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Solution: Asteltoxin, isolated from the cultures of Aspergillus stella...

Question

Asteltoxin, isolated from the cultures of Aspergillus stellatus, exhibits a potent inhibitory effect on the activity of E. coli BF1-ATPase. During S. L. Schreiber’s synthesis of asteltoxin, compound 1 was treated with a strong base to form anion 2 (J. Am. Chem. Soc. 1984, 106, 4186–4188):

(a) Identify the most acidic proton in  1 and justify your choice using any necessary drawings.

Video Transcript

Alright. So, in this molecule, let's go ahead and figure out where the most acidic hidrogen is. So, let's go ahead and give four positions, our yellow, purple, green and blue positions and here what we see guys is that some of these, this are the only positions that have hydrogens, right? The other positions, the other carbons all have four bonds, right? These right here, these all have four bonds so they're not going to have a hydrogen. So guys, let's take a look at green, it would remove a hydrogen from there we know that we are going to get a lone pair here, can that lone pair resonate through here? No, right? Because then that would give us a double bond here and our oxygen would have three bonds and a positive charge and it's a little bit weird what happens to our negative charge, right? Things are a little bit complicated, our oxygen doesn't like to have a positive charge period, so that's probably not going to happen, how about our purple? Well, our purple can go ahead and resonate through here, right? But there's a better one, right? Because that one is gonna have to choose between two paths, if we look at the yellow position, guys wait a minute, that one is already next to a double bond, right? It's not on a double bond which you know it's kind of stable, it's next to a double bond and it can resonate here, create a double bond here, kicking electrons throughout this whole thing all the way up to this oxygen up here, and when we do that, guys guess what? We just did a bunch of resonance structures and that one is super stable, compared to the blue, if you look at the blue, again it has to pick between two paths, right? It has to either go this way or this way and guys what else? It's directly on a double bond, right? So, it's not going to be as stable as the yellow one is if we move that negative charge if we didn't remove that hydrogen, if we remove the hydrogen on the yellow position we get a highly delocalized negative charge over the molecule, right? So guys, the hydrogen right here on our yellow position is going to be the most acidic, alright? Let's move on.