Controlling the geometry of double bonds

Although El reactions show some stereo- and regioselectivity, the level of selectivity in E2 reactions can be much higher because of the more stringent demands on the transition state for E2 elimination. We will come back to the most useful ways of controlling the geometry of double bonds in Chapter 31. 

In this chapter we shall talk about reactions similar to the ones on the previous page and we shall be interested in how to control the geometry of double bonds. Geometrical isomers of alkenes are different compounds with different physical, chemical, and biological properties. They are often hard to separate by chromatography or distillation, so it is important that chemists have methods for making them as single isomers. 

You may think that this method is rather too trivial to be called a method for controlling the geometry of double bonds, as it s only of any use for making cyclic alkenes. Well, chemists are more ingenious than that Corey needed this as-alkene as an intermediate in his synthesis of the juvenile hormone we talked about above (it forms the left-hand end of the structure as shown there),... 

Chapter 31 described the methods available for controlling the geometry of double bonds. 

From this chapter onwards stereochemistry will be our constant companion as it is a major concern of modem organic chemistry. To start with we shall discuss diastereoselectivity alone. Soon, in chapter 15, we shall discover how to control the geometry of double bonds. Later on, in chapters 20-21 we shall discuss how to exercise stereoselectivity in general and in chapters... 

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