Fused Cyclohexane Rings

Attach the methyl groups to the 1 and 3 carbons. Because you are drawing the cis isomer, the two substituents should both be on upward-pointing bonds or both be on downward-pointing bonds. 

Solution to 50b The conformer on the right would predominate at equilibrium because it is more stable—both methyl groups are on equatorial bonds. 

When two cyclohexane rings are fused—fused rings share two adjacent carbons—one ring can be considered to be a pair of substituents bonded to the other ring. As with any disub-stituted cyclohexane, the two substituents can be either cis or trans. The trans isomer (in which one substituent bond points upward and the other downward) has both substituents in the equatorial position. The cis isomer has one substituent in the equatorial position and one in the axial position. Trans-fused rings, therefore, are more stable than cis-fused rings. 

The most abundant member of the steroid family in animals is cholesterol, the precursor of all other steroids. Cholesterol is an important component of cell membranes. (See the box on page 121.) Because its rings are locked in a specific conformation, it is more rigid than other membrane components. 

Cholesterol is probably the best-known steroid because of the widely publicized correlation between cholesterol levels in the blood and heart disease. Cholesteol is synthesized in the liver and is present in almost all body tissues. It is also found in many foods, but we do not require cholesterol in our diet because the body can synthesize aU we need. A diet high in cholesterol can lead to high levels of cholesterol in the bloodstream, and the excess can accumulate on the walls of arteries, restricting the flow of blood. This disease of the circulatory system is known as atherosclerosis and is a primary cause of heart disease. 

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In a fused cyclohexane ring, such as those of steroids, the rings are frozen at room temperature and the axial and equatorial protons of each CH2 group are not chemical-shift equivalent. 

The compound decalin, CiqHis, consists of two fused cyclohexane rings ... 

A tandem radical cylization was used to form an indole ring with a [c,d]-fused cyclohexane ring starting with an N-allyl-2-bromo-3-vinylaniline. <94T2183>... 

Probably the most interesting examples of naturally occurring fused ring systems are steroids—an important class of physiological active organic compounds that include sex hormones and adrenal cortical hormones (e.g., cortison), cardiac glycosides, and bile acids. The skeleton of steroids consists of three fused cyclohexane rings and... 

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