Concept: General Reaction4m
Now let's talk about an oxide reaction on monosaccharides called monosaccharides acylation, alright guys, so monosaccharides have the ability to react at the O position or the oxygen position in several different ways, specifically when we have a base promoted reaction with acid chlorides or with an anhydrides you're able to form polyester derivatives of your glucose, okay? So, here what I have done is I have shown you OAc, maybe you're a little bit not sure of what that is, I'm going to explain in a second.
So it turns out that specifically for this reaction even though lots of different bases would technically work Pyridine is a really good choice for reasons that I'm going to show you once we get to te mechanism, okay? Pyridine is the most commonly base used by far, it's probably the best choice and then specifically even though any acetyl group can be added acetyls groups are the most common carbonyls that are added. Now, acetyl groups are summarized or the structure is OAc, which I have drawn here, just remember what an acetyl groups is guys, it's an O with a carbonyl with a methyl group, okay? So this and the whatever is attached to, this would be an OAc group, and AOc which is O and then an acetyl group after that, okay? And notice that this is an ester, that's what we call polyester derivative, okay? So guy before we go through the mechanism let's just go through the general reaction. So my reaction has, once again I going to start off with beta-d-glucopyranose and I'm going to react it with a base, that base is going to turn these O into good nucleophiles and then it can either react with an acid chloride or anhydride to form these polyester derivatives. Now, guys if you think about it, if you think what we've learned from the other parts of organic chemistry, it's not a far stretch to think about the mechanism that could do this, because we know that is possible for negative charges to come in and do nucleophilic substitution with leaving groups, so that's all that's going to happen here, this is going to be an NAS nucleophilic iso substitution mechanism, but don't worry I'm going to show you that in a second, but I just kind of want to give you a preview that the mechanism is quite easy to think about, all we're going to do is we are going to add either this group from the acid chloride or this group, which is the same thing, from the anhydride and we are going to add that to every single position, so our product in this case would be a fully-acetylated glucopyranose, okay? Also I just want to make one point, this is not actually called the pyranoside because it's not just an R group, it has other Oxygens in it so it's not called pyranoside, only when you specifically have an R group in that position, okay? So anyway, you guys know the general structure, let's see at the mechanism and see if it makes sense.
Alright guys. So, here's the mechanism and what we see is that we're going to start off with our monosaccharide and we're going to react it with what I just drew out as pyridine, so this is what pyridine looks like, and you might be saying but Johnny I mean, I know that pyridine is a base the way the pyridine works by way is that it has a lone pair, so that lone pair can come in and deprotonate any of these OH's, in fact, it's going to do it five times. So, what we're going to get is negatives everywhere, okay? Now, I might be saying Johnny but does it specifically have to be pyridine? Not necessarily guys, but pyridine is a really good choice because in general what you want is a non-nucleophilic base, why is that important? guys, it's important to look at the type of molecule that we're going to use for the second step, in the second step we're using either an acid chloride or an anhydride, okay? What would happen, if I had used OH in the first step? could OH react with my anhydride? hell yeah, it could hydrolyzed that to a carboxylate. Remember, back to the three rules of nucleophilic acyl substitution the OH could come in here and kick off the O and eventually we get a carboxylate. So, I don't want to use bases that are going to react with my second step, which is why pyridine is a really good choice because pyridine is basic but it would never actually like attack here, it's not a good enough nucleophile to do that. So, pyridine is a good choice. Now, that I have these negative charges how can they react with my acyl groups? guys, this is just straight-up nucleophilic acyl substitution where the negative charge could come in, in this case I'm doing it within anhydride and it could grab the bottom, kick electrons up to the top. So, let's just draw this out for a second, okay? So, I'm going to draw pretty much the whole thing. So, now what this is going to attach to is o and that o is going to be attached to, let's see, a carbon that has ch3, that has o negative, that has o and then the rest of this thing here, okay? So, this is the tetrahedral intermediate and all the other O groups are preserved so that I would still have o minus, o minus, o minus, o minus, okay? So, this is my tetrahedral intermediate and then what's going to happen is that this negative charge comes down and kicks out this carbon here or the good leaving group. So, what I'm going to get at the end is a molecule, it looks like this. So, I still have all these O's in the same place but then this one has no O attached to, let's just do it this way, attached to a double bond o and ch3 and then plus my leaving group, which looks like this, okay? And, if you're confused how I got that, just say that this o is this o here, this o carbonyl is this guy right here, and this methyl group is this one here. So, notice that this is what we also call an OAc group and then this would just happen times 4, it would happen everywhere else so that you would get fully acetylated groups in all positions, cool? So, it's not that hard, it's just a variation on what you can do with a basic oxygen. Awesome guys, so we're done with this video, let's move on to the next.