Acetal Protecting Group

Now let's talk about a synthetic application of a settles, and that's to use a settles as protecting groups. So let's look at these two molecules above me, and what we'll find is that these molecules are in equilibrium. Okay, we have learned that if you expose ah, carbon heel, toe, alcohol and acid, you can basically Macon asked How? Okay, So in this case, since there's a cyclic a subtle I would expect to be using a cyclic dial's in combination with acid. So I just put h plus to give me my sick. Like I said, Tell okay, what's important about the difference between these is that there's a huge difference in reactivity between my original carbon eel and my ass. Little notice that my carbon eel has an extremely reactive, partial positive pylon, right, whereas in a Seattle does have die polls. So it does have die polls similar to the carbon Neil, but notice that they're located on Sigma bonds, not on a pie bond. So all these bonds are extremely difficult to break. In fact, in a Seattle is about as reactive as an ether. Just remember how reactive ethers were pretty much un reactive ethers barely do anything. All you could do is combust them Sure they blow up, but other than combustion, they don't react. That's because thes single bonds are extremely difficult to break. So now if I asked you which one is the safer version if I was to run a reaction somewhere else on the molecule, which one is safer toe, have around a carbon ill or has to tell? And the answer is in a settle because a subtle really isn't going to react with almost anything. Okay, whereas a carbon eel is so reactive, we've learned it can even wrecked with water, so we want to protect it. And that's the whole idea behind a protecting group. So assets house are used to protect sensitive Alba highs and key tones from reaction with other agents since they're reversible. The idea being that you can turn the Carbonell in to an acquittal, do your reaction somewhere else, and when you're done, you can go ahead and hide, relies it back to the original carbon eel, sparing it from reaction with your second re agent. So I want you guys to look at this example, and I want you guys to devise a synthesis for this. I want you toe look at these two molecules and figure out what would be the best way to make that first molecules into the second one. Now I'm gonna give you a hint. This is not a one step reaction. If you try to do it in one step, you're gonna fail. So try to think about what reactions you could do in what sequence to make this transformation happen, and then I'll show you.

all right. So the first thing you might have noticed is that I actually switched the molecule on you guys by a little bit. I decided to put a nitrogen here instead of an O. H. It's not really going to change the reaction at all. It's just that I did this for clarification because there were some unwanted side reactions with the O. H that I'm trying to avoid. Okay, so anyway, let's just look at this transformation. How could we make this happen? Well, the transformation that I'm trying to produce is going from an AM I two and a mean And in case you don't remember this reaction. Or maybe you haven't even learned it yet. The way to transfer Thio, transform and and my toe. I mean, it's just use a reducing agent. There's tons of reducing agents that will do this, but I guess the most common is lithium aluminum hydride. So we know that lithium aluminum hydride is a very powerful reducing agent, and it's strong enough to get rid of that carbon. You'll Internet into what I mean. So that's great to know. Awesome. Is that the answer for this question? No, I told you it's not gonna be one step. It's not that easy. It's not just just put LH and you're done, guys. The reason is because LH is a very powerful, reducing agent and it's going to react with my key tone as well, right? I have a key tone, but look in the end product, I need toe also retain that key tone. I can't just get rid of it and turn it into an alcohol. So how can I reduce part of the molecule while keeping the key tone intact? And you guessed it. We're gonna use an A settle protecting group. Okay, so that means that before we do anything, we're gonna have to protect this key tone. So let's go ahead and start. My first free agent is going to be Let's go ahead and use some kind of alcohol in acid to perform an ass. It'll reaction. The most common protecting groups are usually the cyclic, um, a settles. It doesn't have to be cyclic, but that just seems to be the one that eyes kind of like the go to in the lab. Okay, so let's just use on one to death in denial which would be this guy. Okay, so it's a two carbon chain with two alcohol's and acid h plus. Okay, So what that's going to do is transformed my key tone into an asset tell? So let's go ahead and draw that. Everything here is the same, by the way, as the towels don't react with, am I? So that's why nothing is gonna happen down there. But over here, I'm now gonna have Oh, and two carbons in between. So that is my cyclic assets. How protecting group. This is my protecting group. Cool, Awesome, guys. So now I've got my protecting group in place. What's awesome about thes protecting groups? Is that Is that a very reactive molecule now? No, this is basically a die ether, and we know that ethers don't react much at all. So now I can go ahead and reduce this molecule without fearing that I'm gonna lose my ketone. So now let's go ahead to the next step. This is where I use my lithium aluminum hydride, and that's just gonna blast away at this, am I? So what I'm now going to get is it's gonna turn my am I into an Amine, but I'm still gonna have my protecting group in place. Now one of the advantages of my Astal protecting group is that we know it's reversible, right? So that means that what can I use to reverse it and take this off and make it back into the key tone? I can just use dilute acid Dilute H Plus. There's lots of different ways you could write this. It could just be aged three. Plus, that's fine. So we know that instead of this being a one step reaction, this is actually a three step transformation where one I placed my protecting group to I use my reducing agent and then three. I just used H 30 plus to hide relies that as Talbak off and regenerate the original carbon deal. Alright, guys. So that is the way that we use assets. How protecting groups. Let's go ahead and move on to the next page.