Cyclopentane, conformational isomers

Cyclopropane (Fig.J) is a flat molecule in respect of C-atoms, with the hydrogen atoms situated above and below the plane of the ring, so it has no conformational isomers. Cyclobutane can form three distinct shapes-a planar shape and two butterfly shapes (fig.K). Cyclopentane can also form a number of shapes or conformations. The planar structures for cyclobutane and cyclopentane are too strained to exist in practice because of eclipsed C-H bonds. 

The most frequent cyclopentane conformations show carbon atoms that carry two substituents equally disposed above and below the ring plane (Fig. 1.2.7). These are called isoclinal (i). Substituted cyclopentanes usually do not show significant conformational preferences. cw-l,3-Dimethylcyclopentane, for example, is only 0.5 kcal/mol more stable than the trans isomer, compared with an approximate 2 kcal/mol difference between the 1,3-dimethyl-cyclohexanes. 

Diastereomers of 13 were separated before the cycloaddition reaction since only one isomer was expected to adopt the sterically less demanding conformation in the transition state while the other diastereomer would suffer from severe steric congestion. Both diastereomers of precursor 13 were treated with KHMDS and the propynyliodonium salt, 12. The tandem cycloaddition reaction of the substrate 13a, to our pleasant surprise, gave tetraquinane product 11 in 50% yield, while 13b did not yield any cycloaddition product. These results allowed us to determine the relative stereochemistry of 13a and 13b as depicted. The sequential formation of alkylidene carbene and the TMM diradical intermediates transformed the cyclopentane substrate with a linear chain into the tetracyclic compound (Scheme 4). The core stmcture... 

Molecules of cyclobutane, cyclopentane, and cyclohexane are nonplanar. Besides determining configuration at each carbon atom in these compounds, conformational aspects of the whole molecule should also be considered. Figure 2.15 shows the most stable conformations of rings in these molecules. Of course, such molecules can also exist in any intermediary conformations between those shown. In monosubstituted rings of cyclobutane, cyclopentane, and cyclohexane there is no asymmetric carbon atom. In disubstituted isomers of these cyclic compounds, optical activity can appear even if the substituents are identical. The cis forms will not be optically active in any case since they exist in meso form. On the other hand, the trans forms will be optically active (they occur as two... 

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