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6.11: Other Rings

  • Page ID
    191301
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    We have looked at some different six-membered rings, cyclohexanes, and have seen how substituents attached to these rings might influence the shape of the overall molecule. Cyclohexanes are particularly important because of they are quite common in nature. There are other variations of cyclic systems that are worth looking at, including different cyclohexane derivatives and other rings.

    Bicyclics and Other Polycyclics

    Decalins are examples of fused ring systems. In fused ring systems, two rings share an edge. Decalins contain ten carbons total, but the ten carbons are divided into two rings, each of which is six carbons around.

    Skeletal structure of decalin.

    There are two possible isomers of decalin: cis and trans. The easiest way to distinguish the two isomers is to look for the two hydrogens on the shared edge between the two rings. Those two hydrogens are found on the same face of the cis decalin.

    Skeletal structure of cis-decalin.

    Go to Animation CA11.1. A three-dimensional model of cis-decalin.

    The two hydrogens along the junction are seen on the opposite faces in trans decalin.

    clipboard_e1e4a38bb6d003033e693d3ed52991dae.png

    Go to Animation CA11.2. A three-dimensional model of trans-decalin.

    Exercise \(\PageIndex{1}\)

    Draw cis-decalin and trans-decalin using a wedge/dash picture.

    Answer

    Answer to Exercise 6.11.1, with cis- and trans-decalin.

    Exercise \(\PageIndex{2}\)

    Calculate steric strains in cis-decalin and trans-decalin.

    Answer

    Answer to Exercise 6.11.2, with cis- and trans-decalin. cis-decalin has two hydrogens highlighted with the caption "2 x 6 atom ~ 2 kcal/mol". trans-decalin has two hydrogens highlighted with the caption "5 atom ~ 0 kcal/mol.

    Fused ring systems like decalin are very common. In fact, similar ring systems are found in steroids, which are an important class of lipids. Steroids generally have similar structures that include a series of fused rings as seen in the parent compound, cholesterol.

    Skeletal structure of cholesterol, a four-ring organic molecule.

    The similarity is striking between estradiol, a female sex hormone, and testosterone, a male sex hormone.

    Skeletal structures of estradiol and testosterone, both composed of four rings.

    Go to Animation CA11.3. A three-dimensional model of estradiol.

    Go to Animation CA11.4. A three-dimensional model of testosterone.

    Exercise \(\PageIndex{3}\)

    Consider a steroid structure such as cholesterol.

    1. Based on the relationships along the fused edges between the ring, does the structure more closely resemble that of cis-decalin or that of trans-decalin?
    2. The overall shape of the structure could probably best be described as (choose one: wide and wavy / compact and curled up / boxy like a cage).
    Answer a:

    The substituents are always trans along the junctions between each pair of rings. The steroids resemble a series of trans-decalin structures.

    Answer b:

    The overall structure would be more wide and wavy like a trans-decalin, rather than curled or boxy like a cis-decalin.

    Polycyclic systems do not have to be fused along one edge like decalin. They can be connected in a variety of ways. Norbornane is an example of a bicyclic system in which the two rings actually share more than two carbons.

    Skeletal structure of norbornane, systematic name of bicyclo[2,2,1]heptane. Norbornane is composed of two rings fused together, forming a molecule with a total of seven carbons.

    Norbornane can be used to illustrate the systematic way to describe fused ring systems. The suffix describes the total number of carbons in the ring system. The prefix describes how many rings are fused together. The middle part, in brackets, indicates how many atoms form the "bridges" between the "bridgehead" atoms, where the system splits off into two different rings. One number is given to describe each bridge. In norbornane, there are two 2-carbon bridges and a single 1-carbon bridge.

    Exercise \(\PageIndex{4}\)

    Write a systematic name for decalin.

    Answer

    Bicyclo[2.2.0]decane

    Exercise \(\PageIndex{5}\)

    Provide systematic names for the following structures.

    Exercise 6.11.5, a through h.

    Answer a:

    Bicyclo[2.1.1]hexane

    Answer b:

    Bicyclo[3.2.1]octane

    Answer c:

    Bicyclo[2.1.0]pentane (more commonly called "housane")

    Answer d:

    Bicyclo[2.2.2]octane

    Answer e:

    cis-Bicyclo[3.3.0]octane

    Answer f:

    cis-Bicyclo[1.1.0]butane

    Answer g:

    Bicyclo[1.1.1]pentane

    Answer h:

    Bicyclo[4.3.3]dodecane

    Exercise \(\PageIndex{6}\)

    How many different rings can you trace in the structure of adamantane?

    Exercise 6.11.6, with skeletal structure of adamantane.

    Answer

    Answer to Exercise 6.11.6, showing three separate rings of adamantane highlighted in blue, green, and red.

    Although we could sketch out many rings using adamantane, just three rings are needed to include all the carbon atoms in the structure. Thus, adamantane is considered a tricyclic system. The systematic nomenclature of tricyclic systems gets a little more complicated, so we won't worry about that.

    Bicyclic systems are common in nature. For example, the tropane alkaloids all contain a bicyclo[3.2.1] ring system. Alkaloids are a group of natural products that contain basic nitrogen atoms. The tropane alkaloids contain this nitrogen atom in the 1-atom bridge. Tropinone is a simple example.

    Skeletal structure of tropinone. A single-member bridge in the middle is formed by a single nitrogen atom, which has an N-methyl group attached.

    Common examples of natural products containing the tropinone structure include atropine and cocaine. Atropine, derived from deadly nightshade or belladonna, is a heart medication; it is used to increase heart rate if the rate gets too low. Atropine can also be fatal if used incorrectly, causing a wide range of physiological effects; in some dosages, it can even decrease the heart rate. Cocaine, of course, is a narcotic and a controlled substance.

    Skeletal structures of atropine and cocaine. Both are bicyclic ring structures with benzoate groups.

    These compounds have a special place in the history of organic and medicinal chemistry. During World War I there was a shortage of atropine, prompting Sir Robert Robinson to develop an efficient synthesis of tropinone as an inexpensive source of synthetic atropine.

    Larger Rings

    Larger rings of carbon atoms are possible, although they are not as common as five- and six-membered rings. Ring formation is thought to be easiest from a six-carbon chain, because the two ends of the chain can easily wrap around to connect together yet they are not so distant that such an encounter becomes unlikely.

    Intermediate-sized rings, such as cyclodecane, become distorted to avoid steric interactions. Internal steric interactions are an added complication in the formation of some rings.

    Skeletal structure of cyclodecane, a ten-carbon ring.

    Exercise \(\PageIndex{7}\)

    Show the steric interactions within cyclodecane.

    Answer

    If cyclodecane adopted a regular diamond lattice conformation, there would be a whopping 8 atom interaction in the middle of the ring. That interaction isn't even included in our basis set. It would cost at least 6-7 kcal/mol. As a result, the cyclodecane adopts a twisted structure to avoid this interaction.

    At some point, in very large rings, steric problems are not as severe. The hydrogens on the inside of the ring may be far enough away from each other to avoid interactions. Cyclooctadecane is an example of such a ring. Although interior 6-atom interactions are possible, the ring is flexible enough to twist in such a way as to avoid these problems.

    Skeletal structure of cyclooctadecane, an eighteen-carbon ring.

    Exercise \(\PageIndex{8}\)

    Look at the model for progesterone and make a line drawing of it.

    Go to Animation CA11.5. A three-dimensional model of progesterone.

    Answer

    Answer to Exercise 6.11.8, showing the skeletal structure of progesterone.

    Exercise \(\PageIndex{9}\)

    Look at the model for androstanediol and make a line drawing of it.

    Go to Animation CA11.6. A three-dimensional model of androstanediol.

    Answer

    Answer to Exercise 6.11.9, showing the skeletal structure of androstanediol.

    Still pictures of models obtained using Spartan 14 from Wavefunction, Inc., Irvine, California.


    This page titled 6.11: Other Rings is shared under a CC BY-NC 3.0 license and was authored, remixed, and/or curated by Chris Schaller via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.