Diels-Alder Retrosynthesis - Online Tutor, Practice Problems & Exam Prep

In Diels-Alder retro synthesis, identifying the diene and dienophile from a cyclization product involves recognizing the new double bond formed between the second and third carbons. Start by locating this double bond to determine the original diene structure. After identifying the diene, remove the new bonds to isolate the dienophile. This systematic approach simplifies the process, allowing for effective analysis of Diels-Alder reactions, which are crucial in organic synthesis for creating six-membered rings.

Diels-Alder Retrosynthesis

In this video, I'm going to walk you guys through a technique that you might need to use for Diels Alder problems. Sometimes your professor, your textbook, or your online homework is going to ask you to do a Diels Alder retro synthesis. That means that you're going to be given the final cyclization product and then you're going to be asked which diene and which dienophile were required to make this 6 member ring in the first place. Now it turns out this is a really easy type of question to answer if you just have the right technique and that's what I'm here to show you. Here's an example. Which diene and dienophile would you use to synthesize the following compound? It's really easy to get lost in these types of questions because as college students, we hate doing stuff that we haven't been taught to do explicitly, right? And this is a backwards question. You have to think backwards. So I'm going to teach you guys how to think. I already taught you guys how to think forwards and I'm going to teach you how to think backwards as well.

The first thing that you guys want to notice about this product is that there's a certain kind of landmark that you're always going to be able to find on these products because you know about the mechanism. And the mechanism, remember, always makes a double bond between the second and the third carbon. Remember me showing you guys that? So that means the first thing you want to do is you want to find that landmark and you want to orient yourself on that because that's going to provide the structure for the rest of the molecule. The first thing I want to do is identify where was that new double bond. That new double bond is right here. That means that this must have been my second and my third carbon, which means that that must have been where my diene was. My first step is always to find the original diene. The way you do that is by identifying the double bond and then saying well, this must have been the diene to begin with. This must have been my 1 and this must have been my 4. Now that I have that diene, I understand the mechanism better. I know that the diene must have reacted with a dienophile. Remember that the diene always creates 2 new bonds to the dienophile. The next step is going to be to cross out the new bonds because I know that my diene must have made 2 bonds.

Here's my diene once again and it made 2 new bonds. I'm going to cross these out because I know they didn't used to exist. So after I've crossed them out, that means that whatever is attached on the other side must have been the dienophile. So in my next step, I isolate the dienophile. And guess how you do that? Really easy. All you do is you take your x's, right? You still have your diene and you just cross a line right through them. So I'm going to just chop it off right there, right through the x's. And what we see is that now we have a diene on one side and something on the other. The other side must have had a double bond in order to react in the first place. Basically, we're making those 3 double bonds that we started with. So the answer for this question would be that I start off with a diene that looked like this, a 1,3-diene that looked like this Plus, I started off with a dienophile that looked like this. Okay? And now that we've figured that out, isn't it crazy to see how actually it was a dimerization? This is a molecule that reacted with itself through Diels Alder but it wouldn't have been obvious unless you use a system to figure it out. Okay? I know this is a new skill. I know that I might be making it look too easy. Let's test out our skills on this example. Go ahead for the following two problems. Try to figure them out. We'll do them 1 at a time. Go ahead and do A right now and then I'll help you guys and show you the answer.

Retrosynthesis

So guys, this was again just a 3 step process and you always start by finding the original diene which must have been right here because this double bond comes from the original diene. If that's the original diene, that means the new bonds were these 2 right here which means that when I isolate the dienophile, I'm going to chop right through both of those X's. Now, notice that this question did include stereochemistry. In order to get this question right, you needed to include the stereochemistry. Let's go ahead and draw both of these components. I would have a diene that looked like this with the 2 methyls coming off of it and I would have a dienophile that is a double bond because it must have been a double bond. But notice that you can't really see that. Sorry. Must have been a double bond. But notice that my substituents are cis to each other on the ring. Notice that they're both facing up. That means that that original dienophile must have had cis groups. So this is your answer. You must have had a diene that looked like that with the 2 R groups and then a cis double bond dienophile. All right? Awesome. Go ahead and work on the next one and then I'll show you guys the answer.

Retrosynthesis

Alright. So due to limited space, once again, I'm going to take myself out of the video. Don't miss me too much. Right here. Okay. So let's go ahead and do this one once again. It was pretty easy to identify the diene. It was right next to the double bond. So we know that that is the diene. But notice that I gave you guys a bridge compound this time, so it's a little bit more challenging. You have to take out the new bonds in this next step. And I really hope you guys didn't cross out the bridge because that just doesn't make any sense. You guys know that the bridge has to do with extra carbons that must have been on the diene to begin with. So actually the bonds that we want to cross out are the ones that go to the rest of this ring. Meaning that when we go to separate this, we're separating the diene and the dienophile. That means that my original dienophile must have had a double bond here, but it means that my diene had extra carbons. So how big of a molecule was the original diene? How big? It was a ring because we know that rings make bridges. We kind of went over that extensively. The question is how big is that ring? You're just going to draw it exactly the way it is. We're going to show that the diene must have been a 6 membered ring because I've got double bond double bond. But then I've got these 1, 2 extra carbons that didn't even want to be there that are now here. So we had a 6 membered ring that formed a 2 carbon bridge. And then my dienophile is pretty straightforward guys. It's just going to be this ring. Oops. And really, you don't have to worry about cis and trans here because a ring is always going to be cis and trans. Wow, that was really unclear. What I meant to say let me just come back in. You don't have to worry about drawing my dienophile as like a cis or trans ring because rings are always going to be on one side of the molecule. They're automatically cis because they're small. So don't worry about that. This is the right answer. So if you just have this dienophile with this 6 member diene, you got the question right. Okay? Awesome. So I hope that that skill might help you out on the exam. Maybe get you a few more points. Let's go ahead and move on to the next topic.

Here’s what students ask on this topic:

What is Diels-Alder retrosynthesis?

Diels-Alder retrosynthesis is a technique used to determine the starting materials (diene and dienophile) required to synthesize a given cyclization product through a Diels-Alder reaction. By identifying the new double bond formed between the second and third carbons in the product, you can trace back to the original diene and dienophile. This method is essential for understanding and designing synthetic pathways in organic chemistry.

How do you identify the diene in a Diels-Alder retrosynthesis problem?

To identify the diene in a Diels-Alder retrosynthesis problem, locate the new double bond formed between the second and third carbons in the cyclization product. This double bond indicates the position of the original diene. By recognizing this landmark, you can determine the structure of the diene, which typically has conjugated double bonds at the 1 and 4 positions.

What is the role of the dienophile in a Diels-Alder reaction?

The dienophile in a Diels-Alder reaction is the molecule that reacts with the diene to form the cyclization product. It typically contains a double or triple bond that interacts with the diene's conjugated system, resulting in the formation of a six-membered ring. Identifying the dienophile is crucial in retrosynthesis to understand the starting materials needed for the reaction.

Can you explain the mechanism of the Diels-Alder reaction?

The Diels-Alder reaction mechanism involves a [4+2] cycloaddition between a conjugated diene and a dienophile. The diene's π-electrons interact with the dienophile's π-electrons, forming a six-membered ring with a new double bond between the second and third carbons. This concerted reaction proceeds through a cyclic transition state, making it stereospecific and regioselective.

What are some common mistakes to avoid in Diels-Alder retrosynthesis?

Common mistakes in Diels-Alder retrosynthesis include misidentifying the new double bond, overlooking the conjugated nature of the diene, and incorrectly isolating the dienophile. Always ensure you correctly locate the new double bond between the second and third carbons and systematically remove the new bonds to accurately determine the starting materials.