at 1.82 p.p.m. are irradiated, each signal at 3.15 and 2.92 p.p.m. splits into a doublet with a coupling constant, 1.6 c.p.s., and a doublet with 9.6 c.p.s., respectively. Therefore, the assignment of the signals at 3.15 and 2.92 p.p.m. to C-2 and C-4 protons, is clearly established (C in Figure 4). The coupling constants again confirmed the axial- equatorial or equatorial-equatorial relation between C-l and C-2 protons and axial-axial relation between C-4 and C-5 protons. 

The low temperature 13C NMR spectrum of Rh CO), which is iso-structural with Co4(CO)i2 in the solid state, has three equally intense doublets due to the apical, axial, equatorial carbonyls and a triplet due to bridging carbonyls91. These multiplets arise through spin-spin coupling to 103 Rh (100% abundant,... 

In following the temperature dependence of AG°, Booth et al. [85JC-S(CC)467 87T4699]also determined A/f° and AS° for the conformational equilibria of 2-C1-, 2-OMe-, 2-OH-, and 2-NHMe-oxanes (see Table II) and discussed the results in terms of exo- or endo-anomeric effects (Section III,C,8). Employing the NOE and a number of H,H- and C,H-coupling constants as a means of analysis, the preferred rotamers of axial/equatorial-2-OMe-oxane were found to be in the conformations az and 2, respectively, as given in Scheme 1 (90T1525). 

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