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
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:
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:
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).