Cyclopentane envelope

As indicated by Entry 5 in Table 6.2, the lithium enolates of pyrrolidine amides show excellent simple diastereoselectivity, and rearrange in excellent yields [69]. These amides also show a slight dependence of selectivity on the structure of the amide base used [69]. Monosubstituted pyrrolidine amides were poor auxiliaries for this reaction (<76% ds) [69], but C2-symmetric pyrrolidines are highly selective, as shown in Scheme 6.22 [90]. The Si facial selectivity of the lithium enolate and the illustrated zirconium enolate were comparable, but only the zirconium enolate also showed a high preference for the ul topicity illustrated. The two views of the transition structure rationalize both the topicity and the absolute configuration of the product. The enolate Si face is favored because the closer of the two pyrrolidine stereocenters blocks the Re face. The ul topicity is favored because when the enolate moiety is on the concave face of the cyclopentane envelope, a severe interaction between a pseudoaxial hydrogen and a cyclopentadiene is avoided cf. Scheme 6.14 a for another illustration). 

FIGURE 3 12 The (a) planar (b) envelope and (c) half chair conformations of cyclopentane... 

Cyclopentane has two nonplanar conformations that are of similar sta bility the envelope and the half chair... 

Envelope (Section 3 6) One of the two most stable conforma tions of cyclopentane Four of the carbons in the envelope conformation are coplanar the fifth carbon lies above or be low this plane... 

Cyclopentane is nonplanar, with a structure that resembles an envelope (see Fig. 1.5). Four of the carbon atoms are in one plane, and the fifth is out of that plane. The molecule is in continual motion so that the out-of-plane carbon moves rapidly around the ring. 

Cyclopentane is nonplanar, and the two minimum-energy geometries are the envelope and half-chair. In the envelope conformation, one carbon atom is displaced from the plane of the other four. In the half-chair conformation, three carbons are coplanar, vdth one of the remaining two being above the plane and the other below. The energy differences between the conformers are very small, and interconversion is rapid. All of the carbon atoms r idly move through planar and nonplanar positions. The process is called pseudorotation. 

In the envelope confonnation four of the caibon atoms aie coplanai. The fifth cai-bon is out of the plane of the other four. There aie thiee coplanai caibons in the half-chah confonnation, with one caibon atom displaced above that plane and another below it. In both the envelope and the half-chah confonnations, in-plane and out-of-plane carbons exchange positions rapidly. Equilibration between confonnations of cyclopentane is very fast and occurs at rates similar to that of rotation about the caibon-caibon bond of ethane. 

The ring templates can be further used to construct larger, multicyclic systems as illustrated in Fig. 7.8. For norbornane, two fitting conformahons of cyclopentane in the envelope conformation can be joined in order to construct the complete 3D structure of norbornane. In this case, this is the only low-energy conformahon known for norbornane due to its rigidity. 

Figure 4.11 (a) The folded or bent conformation of cyclobutane, (b) The bent or envelop form of cyclopentane. In this structure the front carbon atom is bent upward. In actuality, the molecule is flexible and shifts conformations constantly... 

Two non-planar conformations are believed to exist for cyclopentane which is the envelope form 10.9(Z>), and half chair form 10.9(c). 

Any one of the five atoms of the ring can be either above or below the plane defined by the other four in the envelope conformation. The energy barriers separating them are very low, and in cyclopentane or in proline all of the envelope conformations are freely interconvertible through intermediate skew forms.32 Furanose sugar rings are very flexible but the presence of the bulky substituents reduces the number of possible conformations.33-363 See Chapter 5 for further discussion. 

Any one of the five methylene groups of cyclopentane can be bent out of plane thus cyclopentane is a dynamic system which undergoes a series of conformational changes between different envelope conformations. 

In contrast to the findings for the cyclobutanes where the large amplitude motions mainly consist of conversion between rather rigid forms, the cyclopentanes exhibit more complex conformational and dynamic properties. Pseudorotation is a prominent large amplitude motion prevailing not only in cyclopentane but also in other five-membered rings. If the barrier to pseudorotation is high, distinct conformations may exist. In this case, the envelope conformation which has maximum Cs symmetry... 

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