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multistep synthesis guide 1 2 3 4 5 > tutorials > home      


3. Work backwards

Ever find it easier to get through a maze starting at the finish and working back to the start? The same thing applies to multistep synthesis (working backwards like this is a technique termed retrosynthesis). Look at your product, and think of all the reactions that you know of that could form it, ignoring your starting material.

If your product is an alkene, think potentially of alkene-forming reactions like elimination reactions or the Wittig reaction. Write all these reactions out and look what reactant would be required for each. Now look at your starting material. The reaction that most resembles your starting material is probably the best one to select as a potential candidate.


For example, if you were asked to do the following synthesis:




After completing the first two steps above, you would want to think of ways to make the alkene in the product. Ignore the starting material for the moment. Just brainstorm all the ways you can think of to make the alkene and write them down on your scratch paper. You should get something that looks like this:


Now you have three possible routes to choose from. The route to choose is the one that uses a reactant that most resembles our original starting material. If you did step 2 (accounting for the carbon skeleton), you would know that the product has one carbon more than the starting material. Only the first reaction, the Wittig reaction, accounts for this additional carbon, and since the reactant for the Wittig reaction most resembles our starting material, this would be the reaction to tentatively choose. If it turns out to be wrong, we can always go back and try another route.

Looking at our reaction scheme, we now have something that looks like this:


Now repeat the same procedure for cyclohexanone, thinking of all the different ways you could make the ketone. One pointer here is that the closer you get to completing a retrosynthesis, the more you can reference the starting material in your thinking. At this point, for example, you may want to tune your thinking from "I need to think of all the ways I can make cyclohexanone" to perhaps something more on the lines of "I need a reaction that converts an alcohol to a ketone". If you did step one, you would know several ways (different chromate reagents, KMNO4, Ag2O, etc).

If you get stuck, go back and try one of the other pathways. If the Wittig reaction in our example had let to a dead end, then we could have gone back and tried one of the elimination reactions. Choosing the correct way back is often a manner of feel, and that only comes after working a lot of problems (See tip 5).

 

 Continue Tutorial :: See tip 4


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