12. Alcohols, Ethers, Epoxides and Thiols
t-Butyl Ether Protecting Groups
One way to protect alcohol is to form a reversible adduct with isobutylene via acid-catalyzed alkoxylation, yielding a temporary tert-butyl ether, which is completely unreactive.
Mechanism of t-Butyl Ether Protecting Groups.
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So the first time protecting group that you need to know and probably one of the more common ones is a Turk beauty ether protecting group. Okay, now what this does is it adds in ether to the oxygen, making it un reactive. Because if you guys remember, um, or if you guys just we've learned what function groups in the past alcohol's participate in a lot more reactions than ethers do. So what that means is that if I could turn my alcohol into an either it's gonna be protected as long as it isn't either. Okay, now, the reaction that we usually use for this is an acid catalyzed Al Cox elation. Okay, just so you know, an acid catalyzed a consolation is a lot like a acid catalyzed, catalyzed hydration, except that we're using an alcohol, as are solvent. In this case, the alcohol actually comes from my molecule. So let's go ahead and draw up this mechanism. We're gonna react with a molecule called Isil Beauty Lean, which is just this four member of hydrocarbon with a double bond and what we're gonna wind up getting is an ether. Let's figure out how in our first step. We're gonna protein eight, our double bond through a normal addition mechanism. Okay, What this is going to give me is a Markov Nankov, Carvel cat. I am all right. Remember that Markov knockoff states that your car will cat and goes on the more stable position. Okay, after I've done that, given the electrons to the, uh oh, what happens next? Well, now it's time for my alcohol to step in, so my alcohol is actually going to wind up attacking that Carvell cat iron, Okay. And what? I'm gonna make it something. It looks like this. Where? Now I have a Turk. Beautiful group on one side, the ring structure on the other. I still have one H and a positive charge. Okay, Now, how do you think we could get rid of that positive charge? Smart We could do is we could use the conjugate of my original acid. So I'm gonna go ahead and use the conjugate of my sulfuric acid. I'm gonna de protein eight. And lo and behold, look what I've got. I now have an ether instead of an alcohol. Now, why do you think this might be helpful? Having it looked like that. Well, because it turns out that this either that I'm looking at right here is completely un reactive to strong basis, Like Alcon. I'd remember that. I said an alkaloid would react with an alcohol it won't react with either. Okay, so now that means if I were to introduce my Alcon, I'd to this molecule after the ethers in place. Guess where it's gonna react. Not with the ether, the ethers protected. Now, this is my protecting group. Okay? That's my protecting group. Okay, so now what's gonna happen is that the only thing that it can possibly react with is my alcohol. Hey, lied through an s and two reaction. Okay, so that's the advantage of protecting groups. They allow us to react with just the thing we want and to ignore the thing that we don't want to react with. Okay. Name will be wondering. Well, Johnny, what does the final product look like? Well, what we would do at this point is that we could after this reaction is over, we could remove the protecting group. Why is that? Because we said this reaction has to be easily reversible, right? So what? That means is that? See how this is drawn with a positive arrow? I mean, with a forward looking arrow. Well, actually, it would be truly an equilibrium. It wouldn't be. Just affords arrow. So, for example, here, why I drew a forward zero here. That should really be technically, it should be an equilibrium. Right, Because we know that it's gonna go forwards now, but we can make it go backwards later. Okay? So after we do this step, how would we get it back to the original alcohol? Well, if adding our protecting group of step one, okay. And if adding our Alcon I'd was step two, then we have a third step, and the third step is just to add mild acid. Okay, so I could just say h two s 04 and water, and what that's going to do is that's going to deep protect. Okay. Whenever you protect, you always have to d protect. What does d protect mean? It just means that I'm gonna take that either. Completely off. Okay. Now I'm not going to show you the whole mechanism to d protect. You can imagine it's just the reverse mechanism of everything we've drawn to protect it. Okay, So what that means is that I would actually protein eighth e o first. Then it would leave, and then we'll get protein ated. Okay? The terp it'll group would leave, and then we'll get protein ated all right, and eliminated. All right, so I hope that makes sense. Guys, for the purposes of your test, you will need to know when you have to use a protecting group and when you don't Okay, in terms of synthesis, your professor could ask you Hey, how do I make this final product, okay? And just using that one re agent wouldn't be enough. You would need to use first. You need to protect Second. You could use your Alcon. I'd. And then third, you would have to d protect using acid and water. Okay, so I hope that makes sense. Guys, let me know if you have any questions. If not, let's go ahead and move to the next topic.