Half-chair conformation, of cyclohexane

In the half-chair conformation of cyclohexane, four ac//acent carbon atoms are in one plane with the fifth above this piane and the sixth beiow it. You will this conformation again later—it represents the energy minimum for cyclohexene, for example. 

In order to ensure the stereochemistry of these molecules at C-6 and C-10, they were subjected to X-ray analysis (Fig. 1) which showed that ring A deviates from the preferred half-chair conformation of cyclohexane, is slightly distorted from perfect chair. This is possibly due to constraint imposed by the y-lactone ring. The sign of circular dichroism curve associated with the a, P unsaturated lactone has been taken as evidence for the absolute stereochemistry of this series. Teuflidin (15) showed the same sign and the magnitude of CD curve as teucvidin (14) and, hence, it was assigned to the same absolute stereochemistry. 

The annulation of 4//-thiopyran and cyclohexane rings in 50a results in the planarity of the heterocycle and a half-chair conformation of the carbocycle (81KGS1342). On the other hand, a boat conformation of the 2//-thiopyran ring was found in the crystal of224b [91JCS(P2)2061], Other geometrical parameters were within the limits of the expected values (Fig. 2). 

Figure 8.9. One octant of a sphere on which the conformations of cyclohexane can be mapped. Special conformations C, chair B, boat TB, twist boat HB, half-boat and HC, half-chair. (From Cremer and Pople [1975a,b].)...

Conformational energy of cyclohexane. The chair conformation is most stable, followed by the twist boat. To convert between these two conformations, the molecule must pass through the unstable half-chair conformation. 

The conformations of the six-membered ring systems are better characterized than those of the less stable five-membered analogues. For example, the cyclohexane molecule can occur in two strainless forms, namely in the rigid chair form or in the flexible form (Fig. 2-9). The latter can exist in a variety of shapes of which only the boat and the skew boat (or twist) are regular and easily depictable on paper. The chair form is preferred energetically because it is usually free from steric interactions whereas the flexible forms are not. The half-chair conformation is possible, when a six-membered ring contains either a double bond or an oxiran ring. In the half-chair conformation four adjacent atoms are in the same plane. 

Fig. 2-9. Conformations of cyclohexane. 1, Chair 2, boat 3, skew boat 4, half-chair.

Six-membered rings with more than one sp2 C atom do lose their chair conformation—they become flattened to some degree when there are one or more double bonds included in the ring. Cyclohexene, with just one double bond in the ring, has a half-chair conformation similar to that of its related epoxide, cyclohexene oxide. The usual conformational diagram of cyclohexene is shown below. The barrier for ring inversion of cyclohexene is around 22 kj mol-1 (about half that for cyclohexane ). 

As has been pointed out by Hart <84CHEC-I(7)185>, thiiranes fused to cyclopentanes adopt a boat conformation with C(3) and the sulfur atom on the same side of the C(1)C(2)C(4)C(5) plane. The angle of deviation from this plane is about 30° for C(3) and 60-70° for the S atom. Cyclohexane rings having fused thiiranes prefer a twisted half-chair conformation. Thiiranes fused to larger cycloalkanes can adopt a wide variety of conformations. 

Like all cyclohexene derivatives, L-shikimic acid (61) assumes a so-called half-chair conformation. Introduction of a double bond into a cyclohexane derivative in a chair conformation forces four of the carbon atoms (1, 2, 5, and 6 in shikimic acid) into a plane. This causes distortion to a halfchair form, as, for example, in conduritol B (63a), the depiction of which shows the positions of the substituents. The distortion has little effect on the dispositions of the substituents opposite the double bond, namely, those on C-3 and C-4 in shikimic acid and those on C-2 and C-3 in formula... 

The stereochemistry of these compounds is virtually identical, and the configuration of the substituents is 15(S ), 18(5 ), 19(5 ), 22( ), and 23(5 ). The two cyclohexane rings are chair conformations, whereas the cyclohexene ring has a distorted half-chair conformation. The presence of two more hydroxyl groups in dihydroxyaflavinine generates two additional asymmetric centers, 20(1 ) and 24(5 ). 

Surprisingly, the lowest energy boat conformation is more stable than the lowest energy half-chair by more than 4 kcal/mol. This inversion of conformational preference is a direct consequence of the sp hybridization of the cyclohexane carbons where they join the polycyclic ring system, hold the prow and stem of the boat in close proximity, and raise the energy of the half-chair conformation. ... 

Energy diagram for ring inversion in cyclohexane. The energy of activation is the difference in energy between the chair and half-chair conformations. The skew boat conformations are intermediates. The boat and half-chair conformations are transition states. 

FIGURE 4.16 The relative energies of the various conformations of cyclohexane. The positions of maximum energy are conformations called half-chair conformations, in which the carbon atoms of one end of the ring have become coplanar. 

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