Pseudoaxial and pseudoequatorial

Only structures pertaining to the saturated 1,2-oxathiane 2,2-dioxides have been published. The preferred chair conformer is preserved in both polycyclic <1989AGE202, 1998CEJ1480, 1996CC431, 1994JOC3687> and spiro derivatives <1998EJO2073> with the sulfone oxygens in pseudoaxial and pseudoequatorial orientations. 

Although the termini of the 1,5-diene are sp2-hybridized, their two substituents occupy pseudoaxial and pseudoequatorial positions. Draw the substituents of the double bonds in your chair, again being careful to preserve the double bond geometry. Note that the upper double bond in your top view drawing is now... 

A more rigorous way of dealing with the problem is to flip the chair. When you flip the chair, all the axial substituents at C(sp3) centers become equatorial, and vice versa. Be careful to preserve the stereochemistry about the double bonds the pseudoaxial and pseudoequatorial substituents of the double bonds retain their orientation in the flipped chair ... 

The ability of ROA to measure vibrational optical activity in very low frequency vibrations has important implications for the study of normal modes corresponding to the foothills of the interconversion pathways between conformers. This is bom out by observations of dramatic changes with temperature in the ROA of two low-frequency bands of a-phellandrene, which could be associated with the interconversion of the pseudoaxial and pseudoequatorial conformers 85). 

The stereospecific reduction of the 5,10-dihydro-5-methyl-dil)enzo[b,e]phosphorin-10-one by NaH2Al(OC2H OMe)2 and LiAlHCOBu ) gives the pseudoaxial and pseudoequatorial alcohols (6) and (7) (Granoth, H. Segall, and H. Leader,... 

Calculations (MP2/6-31G ) allowed an estimate of the relative preference for 15 and 16 to place the alkoxy and methyl groups pseudoaxially and pseudoequatorially, respectively. Compound 15 placed the benzyloxy group pseudoaxial, with a 5.3kcalmol 1 preference, whereas 16 resided in a conformation placing the methyl pseudoequatorial with a modest l.Okcalmol-1 preference. 

The Meerwein-Ponndorf Verley reduction of 4-isopropyl-2-cyclohexenone yields a mixture of pseudoaxial and pseudoequatorial alcohols, the trans configuration being favored [d.r. (cis/trans) 35 65]171. 

Stereoelectronic effects dominate in the reactivities of simple 2-tetrahydropyranyl radicals [1]. In these ring systems, anomeric radicals are best characterized as an equilibrating mixture of pseudoaxial and pseudoequatorial radicals, which tend to be slightly pyramidalized because of the presence of the a-oxygen atom. Moreover,... 

The effect of introducing /j -hybridized atoms into acyclic molecules was discussed in Section 2.2.1, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are seen when sp centers are incorporated into six-membered rings. Whereas the energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecy-clohexane ° and to 4.9 kcal/mol in cyclohexanone. The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffraction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. The C(l)—C(2) bond length is 1.335 A, and the C(l)-C(2)-C(3) bond angle is 123°. The substituents at C(3) and C(6) are tilted from the usual axial and equatorial directions and are referred to as pseudoaxial and pseudoequatorial. 

Cyclohexene itself flips rapidly between these two conformations, with a barrier of about 22 kj mol i, about half that of cyclohexane. As in cyclohexane, hydrogen atoms on the saturated carbons of the cyclohexene structure adopt two types of positions, but as they are not quite orientated in the same way as in cyclohexane, we call the two orientations pseudoaxial and pseudoequatorial. ... 

The presence of the double bond in cycloalkenes affects the conformation of the ring. The conformation of cyclohexene is a half-chair, with carbons 1, 2, 3, and 6 in the same plane, and carbons 4 and 5 above and below the plane. Conversion to the alternative half-chair occurs readily, with an energy barrier of 22.2 kJ/mol (5.3 kcal/mol), which is about one half that required for chair-to-chair interconversion in cyclohexane. Substituents at carbons 3 and 6 are tilted from their usual axial and equatorial orientations in cyclohexane and are referred to as pseudoaxial and pseudoequatorial. 

The discussion to this point has concerned only cycloalkanes. Additional considerations come into play with cycloalkenes. For example, cyclohexene is said to adopt a half-chair conformation having methylene hydrogen atoms in four different environments. The hydrogen atoms are considered to be in axial or equatorial positions on C4 and C5, but those on C3 and C6 are said to be pseudoaxial and pseudoequatorial (Figure 3.26). 

The C-1—C-2 bond length is 1.335 A, and the C-1—C-2—C-3 bond angle is 123°. The substituents at C-3 and C-6 are tilted from the usual axial and equatorial directions and are referred to as pseudoaxial and pseudoequatorial The activation energy for ring inversion is 5.3 kcal/mol." The preference for equatorial orientation... 

Scheme 6. Pseudoaxial and pseudoequatorial. ring closure of the zwitterion obtained upon reaction of acrylonitrile with the carbene derived from 7.

In contrast, Nishimura et al. obtained stable monoadducts 62a-g possessing a hydroxy group suitable for further derivatization from the [2 -1- 4]-photocycloaddition of the corresponding carbonyl compounds 60a-g and [60]fullerene (Scheme 26). The adducts 62 were found to adopt one or both of two conformers A and E, which possess pseudoaxial and pseudoequatorial hydroxy groups, respectively (see Table 28.4). Two conformers existed for 62a, c, d, and f, while 62b, e, and g exclusively adopted E, A, and E, respectively. The comformer ratios remarkably depend on the bulkiness of the substituents attached to the aromatic nucleus and the cyclohexene ring. 

FIGURE 2.15 The most stable conformations of cyclobutane, cyclopentane, and cyclohexane rings (pa and pe denote pseudoaxial and pseudoequatorial positions, respectively). 

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