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. So, sometimes your professor, your textbook, your online homework is going to ask you to do a diels-alder retrosynthesis, that means that you're going to be given the final cyclisation product and then you're going to be asked which diene and, which dienophile were required to make this six membered 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. So, here's an example, which diene and dienophile would you use to synthesize the following compound, okay? 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 tell you guys how to think forwards and I'm going to teach you guys how to think backwards as well. Well, the first thing that you guys want to notice with this product is that there's a certain like kind of like 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 kind of orient yourself on that because that's going to provide the structure for the rest of the molecule, so the first thing I want to do is identify where was that new double bond, that new double bond is right here, okay? That means that this must have been my second and my third carbon, okay? Which means that that must have been where my diene was, okay? So, my first step is always to find the original diene and 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 was my 1, and and this one must have been my four, okay? Now, that I have that diene I understand the mechanism better, I know that the diene must have reacted with a dienophile and remember that the diene always creates two new to the dienophile, so the next step is going to be to cross out the new bonds because, I know that my diene must have made two bonds. So, here's my diene once again, okay? And it made two new bonds, I'm going to cross these out because I know what they didn't use to exist, okay? 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, okay? So, basically, we're making those three double bonds that we started with so the answer for this question would be that I start off with a diene that looks like this a 1, 3 diene like this plus I started off with a dienophile that looked like this, okay? And now that we 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? So, I know this is a new skill, I know I might be making it look too easy. So, test out our skills on this example go ahead for the following two problems, try to figure them out, we'll do them one at a time, go ahead and do a right now and then I'll help you guys and show you the answer.

So that this was again just a three-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 two right here, which means that, when I Isolate the dienophile, I'm going to chop, right through both of those X's, okay? Now, notice that this question did include stereochemistry so in order to get this question right you needed to include the stereochemistry.

So, let's go ahead and draw both of these components I would have a diene that looked like this, okay? With the two methyls coming off of it and I would have a dienophile that is a double bond, right? Because it must have been a double bond but notice that you can't really see that, sorry, must have been a double bond, right? But notice that my substituents are cis to each other on the ring versus, if they're both facing up that means that that original diene, dienophile must have had cis groups so this is your answer, you must have had a diene that looked like that with the two R groups and then a cis double bond dienophile, alright? Awesome, so go ahead and work on the next one and then I'll show you guys the answer.

Alright, so due to limited space once again I'm going to take myself out of the video, don't miss me too much home 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. So, you have to take out the new bond 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 okay, meaning that when we go to separate this we're separating the diene and the diene a file, that means that my original diene a file must have had a double bond here, but it means that my diene had extra carbons, right? So, how big of a molecule was the original diene, how big, it was a ring, right? Because, we know that rings make bridges, right? We kind of went over that extensively so the question is how big is that ring and you're just going to draw it exactly the way it is, we're going to draw, let me use red, we're going to show that the diene must have been a six membered ring because I've got double bond, double bond but then it's got these 1-2 extra carbons that didn't even want to be there that are now here, okay? So, we had a six membered ring that formed a two carbon bridge and then my dienophile is pretty straightforward guys, it's just going to be this ring, 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 sister 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 six membered diene you got the question right, okay? Awesome. So, I hope that that skill might help you on the exam, maybe get you a few more points, let's go ahead and move on to the next topic.