Glycoside

Concept: Concept: General Reaction

4m
Video Transcript

In this video I want to teach you guys the most important -O site reaction of monosaccharides and that is called O-glyocosidation. Alright guys, so as you might or may not know, monosaccharides have the ability to react at the -O position or the oxygen position in several different ways, okay? But in acidic conditions, monosaccharides can alkylate selectively, selectively at the anomeric position to produce what we call o-glycosides, that means, this kind upon here is an acetal, where you have an R group specifically attached only to the ÐO position, that's called a glycoside, it gets an O-side ending, okay? This process is also known as Fischer Glycosidation, and it actually has a lot of similarities to another reaction that you might have learned with a Fischer's name on it, a Fischer esterification, you guys remember that? It's very similar because it precedes through a reversible intermediate and it's acid catalyzed, there's oxygen and carbons involved, it's not a very wild mechanism, so once you get it, it's going to be pretty easy, okay? Now, let me show you guys the general structure, the general formula here. General reaction is you take a beta D-glucopyranose and you react it in alcohol and acid, so when I said it's similar to Fischer esterification, because that's the same thing that you would have used for a carboxylic acid, right? It's the same principle of that you're using acid and alcohol with a carboxylic acid, let's say, and remember that for Fischer esterification, you would get an ester, right? You would get OR, what's the same thing that happens here, except that it specifically only happens to this OH and it turns out OH into an OR group that has the same R group here, but it's also, it's only at the anomeric carbon, you're going to get a mixture of anomers, obviously due to mutual rotation. So, you're going to get some kind of combination of alpha and beta anomers, depending on the specific monosaccharides and what, which one is preferred for that setting, okay? At the end, what we get is a new functional group called an acetal, because you have OR groups coming off of both positions; OR, OR and if it's an acetal, that specifically once again gets the name of o-glycoside because it specifically has an R group at that anomeric position, okay? So, you might be saying Okay Johnny, I'm getting what you're saying, but how can you guarantee that it's only going to happen at the anomeric carbon? Why doesn't it happen here or here or here? Well guys, the reason is going to lie in this intermediate, called an oxocarbenium intermediate, which is a very special intermediate, itÕs the important intermediate of this reaction and that's what I'm going to show you in the next video, in the next video I'm going to prove to you guys how this can only happen at the anomeric position and it can't happen anywhere else. So, let's go ahead and go to the mechanism.

Concept: Concept: Mechanism

4m
Video Transcript

Okay. So, as any good acid catalyzed mechanism would go the first step has to be protonation, in this case guys you might notice that the acid I'm using is just going to be ROH positive because it's an easy combination of my alcohol plus my acid, it makes it easy to work with, okay? And it's going to have a very easy conjugate base. So, that's all we're going to use, okay? And guys because of the fact that we're trying to get rid of the anomeric alcohol, right? We're trying to turn it into an OR so that's we protonate, we always protonate the thing that we want to leave. So, my first step is just to take this OH and use it to grab a proton from my acid, what that's going to do is it's going to create a good leaving group and in the next step I can eliminate that leaving group by using one of the lone pairs on this O, one of these lone pairs is going to come in make a double bond and kick out the water and what that's going to make is an intermediate that looks like this with a positive charge here, okay? Now, guys, this is the oxocarbenium cations, let's go ahead and redraw it at the bottom so we can show the resonance structures, there's a soul bond here with a positive charge and we notice there's another resonance structure that would put these electrons up on the O, meaning that I'm going to have a positive charge here, okay? These are the two resonance structures that are possible for the oxocarbenium cation, and by the way, this is why this mechanism can only occur at the one position because notice that if we were to, let's say, do this at the two position right here we would not have any oxygen that we could form this resonance stabilized intermediate with, the only ways you can get the resonance stabilized intermediate is if you're next to the O and this is the only alcohol that's directly next to an O. So, that's why the an American position is the only one that can do this mechanism, okay? So, now we have this resonance stabilized carbo-cation and now it's the time that we can add you would take your ROH, whatever you're trying to add to glycosylate and it's going to attack the positive charge. So, now what I'm going to get is squiggly line, that's the ugliest squiggly line, let's try a little harder, squiggly line R, O, H, R, positive.

The reason I keep putting squiggly line guys is because, I don't know, if it's going to be alpha anamur or beta anamur it's going to be something in the middle, not something in the middle but it's going to be an equilibrium of the two and then finally, we could use our conjugate base to deprotonate and it give us our final answer, which is just OR, okay? So, as you guys can see, O glycosidation selectively adds an acetal group or an R group specifically to the anomeric position and no other positions because no other positions would give us this very important oxocarbenium cation that is resonance stabilized, okay? Awesome guys. So, I hope that made sense, let's move on to the next video.