Stereoselective reactions of cyclic compounds

This chapter is about rings and stereochemistry. Stereochemistry is easier to understand in cyclic compounds and that alone might make a separate chapter worthwhile. But there is something much more fundamental behind this chapter. Stereochemistry is better behaved in cyclic compounds. Suppose you were to reduce this ketone to one of the corresponding alcohols. 

There would be very little chance of any control of stereochemistry at the new stereogenic centre (shown in black). A more or less 50 50 mixture of the two diastereo isomers would be expected. 

However, if we join up the molecule into a ring, things are suddenly quite different, 

The cyclic ketone has a fixed conformation controlled by the determination of the f-butyl group to be equatorial. Reduction can be controlled to give almost exclusively either the axial or the equatorial alcohol as we explained in Chapter 18. Large reagents prefer to approach equatorially while small reagents like to put the new OH group into an equatorial position. These are stereo-selective reactions, and, because the two different outcomes are diastereoisomers, we can call them diastereoselective. 

If your memory of Chapter 18 s discussion of axial and equatorial attack on cyclohexanones is dim, you should refresh it now. We shall use several examples that build on what we said there p. 470). 

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Chapter 33 Stereoselective Reactions of Cyclic Compounds Chapter 34 Diastereoselectivity. 

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