Conformations, anti Newman projections

Conformational studies on ethane-1,2-diol (HOCH2—CH2OH) have shown the most stable conformation about the central C—C bond to be the gauche conformation, which is 9.6 kJ/mol (2.3 kcal/mol) more stable than the anti conformation. Draw Newman projections of these conformers and explain this curious result. 

Figure 19.14 Panel A shows dopamine, 13 from prior Scheme 19.7, in its extended or (E) conformation, which allows for a staggered anti-Newman Projection formula (Ar = 3,4-dihydroxyphenyl). Panel B shows 13 in its folded or (F) conformation, which leads to a fully eclipsed Newman Projection formula of higher energy. Panel C shows ADTN 29, THiQ 30 and apomor-phine 31. d1, d2, d1 and d2 (in the far-right column) refer to distances between the partners for groups (a), (b), and (c) that would exist on the dopamine receptor in order to accommodate dopamine s E or F conformation, respectively. The different spatial arrangements for these two sets of partnering groups are apparent.

Fig. 6.2.3. Representations of isotactic 1(R = CH3) a extended, anti-trans staggered conformation, b Newman projection, c Fischer projection, d Newman projection of gauche staggered conformation...

FIGURE 3 6 The gauche and anti conformations of butane shown as ball and spoke mod els left) and as Newman projections right) The gauche conformation is less stable than the anti because of the van der Waals strain between the methyl groups... 

Higher alkanes having unbranched carbon chains are, like butane, most stable in theh all-anti conformations. The energy difference between gauche and anti conformations is similar- to that of butane, and appreciable quantities of the gauche conformation are present in liquid alkanes at 25°C. In depicting the conformations of higher alkanes it is often more helpful to look at them from the side rather than end-on as in a Newman projection. Viewed from this perspective, the most stable conformations of pentane and hexane... 

Anti conformation (Section 3.7) The geometric arrangement around a carbon-carbon single bond in which the two largest substituents are 180° apart as viewed in a Newman projection. 

The many different conformers resulting from rotation around the carbon-carbon bonds in simple molecules like ethane and w-butane may be shown by Newman projections (Figure 2.7). The most stable is the anti or trans projection where the steric hindrance is minimized. There are a number of eclipsed and gauche arrangements of which only one of... 

The origin of the third diastereomer produced, complex 12, is of particular mechanistic interest. The configuration at Ca of 12 is opposite to that of the other two products 10 and 11 indicating that the opposite face of the enolate 6 has been approached by the epoxide. Two possible alterations of the geometry of enolate 6 inay be invoked to account for this, adoption of the 5yn- -conformer or adoption of the anti-Z-conformer. Examination of the different structures shown reveals that the observed minor product 12 could arise from a matched reaction pair of the ivn-E-enolate and epoxide (Newman Projection G) or from a mismatched reaction pair of the anti-Z-enolate and epoxide (Newman projection I). The absence of diastereomer 13 strongly suggests that the minor product 12 arises from reaction of the. ryn- -enolate, underscoring the extreme reluctance of iron-acyl complexes to form Z-enolates on deprotonation (see scheme on p 955). 

FIGURE 2. Newman projections and energies of the MP2/6-31G calculated geometries of the anti, gauche and ortho conformers of permethyltetrasilane... 

A-l. Draw Newman projections for both the gauche and the anti conformations of 1-chloro-propane, CH3CH2CH2C1. Sight along the C-l, C-2 bond (the chlorine is attached to C-l). 

Figure 9.36 shows Newman projections of the three low-energy conformers for the Ca-Cp bond of an AMX amino acid residue within a peptide or protein. The gauche relationship should give a small coupling (<6 Hz) and the anti relationship should give a... 

Draw Newman projections for the anti and gauche conformations about the C—C bond of these compounds. What other factors, besides steric and torsional strain, influence the stability of these conformations ... 

Closer examination reveals that the syn-periplanar conformation has all the bonds eclipsed, whereas the anti-periplanar conformation has these bonds staggered, as shown in the following Newman projections ... 

Both cis and trans alkenes can be formed from this compound by anti elimination. Draw a Newman projection of the conformation required to form each of these products and. on the basis of these projections, predict which of these products would be formed in larger amounts. 

The Newman projection for the conformation with the methyl group equatorial shows that the methyl group has an anti relationship to both C3 and C5. Figure 3-25 shows the Newman projection along the Cl—C2 bond, with the anti relationship of the methyl group to C3. 

Draw a Newman projection, similar to Figure 3-25, down the Cl —C6 bond in the equatorial conformation of methylcyclohexane. Show that the equatorial methyl group is also anti to C5. (Using your models will help.)... 

In the next example, there is only one proton that can take part in the elimination. Now there is no choice of anti-periplanar transition states. Whether the product is E or Z> the E2 reaction has only one course to follow. And the outcome depends on which diastereoisomer of the starting material is used. When the first diastereoisomer is drawn with the proton and bromine anti-periplanar, as required, and in the plane of the page, the two phenyl groups have to lie one in front and one behind the plane of the paper. As the hydroxide attacks the C-H bond and eliminates Br , this arrangement is preserved and the two phenyl groups end up trans (the alkene is E). This is perhaps easier to see in the Newman projection of the same conformation. 

The second diastereoisomer forms the Z-alkene for the same reasons the two phenyl groups are now on the same side of the H—C-C—Br plane in the reactive anti-periplanar conformation (again, this is clear in the Newman projection) and so they end up cis in the product. Each diastereoisomer gives a different alkene geometry, and they do so at different rates. The first reaction... 

The vicinal (3/) coupling constant between the two black Hs is 11 Hz. This is rather large and can be explained by a predominant conformation shown in the Newman projection, with the two large groups (PhCO and Ph) as far from each other as possible, the two medium groups (Br) as distant as possible, and the two black Hs in the places which are left. The dihedral angle between the black Hs is then 180° (they are anti-periplanar) and a large/is reasonable. 

Convert this drawing into a Newman projection, and draw the conformation having anti periplanar geometry for -H and -Br. 

Draw a Newman projection of the tosylate of (2/ ,35)-3-phenyl-2-butanol. and rotate the projection until the -OTos and the -H on the adjoining carbon atom are anti periplanar. Even though this conformation has several gauche interactions, it is the only conformation in which -OTos and -H are 180° apart. 

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