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Synthesis (3) – Figuring Out The Order

When doing a synthesis question, once you’ve asked yourself : 1) what new bonds formed and broken? and 2) what reactions can I use to accomplish these tasks?, the third question to ask yourself is simply:

“In what ORDER do these reactions happen?”

The order in which certain reactions occur is extremely important in synthesis. Often the presence (or absence) of a certain functional group is critical for various reactions to occur. For instance, in the synthesis below, it’s important to convert the methyl ester to the carboxylic acid before trying to decarboxylate; and it’s important to do the alkylation before the decarboxylation, because the enolate is much easier to form.
Trying to figure out the order in which things are done is MUCH easier at this stage – when we’ve narrowed down so many variables – than it is at the beginning, when there’s so many possibilities.

By the time you get to this step, most of the order in which to do things should be straightforward. Although at this stage, small modifications to the synthesis can occur. Give your final synthetic route a last look-over to make sure there aren’t any problems that could crop up with your synthesis. If there’s some obvious incompatibilities, it might be necessary to re-order things, or even to add in one last class of modifications: protecting groups. More on that soon.

Thanks for reading! James