Conformations of Polycyclic Molecules

The last point we ll consider about cycloalkane stereochemistry is to see what happens when two or more cycloalkane rings are fused together along a common bond to construct a polycyclic molecule—for example, decalin. 

Representations of tram- and cis-decalin. The hydrogen atoms (red) at the bridgehead carbons are on the same side of the rings in the cis isomer but on opposite sides in the trans isomer. 

Polycyciic compounds are common, and many valuable substances have fused-ring structures. For example, steroids, such as cholesterol, have four rings fused together—three six-membered and one five-membered. Though steroids look complicated compared with cyclohexane or decalin, the same principles that apply to the conformational analysis of simple cyclohexane rings apply equally well (and often better) to steroids. 

Norbomane has a conformationally locked boat cyclohexane ring in which carbons 1 and 4 are joined by an extra CH2 group. Note how, in drawing this structure, a break in the rear bond indicates that the vertical bond crosses in front of it. Making a molecular model is particularly helpful when trying to see the three-dimensionality of norbomane. 

Substituted norbornanes, such as camphor, are found widely in nature, and many have been important historically in developing organic structural theories. 

Another common ring system is the norbornane, or bicyclo[2.2.1.)heptane, structure. Like decalin, norbornane is a bicycloalkane, so called because two rings would have to be broken open to generate an acyclic structure. Its systematic name, bicyclo[2.2.1 heptane, reflects the fact that the molecule has seven carbons, is bicyclic, and has three bridges of 2, 2, and 1 carbon atoms connecting the two bridgehead carbons. 

A 1-carbon bridg A 2-carbon bridge Bridgehead carbons 

Which isomer is more stable, cis-decalin or frans-decalin Explain. 

Cyclic molecules are so commonly encountered in all classes of biomolecules, including proteins, lipids, carbohydrates, and nucleic acids, that it s important to understand the effects of their cyclic structures. Thus, we ve taken a close look at some of those effects in this chapter. 

Problem 4.20 Which Isomer is more stable, ds-dccalin or fra/ s-decalin Explain. 

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Conformations of Cyclobutane and Cyclopentane Conformations of Cyclohexane 127 Axial and Equatorial Bonds in Cyclohexane 129 Conformational Mobility of Cyclohexane 131 Conformations of Monosubstituted Cyclohexanes Conformational Analysis of Disubstituted Cyclohexanes Boat Cyclohexane 140 Conformations of Polycyclic Molecules 141... 

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