Synthesis (2) – What Reactions?

In the previous post I  said that the first question to ask yourself when you’re doing a synthesis problem is to say “what’s new”? You want to identify all the bonds that form and break, if possible, all the atoms that are new, and all the atoms that have left.

This is going to be the “to do” list.

Once you’ve got a list, the next question to ask is:

“What reactions do I know that will form or break these bonds?”

Note that answering this second question can be a lengthy process. If you’re stuck, go through your textbook. Look for examples of reactions that might be able to achieve what you’re trying to do.

I wish I could give you a short cut toward making this step efficient. But I can’t. There’s no formula. There’s no short cut. At some point, you just have to know what bonds are formed or broken by each given reaction. I say there’s no short cut, because this data represents experimental results that were discovered – and they’ve been placed in the course material for a reason, as exemplars  of different key types of reactions.

It’s like learning a language: when you learn English, at some point, you just have to learn that the word “sheep” refers to a certain type of animal with fluffy fur that says “baaaa!”. There’s just no getting around it.

Expanding the number of reactions you know is like expanding your vocabulary in a language. The more reactions you know, the more powerful you will be coming up with potential solutions to synthesis problems.

OK, let’s look at the problem from yesterday.

Here, we see that we’re losing an ester, we’re adding carbons, and we’re breaking at least one C-H bond.

Let’s come up with some ideas.

• loss of ester – this could occur through decarboxylation. In order for this to occur we’d need a beta-keto carboxylic acid. We’re starting with a beta-keto ester, so that’s definintely a possibility.
• formation of C-C bonds ; there aren’t too many reactions that form C-C, but one of significance is enolate alkylation. I say this and not the Claisen or Aldol because we seem to be adding carbons that don’t have oxygens on them. This would suggest a straight alkylation. Also, the loss of C-H is consistent with alkylation.
• if we’re alkylating, what kind of alkyl halides would we need? We need to add methyl and ethyl groups. We can’t do this in just one step. So there must be *two* alkylation reactions.

These ideas came from looking at the patterns of different reactions, and recognizing those patterns. It will take some time to see the patterns. That’s why instructors harp on the importance of doing problems – because by doing problems, you’ll get much better at this.

The most fun part (for me anyway) is that there’s not always one right answer.  There can be multiple ways to solve these problems. It’s like traveling from one place to another. You can fly, you can take the Interstate, you can take the back roads. As long as you get to the final destination, t’s OK.
Next: let’s work on the timing.
Thanks for reading! James